applications of core level spectroscopy to adsorbates

Journal of Electron Spectroscopy and Related Phenomena 126 (2002) 3–42www.elsevier.com/ locate/elspec

A pplications of core level spectroscopy to adsorbatesa,b ,*Anders Nilsson

aStanford Synchrotron Radiation Laboratory, 2575 Sand Hill Road, Menlo Park, CA 94025,USAbFYSIKUM, Stockholm University, SCFAB, S-106 91 Stockholm, Sweden

Abstract

In the following review different applications of core-level spectroscopy to atomic and molecular adsorbates will beshown. Core-holes are created through core-level ionization and X-ray absorption processes and the core-hole decays byradiant and non-radiant processes. This forms the basis for X-ray photoelectron spectroscopy, X-ray absorption spectroscopy,Auger electron spectroscopy and X-ray emission spectroscopy. We will demonstrate how we can use the different methods toobtain information about the chemical state, local geometric structure, nature of chemical bonding and dynamics in electrontransfer processes. The adsorption of N and CO on Ni(100) will be used as prototype systems for chemisorption while N2 2

on graphite and Ar on Pt for physisorption. 2002 Elsevier Science B.V. All rights reserved.

Keywords: Core level spectroscopy; Adsorbates; Chemical state; Local geometric structure; Chemical bonding; Electron transfer process

1 . Introduction techniques that are surface sensitive and can be usedto study adsorbed molecules on surfaces [2]. Al-

Most of the important chemical reactions in nature though the methods are surface sensitive it is oftenand technology take place at surfaces and interfaces important to enhance the local information around[1]. In order to understand many of these reactions it the adsorbed entity. Core-level spectroscopy pro-is essential to establish a fundamental knowledge of vides a method to locally study the electronic andhow atoms and molecules interact with surfaces. geometric properties centred around one atomic site.When a molecule is adsorbed new electronic states This is in particular important when investigatingare formed due to the bonding to the surface. The complex systems with many different atomic sitesnature of the surface chemical bond will determine such as molecular adsorbates. Fig. 1 shows a N2

the properties of the adsorbed molecule. In the molecule adsorbed on a Ni surface in a perpendicularpresent review we will demonstrate how we can use geometry. The grey zone represents the chargecore-level spectroscopies to obtain information about density of the valence electrons extending outside thethe chemical state, local geometric structure, nature metal surface and we have a made a cut around theof chemical bonding and dynamics in electron adsorbate to see more deeply into the molecule.transfer processes. Inside we can see one particular molecular orbital

There exist a large number of spectroscopic overlapping both the nitrogen and Ni atoms and thecore electrons that are localized to one atom. Thecore-levels can be used to probe the valence elec-*Fax: 11-650-498-8151.

E-mail address: [email protected](A. Nilsson). trons in two different ways. First, the binding energy

0368-2048/02/$ – see front matter 2002 Elsevier Science B.V. All rights reserved.PI I : S0368-2048( 02 )00141-X

mailto:[email protected]

4 A. Nilsson / Journal of Electron Spectroscopy and Related Phenomena 126 (2002) 3–42

Fig. 1. Schematic view of the local probing character of core level spectroscopies applied to N adsorbed on Ni. The grey area represents2

the charge density outside the metal surface with a cut into the molecular adsorbate. Displayed is one particular molecular orbital extendedover all atomic centers and the two core orbitals on the nitrogen atoms. Electronic transitions between the core and valence electrons areindicated with arrows.

of the core-level as determined through ionization is the core electrons and the electronic structure in aaffected by the valence electrons [3]. Since the two metal in terms of occupied and unoccupied valencenitrogen atoms are non-equivalent, inner and outer states divided by a Fermi level. The different tech-atoms, the core-levels are shifted relative to each niques can be separated into two classes, creationother. Secondly, we can have transitions between and decay of core-holes. We can create a core-holecore and valence levels providing a direct probe of through the absorption of incoming light. The corethe valence electronic structure. Since the core electron can be excited to a bound state or to theelectrons are localized to one atom we can in an continuum where it will become a free particle. If theatom specific way study the valence electrons [4]. latter is the case and we measure the outgoing kineticWe thus now have a tool to look into the nature of energy of the photoelectron we can determine thethe surface chemical bond by disentangling the binding energy of the core-level. The technique thatcontributions from the different atoms. Furthermore, makes use of the ionization process is denotedthe valence electrons will be sensitive to the local photoelectron spectroscopy [5,8–11]. The spectros-geometry and we can expect that the core-levels are copy has traditionally been divided up into ultra-chemically shifted depending on adsorption sites and violet photoelectron spectroscopy (UPS) and X-raymolecular orientations [5]. The excitation process photoelectron spectroscopy (XPS) depending on theinvolving the core electrons will also cause some excitation energy. X-rays have energy high enoughdynamical response to the system involving electron to ionize core-levels, whereas the shallow valencetransfer processes [6] and vibrational excitations [7]. shell can be accessed with lower energy radiation.Overall there are many different properties that can With synchrotron radiation the boundaries betweenbe studied using core-levels. these two regimes becomes shady. We can also

Let us now characterize the different core-level measure the ionization process through the numberspectroscopies. Fig. 2 shows schematic pictures of of absorbed photons versus energy corresponding to

A. Nilsson / Journal of Electron Spectroscopy and Related Phenomena 126 (2002) 3–42 5

Fig. 2. A schematic illustration of core level ionization, excitation and decay processes.

an excitation into the ionization continuum [12]. In the ionization continuum. In the NEXAFS regimethis case we measure the total photoionization cross- the spectroscopy provides information about thesection above a core-level threshold as function of empty electronic states above the Fermi level [13].energy. If the excitation energy is not high enough to The core-hole can decay through two differentreach the ionization continuum we can populate mechanisms involving non-radiant and radiant pro-bound states above the Fermi level. The method is cesses. The first process will lead to electron emis-generally denoted X-ray absorption spectroscopy sion denoted Auger decay and the second to emis-(XAS). It is also divided into two regimes; near edge sion of X-rays. An electron from one of the outerX-ray absorption fine structure (NEXAFS) for bound shells fills the core-hole and a second electron, whichstates and low energy resonances in the continuum, is emitted from the system, takes up the excessand extended X-ray absorption fine structure energy. In the radiant process the core-hole is filled(EXAFS) when the outgoing electron is well above with an outer shell electron and the excess energy is


emitted in the form of a photon. For all core-levels steps, creation and decay, can lead to coupling andboth types of processes contribute to the decay. The the whole process can be considered a one-step eventadded rates of these decay channels determine the [18,19]. Fig. 3 shows the different resonant processeslifetime of the core-hole state and thereby the involving radiant and non-radiant decays. The ex-intrinsic width of the core electron line. In order to cited electron can either participate in the decayobtain information about the bonding electrons we process or be passive as a spectator leading to veryare primarily interested in decay processes involving different types of final states [16].the valence states. The analysis of the emitted In the non-radiant decay the spectator process iselectrons or photons is the basis for Auger electron rather similar to the Auger decay as previouslyspectroscopy (AES) [14–16] or X-ray emission discussed. It leads to a two hole–one electron finalspectroscopy (XES) [17,18]. The final state of the state where the excited electron can interact with thetwo decay processes is rather different, AES leads to two holes in the valence band. The dominant term ofa two-hole state, whereas XES to a one-hole state. If this interaction is the Coulomb screening whichwe are interested in a simple one electron picture lowers the final state energy by a significant amountXES provides a powerful tool for looking at the local of up to 20 eV. Additional energy shifts and splittingsvalence electron structure. The final states in AES are introduced through the coupling of the angularoften leads to strong interaction of the two holes and momenta and spins of the two hole–one electrona simple interpretation in a one-electron picture is final states. When the excited electron is directlynot possible. However, it provides a way for inves- involved in the non-radiant decay process, denotedtigating correlation effects between charge particles. participator decay, the final state is a single-hole

So far we have only considered a core-ionized configuration similar to valence band photoemissioninitial state prior to the decay. However, an initial [16]. Since the experiments in a one-step modelstate with the core electron instead excited into a involve the absorption of the incoming photon andbound state can modify the decay process. The two the emission of an outgoing electron the process can

Fig. 3. A schematic illustration of the coupling between the core-excited electron and different decay mechanisms.


be denoted photoemission and has different abbrevia- in order to slow the hopping rate down into thetions, such as resonant photoemission or resonant femtosecond regime limiting us to physisorptionphotoelectron spectroscopy (RPES). It is also often systems such as N on graphite and Ar on Pt.2

named resonant Auger spectroscopy (RAES) orautoionization spectroscopy (AIS). Under resonantconditions the spectral features disperse with photon 2 . Experimental considerationsenergy on a kinetic energy scale as expected from aphotoemission process and this is often denoted the In performing core-level spectroscopies on adsor-Auger resonant Raman effect [18]. Furthermore, the bates we need access to synchrotron radiation withdirect photoemission and the core-excited process high intensity and resolution together with spec-can undergo interference effects if the matrix ele- trometers to detect the emitted electrons and photons.ments of the two processes are of similar magnitude Since the number of atoms in a surface layer is not[20]. large and some of the spectroscopies (XES) require

The participator involvement of the excited elec- high incident intensity, we need synchrotron radia-tron in the radiant decay leads to a recombination tion from a third-generation facility. Fig. 4 illustratesprocess; the incident energy is released back. This an experimental system optimized for surface studiescan also be viewed as an elastic scattering of the in ultra high vacuum (UHV), where all core-levelincoming photon and if the elastic cross-section is spectroscopies can be applied to the same surfaceenhanced at a core threshold there is a resonant preparation. The instrument was built for beamlineprocess in operation. More interesting is the spec- I511 at MAXlab, Lund, where it presently is located.tator process that leads to a one hole–one electron During 1994–97 it was used at beamline 8.0 at thefinal state similar to an optically excited electron– ALS, Berkeley. Most of the experiments described inhole pair state. There are specific selection rules that the present contribution were performed at the ALS.govern the symmetry of the generated electron–hole Beamline 8.0 at the ALS consists of an undulatorpair which lead to that the parity is conserved for with 98, 5-cm periods and a spherical gratingfree molecules [18] or thek-vector is conserved in monochromator with three interchangeable gratings.solids [21,22]. The process can also be viewed as The total beamline covers an energy range betweeninelastic scattering of the incident X-rays where the 100 and 1000 eV with a maximum energy resolution

13 14lost energy is given to create the electron–hole pair. E /DE of 5000–10 000. A photon flux of 10 –10The method is therefore often called resonant inelas- photons/s can be achieved with a medium energytic X-ray scattering (RIXS) or resonant X-ray emis- resolution of 1000, important for XES experiments.sion spectroscopy (RXES). The cross-section for In order to match the size of the incoming beam withinelastic scattering well below the core threshold in the size of the entrance slit of the spectrometers thethe soft X-ray regime is negligible and therefore, as photon beam was focused down to 1003100 mmin photoemission, no interference effects are ob- using a pair of spherical mirrors.served between the direct and core assisted pro- One of the unique characteristics of synchrotroncesses. radiation is the polarization of the light and in

In the present contribution we will give examples beamline 8.0 the radiation on axis is nearly 100%of how we can use core-level spectroscopy to linear polarized. Since many adsorbates are wellinvestigate different aspects of adsorbate systems. oriented on the surface we want to make use of theWe will, in particular, address the nature of the polarization by allowing the light incident on thesurface chemical bond; different adsorption sites, sample to have theE-vector at different angles withmolecular orientations, vibrational excitations and respect to the surface plane. For one particularE-electron transfer processes. We will use the adsorp- vector orientation we want to be able to measure thetion of CO and N on Ni as our prototype system to emitted electrons or photons in any angle relative to2

illustrate how we can build up a consistent under- the surface. Since theE-vector orientation is fixedstanding. In the case of electron transfer processes from the insertion device we have to rotate both thethe coupling to the substrate has to be much weaker sample and the spectrometers. In order to reach high


Fig. 4. Schematic overview of beamline 8.0 at the ALS and the Uppsala Surface Science endstation.

symmetry orientations such as normal emission with differentially pumped rotatable seals. In order tothe E-vector either in plane or out of plane we need allow rotation of the chamber without too muchto have grazing incidence of the light to surface. By stress on the vacuum chamber the electron and X-rayrotating both the sample and the spectrometers spectrometer is supported via wires to a cranearound the axis of the incoming light all orientations located above the instrument. The wires are con-can be reached. However, for pure high symmetry nected to a spring block that has a counter weightorientations the angle of incidence of the light should force matching the weight of each spectrometer. Thebe close to zero. In the described set up we used an wires are attached at the center of mass on theincidence angle of 3–58. The grazing incidence also spectrometers and can glide on the crane duringincreases the sensitivity of the adsorbate layer with rotation. This means that the spectrometers have norespect to the substrate. Soft X-rays have penetration force on the vacuum chamber and the rotation can be

˚depth of the order of 1000 A and by using grazing made very easily. The angle of the chamber can beincidence the radiation is adsorbed more in the set within 18. There is a preparation chamber behindsurface region where the adsorbate is located. the analysis chamber separated with a valve. A

Fig. 5 shows a projection of the analysis chamber motorized long travel manipulator allows for motionof the experimental endstation along the incident of the sample between the two chambers. Thebeam direction containing an electron spectrometer, preparation chamber contains equipment for sampleX-ray spectrometer and a partial yield electron preparation and characterization with an ion-sputterdetector. The whole vacuum chamber is made rotat- gun, gas inlet system, evaporators, mass spectrome-able around the axis of the incoming beam using ter and low energy electron diffraction (LEED)


Fig. 5. Frontview graph of the Uppsala Surface Science endstation displaying the rotatable analyzer chamber with spectrometers.

system. The substrates are single crystals that are two channel plates in tandem and a phosphorousmounted on a sample holder where temperatures screen. A CCD camera registers the light pulses andbetween 30 and 1500 K can be achieved through the readout system is interfaced to a computer. Onecooling with liquid He or N and heating performed important consideration in using the optical readout2

radiatively or by electron bombardment. The base is the nonlinear response of the detector system due211pressure in both chambers varied between 5310 to multiple counting and saturation effects [25].

210and 1310 Torr. For many adsorbates, the high However, using a careful calibration procedure theseintensity beam from a third generation synchrotron effects can be eliminated.source is destructive. In these cases it is necessary to The X-ray emission spectra were measured with ascan the sample during data acquisition. multigrating, grazing incidence spectrometer with a

The electron spectra were recorded using a Scienta movable multichannel plate detector [26]. The grat-SES 200 electron spectrometer that consists of a ings have different line densities of 1200, 400 and200-mm radius hemispherical analyzer incorporating 300 l /mm and optimized blaze angles for thea multichannel detection system and a retarding spectrometer to cover a spectral range between 50electron lens [23]. The maximum resolution obtained and 1000 eV. The whole spectrum, with an energyfrom such an analyzer is 2.7 meV as determined in region around a core transition, is measured with thethe gas phase for the Xe 5p line using a He lamp. detector in fixed position without moving any com-However, for core-level studies of adsorbates a ponents. In order to control the energy resolution theresolution of the order of 50 meV is required to entrance slit is continuously variable from 0 to 100match the 100–200 meV inherent lifetime broaden- mm. For the C, N and O K-edges the spectrometerings from the C, N and O1s levels [24]. It is more resolution was set to 0.5 eV.important for the analyzer to have a high transmis- The X-ray absorption detector was used in thesion and electron detection efficiency since both the partial electron yield mode, effectively blocking slow1s ionization cross-section and number of adsorbed photoelectrons and secondaries by applying anmolecules are relatively low. The detector consists of electrostatic bias [12]. For XAS at the C, N and O


K-edges only electrons with kinetic energies larger effect on the valence electrons of a core-ionizationthan 200, 300 and 400 eV were selected, respectively. event will therefore be nearly the same as if a unitConsequently, Auger electrons from the corre- charge were added to the nucleus. Many of thesponding core-hole decay gave the dominant contri- properties of a core-ionized atom are therefore thebution to the signal. same as the properties of the next element in the

periodic table. This is theZ11 or equivalent coreapproximation. This approximation was used alreadylong ago when interpreting optical spectra. There are3 . Creation of core-holeseffects due to the interaction of the open shell natureof the core and valence states leading to multipletWe can create core-holes in two different wayssplittings that theZ11 approximation cannot de-resulting in ionization or excitation of the corescribe. However, the total energy of the core-holeelectron (see Fig. 2). In the case of ionization thisstate is usually well estimated using this approxi-can be studied through X-ray photoelectron spec-mation [27].troscopy (XPS) or X-ray absorption spectroscopy

We will use the equivalent core approximation in(XAS) depending on the detection scheme. In XPSseveral aspects in the present chapter. Chemicalwe measure the energy of the outgoing photoelectronshifts in adsorbates can be interpreted in terms ofat one specific photon energy (hy) and determine thedifference in adsorption energies ofZ and Z11core-level binding energy throughmolecules. The vibrationally resolved C1s spectra of

E 5E 2 hy (1)B kin adsorbed CO can be interpreted as vibrational excita-tions in adsorbed NO. The energy position of theWe can also study the ionization event in XAS2p* resonance in adsorbed CO measured using XASmeasurements through the variation of the photoioni-can be compared with the 2p* level in adsorbed NO.zation cross-section as a function of photon energy.

When an atom or molecule adsorbs on a surface,However, near threshold we determine the boundnew final state relaxation (screening channels) openstate close to the Fermi level. In both spectroscopiesup which are not present in the free atom orthe transition energyE is given as a difference inmolecule. The surface-induced relaxation can lowertotal energy before and after the spectroscopic eventthe final state energy by several eV. There are two

final initialE 5E 2E (2) different types of substrate-induced screening for anTrans tot tot

adsorbate: metallic screening and image potentialSince we are considering the total energy of the screening.

system we include all many body effects from the Metallic screening involves a charge redistributionremaining electrons in the system. For an adsorbed of the conduction electrons in the metallic system inmolecule on a surface the total energy means a order to reach a locally neutral core-hole site. Thetreatment of all electrons both on the molecule and in totally screened state can be looked upon as a state inthe whole crystal. In the creation of a core-hole there which the screening electron has been taken fromwill be substantial relaxation of the remaining elec- infinite distance. Since for a metallic system thetrons due to the change of Coulomb potential at the appropriate reference level for the binding energyionized site. It is essential to understand how the scale is the Fermi energy, we can consider theelectronic structure and the total energy of the core-ionization process as a core-excitation to thesystem will be changed due to the presence of the Fermi level. The onset in X-ray absorption spec-core-hole. troscopy (XAS) corresponds to the creation of a final

One way of getting total energy information on the state where a core electron has been placed in theproperties of the core-hole state is to use the so- lowest unoccupied state, i.e., at the Fermi level. Thiscalled equivalent core approximation. The radius of a means that the core-ionized final state is more or lesscore orbital is much smaller than that of a valence identical to the state obtained at the X-ray absorptionorbital. The core electrons are therefore located threshold, see Fig. 6. There is only a negligiblealmost entirely inside the valence electrons. The difference due to the fact that one electron is actually


Fig. 6. A schematic illustration of metallic screening as a relaxation channel through electron transfer in the final core hole state afterphotoionization. The core-ionized state can be compared with the lowest state reached in X-ray absorption spectroscopy (XAS).

removed in the photoemission process which is notthe case in XAS. This will change the Fermi level byan amount inversely proportional to the total numberof valence electrons in the system. Due to thisrelationship the Fermi level position for the un-occupied valence states as probed by XAS can bederived from the corresponding photoemission bind-ing energy.

Fig. 7 shows how the XP and XA spectra arerelated in two different cases of chemisorption for astrongly chemisorbed atom, C on Ni(100). Theadsorption energy is of the same order of magnitudeas the cohesive energy of Ni metal. It is clearly seenhow the XPS peak defines the threshold of the XASspectrum. There is even a change in slope of theleading absorption edge at the peak position of thephotoemission line. The same relationship holds alsofor molecular chemisorption.

The metallic screening of the adsorbate involvescharge transfer from the substrate to the molecule.The degree of charge transfer does not need tocorrespond absolutely to one electron and the core-ionized atom may in many cases be slightly ionic orpolarized in the bond to the surface. Information onthe electronic structure of the lowest core-hole statescan be obtained from theZ11 approximation. Foradsorbed atomic carbon, the final state is replaced by

Fig. 7. The XPS and XAS spectra for the p4g(232) phase ofan atomic adsorbed nitrogen atom. The local electron atomic carbon chemisorbed on Ni(100), The energy scale refers topopulation does not differ by exactly one unit binding energy relative to the Fermi level for XPS and to photonbetween carbon and nitrogen since the two atoms energy for XAS.


have different electronegativities. When ionizingatomically adsorbed C, N and O the final state atomswill always have a larger electronegativity than theground state atoms, leading to a screening by morethan one electron. In the case of a chemisorbed raregas atom, the final state is similar to an adsorbedalkali atom, which is ionic or extremely polarized,and we could anticipate that the charge transfer ismuch less than one electron.

If the interaction between the adsorbate andsubstrate is weak as for a physisorbed molecule(small orbital overlap), the ionized state cannot bescreened by electron transfer from the substrate. Therelaxation occurs only by polarization of the sub-strate, which in the case of a metallic substrate is

Fig. 8. The N1s spectrum for N adsorbed on Ni(100) in a2denoted image potential screening. The ionized atomc(232) structure. The spectra were recorded at 80 K. The two

or molecule is now left in a positively charged state different nitrogen atoms are marked in the figure.and charge redistribution in the metal causes anegative polarization charge to build up in thesubstrate. Hence the core-hole state is not in the based on different observed angular variations of thelowest possible state. There is no relation of the photoemission intensities from the two nitrogenenergy scales between the XPS and XAS spectra [5]. atoms due to photoelectron diffraction effects [3,28].However, there are electron transfer processes that The 1.3-eV shift can be understood in terms of thecan take place long after the core-hole has been adsorption energies of the different final statecreated but prior to the decay that can affect the species. Using theZ11 approximation, the N1sdecay spectra. This gives rise to the ‘core-hole clock ionized final state can be replaced by an adsorbedmethod’ discussed in Section 4.1.1. NO molecule. Core-ionization of the outer nitrogen

atom leads to adsorbed NO with the nitrogen atom3 .1. X-ray photoelectron spectroscopy (XPS) closest to the surface whereas the ionization of the

inner nitrogen atom leads to NO with the oxygen endIn this section we will discuss some examples of down. The stable configuration of adsorbed NO has

core-ionization providing structural and vibrational been shown to be with the nitrogen atom downwardsinformation in adsorbed N and CO on Ni(100). We [29]. Ionization of the outer nitrogen atom then leads2

will also demonstrate the future potential of XPS for to the most favorable final state, even though thehigh-pressure studies using differential pumping for core-hole is further away from the surface. Thismore realistic surface and interface studies. demonstrates that the energetics of the final state

screening in a chemisorbed molecule is not directly3 .1.1. Chemical shifts related to the distance of the core-hole to the surface,

Let us start to discuss the chemical shift between but that it depends on the overall electronic structurethe two nitrogen atoms in adsorbed N on Ni(100) of the core-hole state. The difference in adsorption2

with the XP spectrum shown in Fig. 8 [3]. Due to the energies between the two NO orientations on Ni isperpendicular adsorption geometry, the two nitrogen equal to the binding energy shift of 1.3 eV [28].atoms are inequivalent and two chemically shifted Next we will demonstrate how binding energyN1s peaks are seen at 399.4 and 400.7 eV binding shifts can be used to determine the adsorption site ofenergies. The broader feature at higher energy is due CO on Ni surfaces. In the coadsorption system CO1

to shake-up satellites. The component with the H on Ni(100) a wide range of adsorption siteslowest binding energy corresponds to the outermost becomes accessible [30]. There are three differentnitrogen atom [28]. This identification has been phases of CO in which the molecules occupy on-top,


bridge and hollow sites. Fig. 9 shows the C1s and on top.bridge.hollow. This is an empirical rela-O1s spectra from two different phases of the coad- tionship that could be used in the study of CO

] ]Œ Œsorption system, a c(2 23 2) overlayer structure adsorption on metal surfaces [30–32]. In vibrationalwith both on-top and hollow sites and a disordered spectroscopy a similar empirical rule has been usedphase with only bridge site. In the lower spectra two for many years based on the CO stretch vibration.different adsorption sites are populated giving C1s The shifts can be understood from total energybinding energies of 286.3 and 285.1 eV and O1s of considerations. In the initial state prior to ionization532.9 and 530.3 eV. The two spectral peaks are there are only small differences in adsorption ener-assigned to on-top and hollow sites. Another phase gies (a few 100 meV) for the different sites. Hence,can be populated with a single spectral peak in the major contribution to the shift must come frombetween the previous peaks with C1s and O1s changes in the energy of the core-ionized state.binding energies of 285.5 and 531.3 eV corre- Within theZ11 approximation, the C1s ionizationsponding to bridge site. The fine structure seen in the produces a NO-like final state and the O1s ionizationC1s spectra is due to vibrational effects discussed in a CF-like final state. The carbon part of the CFSection 3.1.2. molecule contains ‘three unpaired’ electrons avail-

These examples clearly show that there are strong able for bonding, while the ‘fluorine’ part of thesite-dependent binding energy shifts for adsorbed molecule is completely saturated due to the largeCO. The shift is about twice as large for O1s than for difference in electronegativity between the twoC1s. The binding energy decreases with increasing atoms. For a free carbon atom with four unpairedcoordination to the substrate atoms e.g., in the order electrons the adsorption site is such that the coordi-

nation to Ni is optimized. On Ni(100) the interactionis so strong that a reconstruction of the Ni latticeoccurs with the carbon atoms in the hollow position.Also CF can be anticipated to maximize the coordi-nation in a similar way. From this it can be con-cluded that the O1s final state molecule has itsequilibrium adsorption site in the hollow position,which is in agreement with the O1s shift measure-ments. In order to move the CF molecule to thebridge and on top sites the spectra show that energyof 1 and 2 eV is required. There is no largedifference in electronegativity between the CO andNO molecules. NO is therefore expected to adsorb ina manner similar to CO, only with slightly largeradsorption energies due to the extra unpaired valenceelectron. Therefore, the variation in the final stateenergy is smaller for C1s than for O1s. Thesechemical arguments have been supported by CIcalculations of Ni clusters simulating the ionizationfor on top and hollow adsorption sites of CO [32].

3 .1.2. Vibrational effectsThe core-level line profile in adsorbates is given

by the excitation of vibrations or electronic excita-tion processes [5]. The latter are often denotedshake-up excitations and can give rise to discreteFig. 9. The C1s and O1s spectra from the CO–H coadsorptionsatellites and asymmetries of the main line. How-system on Ni(100). The spectral structures corresponding to

different adsorption sites are indicated in the figure. ever, in cases when the main line is only negligible


affected by electronic excitations, vibrational effectswill determine the line shape. When atoms andmolecules are adsorbed on surfaces the vibrationalmodes that are present in the free molecule remainalso in the adsorbate, usually with only slightlymodified parameters. Furthermore, due to bonding tothe substrate, new vibrational modes will appear.These are related to the translational and rotationaldegrees of freedom of the adsorbate.

Let us first consider why vibrational excitationsappear upon core-ionization and describe theFranck–Condon principle. If the core-ionizationleads to changes in the interatomic distances and/orthe force constants, vibrational motion is excited inthe final state. The ionization is usually treatedwithin the Born–Oppenheimer approximation, whichimplies that the nuclear motion can be decoupledfrom the electronic motion. This allows the totalwave function to be factorized into one nuclear andone electronic part, which are solutions to separateequations. Fig. 10 shows the potential energy curvesfor the nuclear motion for two different cases. Thelower curve in each figure corresponds to the initialstate and the upper curve to the final state of aparticular electronic transition, such as a core-ioniza-tion process. The vibrational states are characterizedby quantum numbersn andn9 for the initial and finalstates, respectively. In Fig. 10 it is assumed that onlythe n50 vibrational state is populated. For thetransition probabilities to the various vibrationalstates the Franck–Condon principle is generallyapplicable which states that the relative transitionprobabilities for the same electronic transition aregiven by the square of the overlap between the initial Fig. 10. Initial and final state potential energy curves describingand final state nuclear motion wave functions. The photoionization in two different cases: (a) corresponds to a small

and (b) to a large geometry change due to the ionization, givingsituation in Fig. 10a refers to a situation where therise to small and large line widths, respectively.initial and final state potential energy curves are very

similar. The two sets of wave functions are therebyapproximately the same. In this case only then50 to 11 [7]. These can be directly assigned to excitationn950 overlap is non-zero. In Fig. 10b the potential of vibrational motion in the adsorbed CO molecule.energy curves are quite different and then50 wave The C1s spectrum is dominated by the first vi-function is non-orthogonal to a whole series of final brational component, whereas the O1s spectrumstate wave functions withn950, 1, 2, etc. This leads shows a rich progression. The C1s main line has ato the excitation of a number of different vibrational splitting of 217.8 meV and the O1s main line exhibitsfinal states. a splitting of 173 meV. To determine which vi-

If we inspect the C1s and O1s spectra for CO brational mode is excited in the core-ionizationadsorbed on Ni(100) with high resolution (50 meV) event, we look at the ground state vibrational modesthere are clearly well-resolved peaks, shown in Fig. of CO adsorbed on Ni(100) in the c(232) super-


allows us to show the effect of charge transferscreening in the adsorbate. This is due to the metalliccharacter of the system that leads to a locally neutral,core-excited final state [5]. For gas phase CO theO1s XAS and XPS vibrational progressions differsignificantly. Within the Franck–Condon picture theexcitation of vibrational motion is due to the suddenchange of the internuclear equilibrium distanceDrupon core-ionization. For gas phase CO the vi-brational progression in O1s XPS implies a smallchange of internuclear equilibrium distanceDr be-tween initial and final state, whereas in O K-edgeXAS Dr is large showing high vibrational excitation.The bond length for O1s core-ionized CO is slightlylarger than that of the ground state molecule [38].However, population of the antibonding 2p* orbitalin XAS leads to a substantial additional elongation ofthe internuclear equilibrium distance [38]. Turning tothe adsorbate O1s XPS spectrum we find that thevibrational progression is very different from the gas

Fig. 11. High resolution C1s and O1s spectra of c(232) CO on phase O1s XPS spectrum but similar to the gas phaseNi(100). The spectra show fitted vibrational fine structures. O K-edge XAS spectrum. This immediately dem-

onstrates again the importance of charge transferstructure. In this phase the molecule occupies, in an screening into the adsorbate LUMO with largely COupright position, on-top sites with the carbon end 2p* orbital character yielding a locally neutral core-down [33]. The carbon–oxygen stretch, molecule– excited final state. In the case of C1s spectra on thesubstrate stretch and frustrated rotation have been right side of Fig. 12 we find quite comparablemeasured with electron energy loss spectroscopy vibrational progressions for all the C1s XPS and C(EELS) to 256, 59.5 and 35 meV, respectively [34]. K-edge XAS data on gas phase CO and CO adsorbedThe frustrated translational mode has been deter- on Ni(100). This shows that the changes of internu-mined using inelastic helium scattering to 3.5 meV clear equilibrium distanceDr between the initial and[35]. The vibrational energy of the core-ionized final states are comparable in these processes. Themolecule will be slightly different but the only mode creation of a C1s core-hole causes a shortening ofclose to our observed splitting is the carbon–oxygen the intramolecular bond, contrary to the creation of astretch mode. O1s core-hole, where the intramolecular bond is

Both the different progressions and splitting dem- elongated. The population of the antibonding 2p*onstrates again that the electronic relaxation process orbital in C1s XAS causes an elongation of theleads to significant differences in the final states for intramolecular bond [38]. For adsorbed CO there isC or O ionization. We can further clarify the mecha- therefore a balance of a shortening due to the C1snism of metallic screening by comparing the adsor- core-hole and the elongation due to the screeningbate XPS spectra with gas phase CO data. In Fig. 12 electron in the 2p* orbital. The net effect of bothwe compare the vibrationally resolved C1s and O1s contributions is a change in internuclear distanceDrXPS spectra for c(232)CO/Ni(100) with gas phase of similar magnitude as the shortening in the XPSCO data from XAS [36] and XPS [37]. On the left final state, leading to low vibrational excitation in theside of Fig. 12 the spectra related to O1s excitation C1s XPS main line.and ionization are shown, whereas on the right side We can now make a nice comparison with thedata from C1s excitation and ionization are pre- vibrational properties of theZ11 adsorbate in thesented. The direct comparison to gas phase data case of C1s ionization. The NO molecule occupies


Fig. 12. The importance of metallic screening is illustrated by comparing the vibrational progressions in the XPS main line of adsorbed COwith XAS and XPS data of gas phase CO.

on top sites in the NO/Ni(100)–6L phase like the sorption system described in Section 3.1.1 we canCO molecule on Ni(100) in the c(232) structure. In measure the C1s spectra for different sites, shown inthis phase the ground state N–O stretch mode has a Fig. 13. A lowering of the vibrational splitting in thevibrational energy of 208 meV [34]. When compar- C1s line is found with increasing substrate coordina-ing the intramolecular stretch energies between C1s tion. In more highly coordinated sites the vibrationalionized CO and NO one has to correct for the sublevels are less resolved. This is due to smallerdifferent atomic masses using a triatomic (Ni,C,O) vibrational splitting, but also due to larger additionallinear force model. There is an excellent agreement broadening. We can numerically fit the spectra frombetween the measured vibrational energies of C1s the vibrational envelope using the Franck–Condoncore-ionized CO on Ni(100) in on-top position at principle [7]. The displacement of internuclear217.8 meV and the renormalized ground state vi- equilibrium distanceDr upon C1s ionization forbration in NO/Ni(100)–6L at 218 meV. This dem- adsorption on bridge and hollow sites is 0.045 and

˚ ˚onstrates the very high accuracy of the equivalent 0.066 A, respectively, in comparison to 0.039 A forcore approximation for bonding properties. on-top. We can note that there is an increase ofDr

We can now look at how the vibrationally resolved with increasing substrate coordination. Employingspectra change when the CO molecule can occupy the equivalent core approximation and the knownother adsorption sites [7]. Using the CO–H coad- adsorption geometry of CO/H/Ni(100) in bridge


tially a stepped (100) surface, a loss feature at 150meV has been observed and assigned to NO onhighly coordinated step edges [39].

3 .1.3. XPS at intermediate pressuresMost of the important chemical processes that take

place on interfaces proceed at much higher pressures.Often an UHV environment allows studies, in acontrolled fashion, of fundamental aspects of mole-cule–surface interactions. However, it is essential tocompare with studies performed at higher pressures.Photoelectron spectroscopy involves the detection ofelectrons that strongly interacts with matter. Thismeans that the escape depth in gas phase at highpressures is rather short. We can minimize therequired distance by using a gas cell and differentialpumping to lower the pressure in the space betweenthe cell and the spectrometer. A test experiment wasperformed in 1986 to demonstrate that this can beachieved [40]. Fig. 14 shows the set up and thespectrum of a Ag sample immersed in oxygen gas of1 Torr. The incoming X-rays pass through a thin Alwindow and the outgoing photoelectrons throughslits with several stages of differential pumping. The

27pressure in the spectrometer could be kept at 10Torr during the measurements.

The final state relaxation is very different for anFig. 13. Vibrational fine structure as a function of substrateionized species on a surface or in the gas phasecoordination. The C1s for CO molecules in on-top, bridge and

hollow sites. giving rise to a large chemical shift. In the spectrabelow there is a separation of 5–9 eV between thegas phase and the surface contributions. This inher-ent condensed-gas phase chemical shift will always

and hollow sites we attempt to assign NO adsorption be present upon core-ionization allowing for an easysites by comparing vibrational splittings between C1s separation of the two contributions. The two peaks inionized CO and ground state NO. Adjusting for the the gas phase contribution are due to exchangedifferent nuclear masses, we predict from our C1s interactions between the open valence and core shellsXPS measurements that the NO vibrational stretch in the final state since O is a paramagnetic molecule2

energies in the NO/H/Ni(100)system should be 175 [41]. The high binding energy feature in the surfacemeV and 150 meV for adsorption in bridge and spectrum is only present at high pressures.hollow sites, respectively. EELS data for NO/H/ With more efficient differential pumping it can beNi(100)–0.5L shows a loss peak between 160 and anticipated that higher pressures up to 10 Torr can be180 meV [39]. This is consistent with NO molecules obtained. This would allow studies of system involv-in bridge sites. For NO or NO/H on Ni(100) no ing liquid water. Recent attempts at the ALS inadsorption phase is found which exhibits an EELS Berkeley have demonstrated that such pressurespeak at 150 meV. This makes it unlikely that NO allow studies of water (M. Salmeron (2001) privatewould adsorb in 4-fold hollow sites on this surface. communication). We can foresee an interesting de-However, for 0.5L NO on Ni(510), which is essen- velopment for surface studies of surface reactions in


Fig. 14. The left panel shows an experimental schematic of the apparatus (left panel) used to perform X-ray photoelectron spectroscopy in a1-Torr atmosphere. A differentially pumped sample cell was utilized to measure photoelectron spectra from a silver sample with an ambientoxygen pressure of 1 Torr. The right panel shows the O 1s spectrum resolving contributions from the gas phase and the Ag crystal.

catalysis, liquid interfaces, molecular environmental position of the Fermi level in the spectrum, to whichscience and biomaterials. the valence electron states are related, is provided by

the core-level photoemission binding energy, as3 .2. XAS discussed in Section 3. The final state rule states that

In the following section we will describe a few the spectral features reflect the eigenstates of theaspects of XAS on adsorbates. If we want to use final state Hamiltonian [42,43]. In the case of XAS aXAS for studying the empty electronic states in core-hole is present in the final state. The influenceterms of chemical bonding it is important to consider of the core-hole may be very different for differentthe final state rule. The influence of the core-hole systems. One way to estimate the static influence ofwill modify the empty orbital structure in terms of the core-hole, i.e., the shift in position and change ofsymmetry and energy shifts. Core level shifts can the shape of the spectrum, is to use theZ11also be observed in XAS spectra and can be used to approximation, which implies that the core-excitedselectively generate core-holes on different atoms of atom is replaced by theZ11 atom. In XES, on thethe same element for decay spectroscopy studies, other hand, the core-hole is filled by a valencedescribed in Section 4. The dipole selection rule and electron leading to a similar final state as in valencethe polarized nature of the synchrotron light make it band photoemission. Ground state-related informa-possible to use XAS for studies of molecular orientation about the occupied states could in this way betions on surfaces [12]. more directly obtained.

The electronic structure of the interaction of CO3 .2.1. The final state rule. When we want to derive with metal surfaces will be discussed in Section 4.information about the electronic structure using X- There is a hybridization between the orbitals in theray spectroscopy it is important to understand in- metal and on the molecule leading to mixed states.fluences of the core-hole. In a one-electron picture The 2p* is the first unoccupied orbital of the systemthe X-ray spectrum reflects the occupied (XES) or and will have both molecular and Ni3d contributions.unoccupied (XAS) local density of states. The We can probe this orbital in a number of different


ways. Either through a local core-excitation on the C When probing these states through core-excitationsor Ni sites using XAS or the total projected un- on the C or Ni sites we observe energy differences.occupied states using inverse photoemission (IP). In the C1s XA spectrum the 2p* resonance appears

Fig. 15 compares the C1s and Ni2p XA spectra at 1.5 eV above the Fermi level, a shift of 2 eV in[13,44] with IP spectra [45]. As discussed in Section comparison with the IP spectrum. This can be3, the core-hole perturbs the empty density of states understood from the final state rule. The spectralin an XA spectrum. When plotting the XA spectra shape of the XA spectrum is governed by the densitythe corresponding core-level photoemission binding of states in the final state. We can estimate the effectenergies have been subtracted to provide an energy of the final state using theZ11 approximation. Atscale relative the Fermi level. In the IP spectrum of the bottom of Fig. 15 the NO on Ni IP spectrum isCO on Ni the empty 2p*–3d hybrid states can be shown and as can be seen the 2p* states appears atobserved at around 3.5 eV above the Fermi level. 1.5 eV above the Fermi level. This nicely correlates

with the position in the C1s XA spectrum and againvalidates theZ11 approximation.

Next we turn to the Ni2p XA spectrum which alsoprobes the 2p*–3d hybrid states but now on the Nisite. The spectrum has been obtained using onemonolayer of Ni on Cu as a model substrate [44].When CO is chemisorbed a new CO-induced (2p*derived) feature appears in the Ni2p XA spectrum atan energy of 3 eV above the core-ionization thres-hold. In this case the perturbation due the core-holeis not as large. Using theZ11 approximation, thechange in the 2p* position when CO is insteadadsorbed on Cu is small. This is shown in the IPspectrum of CO on Cu [46] at the top of Fig. 15.

3 .2.2. Chemical shifts. We showed in Section 3.1.1XPS chemical shifts between the two inequivalentnitrogen atoms in adsorbed N and for CO adsorbed2

in different sites. Since the core-level binding energydefines the onset of the XA spectra the question is ifwe will have corresponding shift of the 2p* reso-nance in adsorbed N and CO. Furthermore, the2

width or the density of state profile of the resonancecould also depend on the chemical surrounding.

How can we separate the different contributions tothe resonance from the two atoms in N ? The core-2

excited states obtained in the XAS process willsubsequently decay by resonant Auger or, autoioni-zation described in Section 4.1. Resonant Augerspectra have been recorded at several photon ener-gies around thep resonance [47]. It will be dis-cussed in Section 4.1.1 how the lifetime of theexcited electron depends on the strength of thecoupling between the adsorbate and the substrate.For chemisorbed systems the delocalized nature ofFig. 15. C1s and Ni 2p XAS spectra of CO adsorbed on Ni and

inverse photoemission spectra of CO on Ni and Cu and NO on Ni. the excited electron leads to similar core-excited


states prior to decay. We can expect that the decay obtained and are shown in Fig. 16. The position ofspectra are different for the two different nitrogen the 2p* resonance was estimated to 400.6 and 401.0atoms, as is indeed found. The intensity variations eV for the outer and the inner N atom, respectively.depend on the core-excited site and the resonant The splitting of 0.4 eV between the two resonances isAuger spectra can thus be used to separate the much smaller compared to the 1.3 eV splitting in theindividual contributions to thep resonance from the core-level binding energy positions. Furthermore, thetwo inequivalent N atoms [47]. widths of the two resonances are rather different, 1.6

The XA spectrum of the 2p* resonance of N eV for the outer atom and 0.9 eV for the inner atom.2

adsorbed on Ni(100) is shown in Fig. 16 [47]. The The fact that the XA positions are separated byspectrum consists of a main peak at a photon energy less than the XPS peaks is a result of the differencesof 401.0 eV with a shoulder at 400.4 eV. This can be in the final state density of states of the two atoms.compared with the core-level binding energies of the We can use theZ11 approximation to again de-two atoms at 399.4 and 400.7 eV described in scribe the influence of the core-hole corresponding toSection 3.1.1. We can see that the XA threshold for NO for core-excited N . Excitation on the outer atom2

the two main structures corresponds to the core-level leads to NO with nitrogen down which is the normalbinding energies. Using the resonant Auger decay chemisorption geometry for adsorbed NO [29]. Forprocess to decompose the XA spectrum, the inthis NO geometry we generate a rather strong surfacedividual XA spectra for each nitrogen atom could be bond resulting in a large hybridization of the 2p*

with the metal electrons with a broad resonance. It isinteresting to note that the same final state can bereached from core of the carbon atom in adsorbedCO, as described in the previous section. The widthof the C1s 2p* resonance is 1.7 eV [13], closelyresembling what is obtained for the outer nitrogen inN . Furthermore, the peak position is at 1.5 eV above2

the core-level binding energy compared with 1.2 eVfor the outer nitrogen. Part of the difference in thelatter comparison comes from vibrational effects, asdescribed in Section 3.1.3. The peak position in theXPS nitrogen spectrum corresponds to a more highlyexcited vibrational state is in higher vibrationalexcited state compared with C1s in CO resulting in asmaller difference between the XAS and XPS posi-tions. In the case of the XA spectrum from the innernitrogen it corresponds to NO adsorbed with oxygendown leading to an extremely weak binding to thesubstrate. The resonance maximum is located 0.4 eVabove the core-level binding energy position. Theweak bond for NO with oxygen down leads to smallhybridization of the 2p* level. The free NO mole-cule is paramagnetic with one electron in the 2p*level. When the molecule weakly interacts with ametallic system we can expect the half filled 2p*level to intersect the Fermi level. Furthermore, if welook at the shape and width of the 0.7 eV resonancewe can make a connection with the gas phase XAFig. 16. Separation of the N1s XAS 2p* resonance for thespectrum. Both shape and width are similar to thec(232) N /Ni(100) into individual XAS spectra from inner and2

outer atoms. vibrational envelope of the corresponding excitation


of the free molecule implying that the hybridization resonance widths and vibrational envelopes. We canbroadening is rather small in the inner nitrogen 2p* note that the population of both sites is equal in theXA spectrum. overlayer and the weaker intensity for the hollow site

Fig. 17 shows 2p* resonance XA spectra of CO in is due to much broader width and decreased oscil-different adsorption sites for the CO–H coadsorption lator strength. The latter is related to an increasedsystem on Ni(100) corresponding to the XPS spectra population of the 2p* level in the ground state due todescribed in Section 3.1.1 [48]. The corresponding hybridization with the occupied orbitals, see SectionXPS binding energy positions are indicated by 4.2.2. In the disordered phase with only bridgearrows and taken from Fig. 8. The on-top site in the adsorption the resonance position is in between on-c(232) structure can be prepared by CO adsorption top and hollow similar to the core-level bindingon the clean or hydrogen precovered Ni(100) sur- energy shift. The molecule–surface interaction isface. The former leads to much stronger CO–metal large in the bridge site resulting in a broad reso-interaction reflected in broader resonances. The nance.resonance width and position in the hydrogen preco- We can observe a trend in that the increased 2p*vered surface are similar to the resonance in the width both in adsorbed N and CO reflects the2

weak adsorption system CO on Cu(100) [49]. The interaction strength. However, a large electronic] ]Œ Œc(2 23 2)structure consists of both on-top and interaction strength does not necessary lead to a

hollow sites. We can directly see in Fig. 17 two strong chemical bond. There are interactions that canchemically shifted resonances in both the C and O have different signs in terms of changes in the totalK-edge spectra, corresponding to the different sites. energy. In Section 4.2.1 we will describe attractivep

The resonance shifts follow the core-level binding interaction and repulsives interactions that counter-energy shifts in a manner similar to N with the balance each other. Both of these terms can be2

hollow site at lower energies However, the shifts can increased, while the total bonding energy remains.

be slightly different depending on variation of the unchanged.

Fig. 17. XAS of the C1s and O1s 2p* resonances for three phases of the CO–H coadsorption system on Ni(100). The spectra for the COc(232) phase on Ni(100) is shown for comparison. The respective 1s XPS binding energies are indicated.


3 .2.3. Molecular orientations. One of the mainapplications of XAS to studies of adsorbed mole-cules is to determine molecular orientations, which isbased on the application of the simple dipole selec-tion rule for XAS [12]. In this context the techniqueis often called NEXAFS. The resonance intensity inan XA spectrum associated with a particular molecu-lar orbital final state is largest if theE-vector of theincoming light points in the same direction as thep-component in the final state orbital on the excitedatom and vanishes if theE-vector is perpendicular tothis direction. Hence, by using linearly polarizedsynchrotron radiation, information about the adsorp-tion structure of molecules can be obtained byperforming angle-resolved measurements. A strong

Fig. 18. C1s, N1s and O1s XAS of glycine adsorbed on Cu(110)polarization dependence of the XA spectra indicatesspectra measured in three directions. The corresponding XPSa well-defined adsorption geometry of the surface–binding energies are marked with vertical lines.molecule system.

We like to show how we can determine theorientation of the simplest amino acid, glycine oriented close to parallel to the surface. However, weadsorbed on Cu(110). The glycine molecule contains can also observe an appreciablep* intensity in thetwo functional groups, a carboxylic group and an in-plane spectra along the [001] direction. Theamino group. The molecule adsorbs with the molecu- angular dependence of the resonance intensity goes

2lar skeleton intact except for the removal of the asI~cos u [12] whereu is the angle between theacidic hydrogen. The Cu(110) surface has 2-fold E-vector and the direction of the atomic orbital.symmetry, where the surface Cu atoms are oriented From the intensity difference we can derive that theinto rows. This means that we have three directions COO plane is tilted by around 308 with respect to thedefined for the p-components, p , p and p respec- surface in the direction perpendicular to the rows.x y z

tively. We can align theE-vector out of plane ([110]) In the C and O K-edge spectra we can observe twoand in plane either parallel ([110], or perpendicular molecular shape resonances,s and s , connected1 2

([001]) to the Cu rows and determine how the with the carboxylic group. We can see that thedifferent functional groups are oriented with respect resonances have opposite polarization dependenceto these symmetry directions. having contributions of either p or p character. Wex y

In Fig. 18 we show the C, N and O K-edge XA can consider these resonances as arising from de-spectra resolved in three directions [50]. We can note localized molecular orbitals derived from the appro-that there is a strong polarization dependence found priate point group symmetry. The glycine moleculein both the C and O K-edge spectra indicating a well does not have any symmetry, however, the localoriented overlayer. In both C and O spectra we can probing using core-levels means that we can consideridentify energy regions withp*- and s*-related the symmetry of the particular functional group. Weresonances indicated in Fig. 18. The carboxylic can make a comparison with formate adsorbed on agroup contains ap electron system delocalized over surface [50]. The constituting orbitals can thereforethe COO unit. We can derive the orientation of the be viewed as a formate, a orbital with a p-com-1

COO plane with respect to the surface plane from the ponent in the COO plane pointing towards thepolarization dependence. Since the C and O 2p carbon atom situated at lower energy and a b -type2

components in thep system will be perpendicular to orbital pointing along the O–O direction. Thes2

the COO plane. From Fig. 18 we can observe the shows a polarization dependence indicating that themaximum intensity of thep* resonance in the out of O–O direction is in a plane along the Cu rows andplane spectra indicating that the COO group is thes shows that the COO plane is tilted close to the1


surface normal. Both these observations support the substrate-induced antibonding orbitals is moreoverconclusion made from thep system. found to be present at the Fermi level. We therefore

Two shape resonances associated with the C–C conclude that the amino group is directly involved inand C–N bonds are furthermore expected to appear the bonding to the surface. Although the antibondingin the spectra. The interaction between the corre- states are to a large extent occupied, some contribu-spondings orbitals will result in new hybrid orbitals. tion is expected to appear near threshold in theEmpirically, s resonances due to C–C and C–N absorption spectra. Inspection of the data reveals thatsingle bonds are found at lower energy compared to this is the case. Intensity is observed all the wayresonances associated with C–O double bonds [12]. down to the Fermi level in the N K-edge [110]From a comparison between formate and acetate, spectrum. The lowest unoccupied molecular orbitalwhere the latter has an additional methyl group thes (LUMO), observed close to the Fermi level in theshape resonance related to the internal C–C bond has [110] spectrum, can hence be interpreted as thebeen identified with a position indicated in Fig. 18. unoccupied part of the antibonding substrate-inducedThis resonance intensity is enhanced in the [001] state, to be discussed in Section 4.2.3. The hydrogenspectrum as expected since the bond is an extension bonding to the adjacent molecules makes the identifi-of the COO plane with a small contribution also in cation of the C–N shape resonance not straight-the out of plane spectrum due to the tilted geometry. forward. From a tentative interpretation of the N1sHowever, the spectrum will also contain a strong spectrum a straight C–C–N geometry could beC–N s resonance contribution. We note that there is obtained. However, it turns out that the spectra canonly negligible intensity due to this resonance in the be theoretical simulated showing excellent agreementspectrum excited with the electric field vector along with the experiment if a bent C–C–N geometry isthe direction of the Cu rows. This indicates that also assumed [52]. This also demonstrates how importantthe C–N axis likely is oriented along the [001] it is with a close interplay between experiment anddirection. theory.

The interpretation and assignment of thes reso- Fig. 19 shows the orientation of the glycinenance in the N K-edge spectra of the amino group is molecule on Cu(110) as obtained from the ex-more complicated. From a comparison with adsorbed perimental and theoretical simulated XAS or NEX-ammonia we can conclude that NH resonances are AFS study. The same structure was obtained usingstrongly affected by the presence of the substrate. density functional theory (DFT) calculations toRecent theoretical simulations have indicated strong optimize the geometry providing the lowest energyeffects in spectra due to hydrogen bonding between [50,52]. In addition, the molecular orientation hasmolecules within the overlayer [51]. Similar to what also been investigated with reflection absorptionis observed for adsorbed ammonia, we find thespectra to be dominated by broad features due tohydrogen bonding. Near threshold, more narrowstructures are found. These are interpreted as due toN–H s resonances, similar to the ones observed forfree ammonia. From the C and O K-edge spectra weknow that the carboxylic group is highly tilted on thesurface. It is therefore natural to assume that theamino group contributes to the bonding to thesurface. From angle-resolved X-ray emission studiesof glycine [52,53] and ammonia [54] adsorbed onCu(110), evidence of new, substrate-induced valencestates has been found, see Section 4.2.3. These statesare interpreted as due to Cu-amino lone-pair hybridconfigurations and are observed along the surface Fig. 19. The adsorption structure of glycine chemisorbed onnormal, i.e., the [110] direction. Intensity of the Cu(110).


infrared spectroscopy deriving a similar adsorptionstructure [55]. The positions of the oxygen andnitrogen atoms relative to the Cu surface atoms arein close agreement with an energy scanned photo-electron diffraction study [56]. We can see that themolecular skeleton bridges over two Cu rows on thesurface in order to bond through both the carboxylicand amino functional groups. XAS or NEXAFS is apowerful tool to derive molecular orientations fromthe intensity variation of the spectral resonances withrespect to differentE-vector orientations relative tothe surface plane.

4 . Decay of core holes

The generated core-holes can decay in two differ-ent ways (see Fig. 2). In the non-radiant decay weget emission of Auger electrons and in the radiantdecay, emission of photons. The rates of the twoprocesses determine the lifetime of the core-hole. For

Fig. 20. Comparison of O1s core-hole decay spectra from radiantthe C, N and O 1s levels the Auger rate is muchand non-radiant processes on O/Ni(100) c(232). The electronfaster than the radiative decay process and thusbinding and emission energies are put on a common energy scale.completely determines the inherent lifetime broaden-The vertical bars in the AES spectrum indicate the relative energy

2 4 1 5 0 6ing. In the case that the decay process involves the position of the Ne 2s 2p , 2s sp and 2s 2p atomic multipletvalence shell we can obtain information about the configurations. The dotted line indicates relative shifts of some

multiplets due to atomic number corrections between the Ne andoccupied electronic structure in an atom-specificO atoms.way. Since the final state after the decay process

contains no core-holes, we probe the electronicstructure without the core-hole modifications seen in in Fig. 20 to represent the O2p partial density ofXAS, described in Section 3.2.1. The two decay states with an overall width of 8 eV. The selectionprocesses give rather different final states. The X-ray rule governing the Auger process allows both O2semission decay generates a hole in the valence shell and 2p states to be observed. The O2s states havewhereas the Auger decay process generates a double- much higher binding energies, around 20 eV andhole state. If the two holes are both in the valence therefore provide the much larger width of 50 eV inshell the difference in the final state becomes rather the AES spectrum. Since the Auger decay processlarge between the two spectroscopies. involves the ionization of two valence electrons we

Fig. 20 shows a comparison of the AES [57] and can expect that the spectrum can be simulatedXES [58–60] spectra of atomic oxygen adsorbed on through a self-convolution of the O projected localNi(100). The core-level binding energy is 530.2 eV density of states. However, it has been shown thatcorresponding to the energy of the Fermi level in the the main peaks in the spectrum in Fig. 20 cannot bedecay spectra, as discussed for XAS in Section 3. We explained from a simple self-convolution [57]. In thecan observe an extremely large difference in width of energy region 475–515 eV, five features are dis-the spectra. First of all there is a large difference in cerned in the spectrum. Upon comparison with anselection rules for the two decay processes. The XES Auger spectrum of Ne gas [61] with the energy scaleprocess follows the dipole selection rule such that reduced due to difference in atomic number, it isonly O2p valence electrons can decay into the O1s observed that these features are almost identical incore-hole. We can regard the XE spectrum displayed terms of both relative intensities and relative energy


positions as in the case of Ne. This can be under- different with large energy shifts of the main peaks.stood based on that the Auger decay starts with a This large spectral influence provides a probe if the

6local 2p configuration due to the screening of the excited electron is still present on the core-excitedcore-ionized oxygen atom [57,62]. The bars in the atom when the decay takes place. We can think that

2 4spectrum correspond to different parts of the 2s 2p we start a clock at the time when the core-hole is1 5 6(bars 1 and 2) 2s 2p (bars 3 and 4) and 2s82p (bar generated and then we probe during the decay what

5) multiplet patterns of different atomic oxygen has happened at a time later, corresponding to theconfigurations. The shape of the spectrum is mainly lifetime of the core-level. Since the core-hole life-determined from the hole–hole interaction providing time is of the order of femtoseconds we have anmany different terms of local atomic configurations. extremely fast probe to study charge transfer pro-However, the weak features around 520 eV can be cesses between an adsorbate and substrate. Thisgenerated from a self-convolution of the XES spec- process has been denoted the ‘core-hole clock’trum [57]. method.

From the example above we have demonstratedthat if we want to obtain a simple one-electron 4 .1.1. Femtosecond probing of charge transferpicture of the local electronic structure XES is the processesspectroscopy to utilize. The dipole selection rule and For molecules that are physisorbed on surfaces thethe one-hole valence final state makes the spectrum overlap of the molecular orbitals with the substrate isrelatively easy to interpret. The question is why do rather small. We consider most of the adsorptionwe want to use AES at all. First of all the hole–hole energy to come from dispersive forces. Many of theinteraction provides a possibility to study complex properties of the free molecule are therefore pre-correlation effects. However, rather detailed calcula- served. However, the response of the system upon ations are necessary for the understanding of these perturbation caused by the spectroscopic process canspectra. There is another important utilization of be rather different compared with that of the freeAES using resonant excitation. We discussed in molecule. The interaction with the substrate in theSection 1 that we can have both spectator and final state can generate rather complex spectralparticipator decay following a bound core-excitation. profiles [63].The influence of the excited electron in the spectator Let us go back and look at the core-hole creationspectra becomes rather large due to the multiplet process for N physisorbed on graphite. The re-2

splittings observed in AES spectra. The spectra are sponse from the substrate is very different for thevery different if the excited electron is present in the excitation and ionization processes. Fig. 21 comparesfinal state after the decay process. This can be used N1s XPS and XAS spectra [6]. The XAS peak atto study the time scale of charge transfer processes 401.0 eV is due to the excitation of a 1s electron toafter core-excitations. the lowest unoccupied orbital, 1p , in N . The fineg 2

structure in the spectrum corresponds to vibrational4 .1. Auger electron spectroscopy (AES) excitations. It is immediately seen that the XAS peak

appears at lower energy than the XPS peak. A fullyWe will in the following section describe how we screened XPS final state, neutralized by the transfer

can use AES spectra after resonant excitation to of an electron from the substrate to the 1p orbital,g

study time-dependent processes. As described above, would have an energy corresponding to the X-raythe spectral shape of the AES spectra is often absorption threshold (see Section 3). Hence, the XPSdetermined by the open-shell exchange interaction of final state at 403.9 eV is not the lowest possible

21the two-hole state. The presence of an additional core-hole state. It is a 1s ionized state with noexcited electron above the Fermi level will severely charge transfer screening from the substrate, butchange the multiplet structure in a spectator decay screened by the image potential [64]. No peak isspectrum. Furthermore, if the excited electron par- seen at the binding energy of the screened final state.ticipates in the decay process providing a simple The electrostatic interaction with the substrate in thesingle hole final state the spectrum become very ionized state creates a rather large final state bond


screened core-hole state prior to the decay. Fig. 22compares the decay spectra for four different situa-tions; a resonant Auger spectrum of N on graphite2

excited at the 1p resonance, the correspondingg

Auger spectrum after core-ionization, an Auger

Fig. 21. The N1s to 1p XAS and N1s XPS spectra of oneg

monolayer N physisorbed on graphite. The energy scale is photon2

energy for XAS and binding energy for XPS.

energy of around 1.5 eV. This will result in extradegrees of freedom in terms of excitation of mole-cule substrate vibrational motions on top of theinternal vibrational excitations seen for the freemolecule. Therefore no fine structure can be seen inthe XPS spectra. However, the core-excited neutralstate interacts with the substrate through dispersiveforces creating only small changes in the bondingenergetics and a potential curve of the final statesimilar to that of the initial state resulting in a

Fig. 22. N1s hole decay spectra of N ; 401.0 eV photon energy2spectrum in close resemblance to the free molecule 21 1corresponds to excitation to the 1s 1p state, while 500 eVgspectrum. 21photon energy corresponds to the 1s intermediate state. General-The interesting question is if there can be any ized final state configurations are given for the Auger and resonant

relaxation of the core-ionized state to the lowest Auger processes.


spectrum for N which has been physisorbed with a molecular orbitals. At timet50 the core-ionization2

event of the N molecule takes place. There is adouble spacer layer of Ar between the N and the 22

probability for a charge transfer process from thesubstrate and finally an Auger spectrum for N in the2

substrate to the adsorbed, ionized molecule prior togas phase [6]. The resonantly excited spectrum of N2

the core-hole decay. We can denotet by theon graphite is very similar to the core-excited gas CT

timescale for the charge transfer process. The chargephase spectrum [65]. The twin peaks at 390.3 and21 transfer-screening electron goes into the 1p orbital,391.3 eV kinetic energies correspond to the 1p gu

21 which is the only unoccupied orbital of the ionicand 3s states, respectively, created by participatorg

molecule below the Fermi level of the substrate. At atransitions. At lower kinetic energies we find mainlylater time corresponding to the core-level lifetimet ,two-hole one-particle final states formed by spectator G

the core-hole decays. We can decompose the decaytransitions [66]. The shape of the gas phase Augerspectra into two model spectra that arise fromspectrum [67], shown at the top of the figure, isdifferent initial states prior to the decay, ionized orcompletely different from the resonantly excitedneutral states. The resulting final state of the Augerspectrum since the initial state of the core-hole decay

21 decay process is either a local double charge state oris now a 1s ionic state. The main intensity is founda single charge state, respectively. The latter part ofaround 363 eV kinetic energy, approximately 20 eVthe spectrum contains 3865% of the intensity [6].lower than in the corresponding (spectator) transition

From the relative intensities of the neutral andin the adsorbate resonantly excited spectrum. Thisionic decay processes in the adsorbate Auger spec-decrease in kinetic energy is due to the differenttrum the neutralization rate can be determined. If wereference energies, the presence of the spectatorassume an exponential Auger decay of the core-holeelectron and the substrate screening for the adsorbedgoverned by the lifetimet , and an exponentialmolecule. Turning now to the N on graphite Auger G2

neutralization governed by the charge transfer timespectrum it is immediately clear that it containst , it is straight-forward to see that the probabilityspectral features typical both for resonantly excited CT

for Auger decay from the neutralized state is givenand core-ionized Auger transitions. At 390 eV kinetic2121 21 by the ratio (11t /t ) [6]. The lifetimet of theenergy a feature reminiscent of the 1p and 3s CT G Gu g

215single-hole final states is observed. Around 373 eV core-hole is 5.5310 s (G50.12 eV) for free N2

kinetic energy, peaks are seen which have to be [68,69]. This yields a neutralization timet 593CT215identified with free molecular like N Auger transi- 10 s. This can be compared to the similar charge2

tions. The kinetic energy is 3 eV higher than for the transfer processes in ion neutralization spectros-corresponding transitions in N /Ar /Ag(110), due to copies, which yield typical neutralization times for2

215differences in work function and image potential ions incident on surfaces of the order of 10 s [2].screening. We can thus identify two distinctly differ- In the previous example we showed that there is aent parts in the spectrum; one due to transitions from charge transfer process from the substrate to adsor-

21 1a neutral 1s 1p state, and another one due to bate to obtain the lowest core-hole state prior to theg21transitions from an ionic 1s state. Since no neutral decay. We can also envisage the reverse process

state was seen in the XP spectrum the neutralization where an excited electron is transferred from themust occur after the primary ionization but at the molecule to the substrate. In order for this reversetime scale of the 1s hole lifetime. From the dis- transfer to happen the core-ionized state should haveappearance of the spectator and participator features a lower energy than the core-excited state. Thiswhen the N molecules are separated from the situation arises in adsorbed Ar on surfaces where the2

substrate by the Ar layers it can be concluded that 4s resonance is located well above the Fermi level.the electron comes from the graphite substrate and In Fig. 24 the Ar2p to 4s XAS and 2p XPS3 / 2

not from the surrounding molecules in the N spectra are shown on a common energy scale for Ar2

overlayer. adsorbed on Pt(111) [19]. We can observe that theThe ‘core-hole clock’ process is summarized in energy of the ionic XPS final state is 3.9 eV lower

Fig. 23. The figure illustrates a general DOS picture, than that of the neutral XAS final state. The spectralalthough the physisorbed N system has discrete shape of the 4s resonance is clearly modified com-2


Fig. 23. Schematic description of the core-hole clock method for N physisorbed on graphite. The electronic structure of the N molecule is2 2

illustrated in a DOS picture.

pared with the gas phase due to interaction with the to the maximum of the 4s resonance. The spectrumsurface. The resonance is broadened and it has a tail reveals two sets of spectral features, which may beextending all the way to the energy of the Ar2p identified as demonstrated in the lower part of the3 / 2

XPS peak, corresponding to the Fermi level, due to figure. The Auger spectrum (crosses) corresponds tointeraction with the substrate. The intensity in the a 2p off-resonance Auger spectrum of the Ar on3 / 2

XAS spectrum at the energy of the XPS peak is very Pt system. The different peaks in the spectrum are4small. The interaction of the 4s state with the due to multiplets of the 3p configuration. The gas

substrate is manifested in the decay spectra. We may phase 4s resonantly excited spectrum (dashed line)view the 4s derived states as quasi-localized atomic gives rise to a different fine structure corresponding

4 1states. Due to the presence of the substrate, delocali- to the 3p 4s spectator configurations. By summingzation of the 4s-derived charge may occur via charge the gas-phase Auger and spectator resonant excitedtransfer. This can be described as a transition to the spectra with appropriate weight factors, we obtainenergetically most favorable core-hole state occur- the model spectrum given by the solid line. As canring within the core-hole lifetime. be seen, this spectrum mimics almost all features of

Fig. 25 shows the resonant Auger spectrum for Ar the experimental Ar on Pt spectrum. In the sameon Pt(111) with an excitation energy corresponding manner as for the Auger spectrum of N on graphite,2


21 1Fig. 24. The Ar2p to 4s XAS and Ar2p XPS spectra shown on3 / 2 Fig. 25. The Ar/Pt(111) 2p 4s resonant Auger spectrum has3 / 2a common energy scale for Ar adsorbed on Pt(111). For XPS the been decomposed into two parts. One part corresponds to decay to

4 1binding energy relative to the Fermi level has been used. a spectator 2p 4s configuration, while the other part is found to4correspond to Auger type 2p final states.

the resonantly excited spectrum of Ar on Pt may bethought of as consisting of two parts, one with The shape and position of the (empty) density-of-Auger-like final states and one with spectator-like states in the substrate will determine the relativefinal states [19]. The branching ratio between the two amount of charge-transfer via the charge-transferfinal states depends on the properties of the inter- (hopping) matrix elements. We consequently con-mediate state. From this ratio we can determine the clude that the 4s–substrate interaction in the inter-

215charge transfer time,t 54.7310 s, in the same mediate state, increases in the series Pt–Au–Cu–Ag.CT

way as described above. Similar studies of Ar It is interesting to note that this is also how the workadsorbed on Ru have been performed with the same function of the metals is related. Pt has a high andcharge transfer effects [70]. Ag a low work function. The long charge-transfer

In Fig. 26 we show resonant Auger spectra for Ar time for Ar on graphite may be understood based onadsorbed on Ag, Cu, Au and Pt [71]. The solid lines its semi-metallic properties. Because of the relativelyshow the corresponding Auger decay spectra from low density of states above the Fermi level for thethe core-hole. In Fig. 27 we also show the resonant graphite substrate the overlap is small and theAuger spectrum for Ar coadsorbed with K on charge-transfer is slow. The coadsorption system,graphite. Let us start with the adsorption on the Ar/K/ graphite, constitutes a special case here. Themetal substrates. The separation between the ionic presence of K shifts the Fermi level upwards by 2.8XPS final state and the neutral, core-excited state eV. This results in a situation were the neutral core-decreases monotonically in the sequence Pt, Au, Cu, excited state is 0.7 eV lower than the ionic XPS finaland Ag. The values are 3.9, 3.4, 2.8, and 2.5 eV, state. Naturally, we will not have any adsorbate torespectively. This is found to correspond to a mono- substrate charge transfer in that case.tonic increase of the charge-transfer rate (or decrease We have demonstrated how we can use Augerof the charge-transfer timet ) in the decay spectra. decay spectra both, resonant and non-resonant toCT


Fig. 27. (a) Ar 2p resonant Auger (dotted) and Auger spectra3 / 2

(line) for Ar adsorbed on graphite. Features due to decay from2p excited states have been subtracted from the Auger part.1 / 2

Determined charge transfer ratest are indicated in the besideCT

each spectrum. (b) Ar 2p resonant Auger spectrum for 0.23 / 2

monolayer Ar and 0.1 monolayer. K coadsorbed on graphite.

obtain information regarding time scales for chargetransfer processes between adsorbate and substrate. Itis the signature of the presence of an additionalelectron above the Fermi level in the spectra togetherwith the time scale of the decay of the core-hole thatallows determining these rates. We can use thecomplexity of the Auger final states in a favorableway to locally distinguish the presence of extraelectrons. However, it is important to be aware thatthe description in a time picture is a simplification of

Fig. 26. Ar 2p resonant Auger (dotted) and Auger spectra (line)3 / 2 the processes in these spectra. The intensity in thefor Ar adsorbed on Pt, Au, Cu and Ag surfaces. Features due to

decay spectra is given by the transition momentsdecay from 2p excited states have been subtracted from the1 / 2between the intermediate state and the various finalAuger part. Determined charge transfer ratest are indicated inCT

the beside each spectrum. states after the decay. The nature of the extra


electron in the spectator state will also affect thematrix elements. However, it is clear that our andother studies of similar systems can see chemicaltrends indicating that final state effects do not changethe picture too much [70,71]. Overall, resonant AEScan be used to obtain information of ultrafast phe-nomena on a time scale not accessible today withpump-probe laser spectroscopy.

4 .2. X-ray emission spectroscopy (XES)

If we want to derive a simple one-electron pictureof the occupied electronic states projected on theadsorbate XES is a powerful technique [60,72]. Inmolecular adsorbates with nonequivalent atoms italso allows for a projection of the molecular orbitalson the individual atoms within the adsorbate. Fur-thermore, using angle-resolved XES measurementswe can separate orbitals with different symmetries[72]. In all we can obtain an atomic view of theorbitals involved in the formation of the chemicalbond to the surface.

In a one-electron picture the XES spectrum re-flects the occupied valence electron states. In orderto put the spectra on a valence binding energy scalewe need to identify the position of the Fermi level. Fig. 28. Comparison between XES and valence band photo-The same procedure can be used as discussed foremission spectra for the c(232) phase of N adsorbed on Cu(100).

XAS (Section 3) to measure the core-level bindingenergy in photoemission with respect to the Fermilevel. The final state in XES contains a quasiparticle tageous when probing the extended nature of thevalence hole similar to the final state in valence band electronic states. Angle-resolved photoemission is aphotoemission. This is demonstrated in Fig. 28 well established method for band mapping. The twoshowing a comparison between a valence band methods are complementary to each other.photoemission spectrum and an XES spectrum of Contrary to photoemission the initial state in theatomic N adsorbed on Cu(100) [73]. The figure XES process contains a core-hole. An importantshows photoemission spectra both for a clean question is if the core-hole could have some in-Cu(100) and N covered surface. The dominating fluence on the XE spectrum. Based on the ex-structure around 2–5 eV is due to the Cu d-band. perimental results from XES on simple metals, vonUpon adsorption there are some new weak features Barth and Grossmann formulated what has becomeseen at around 1 and 5 eV binding energies. The known as thefinal state rule [43]. In principle thissame structures are clearly seen in the XE spectrum rule states that the spectral distribution is governedthat has been put on a binding energy scale by by the final state electronic configuration (without asubtracting the core-level binding energy from the core-hole) times a dynamic factor giving a singularemission energy. We can clearly see the difference in behavior near the Fermi level. Recent cluster calcula-the adsorbate sensitivity. In XES we project the tions of the XES process for CO on Cu give noelectron states on the adsorbate atom, whereas in significant modification of the interpretation usingphotoemission the electronic structure of the whole ground state frozen orbitals [74]. There are no strongsystem is seen. The latter aspect can also be advan- enhancements observed close to the Fermi level. In


the following discussion we regard the XES process the decay process gives rise to X-ray satellites andto reflect the ground state electronic structure. these can have a substantial overlap in energy with

For oriented systems the symmetry of the states the main lines [58]. This will make the assignment ofcan be obtained from angle-dependent measurements the ‘one-electron states’ difficult.[72,74,75]. The polarization of the emitted light will Since we are utilizing core-excitations for thebe aligned in such a way that theE-vector points in creation of the core-hole the additional electronthe same direction as the p-component in the final should generate spectator or participator like finalstate orbital localized on the excited atom. However, states, shown in Fig. 3. However, we demonstratedsince we cannot analyze the polarization of the in the previous section with resonant Auger studiesemitted X-rays, symmetry information only comes of physisorbed systems that there are two types offrom the fact that theE-vector is always perpen- final states in decay spectra. One where the exciteddicular to the direction of the emitted X-rays. Thus, electron is present when the decay takes place andemission detected in a direction normal to the surface the other where the electron has been transferred tois due only to transitions involving valence states the substrate. The latter generates similar decayparallel with the surface plane i.e., p , whereas spectra as a nonresonant Auger process. When thexy

detection at grazing angles gives spectral intensity adsorbate coupling to the substrate increases uponrelated to both in-plane p and out-of-plane states chemisorption, it is only the Auger type of decay thatxy

p . Subtracting around 50% of the spectra recorded can be seen [71,76,77]. This means that for XESz

in normal emission from the ones taken in grazing studies of chemisorbed systems we can neglect theemission (both normalized to the same area) only influence of the excited electron in the spectra.contributions due to out-of-plane states p are left. In However, there are exceptions in adsorbate systemz

the case of molecular adsorbates such as CO or N , with a large degree of symmetry, in particular when2

this procedure provides a separation of states withp several atoms are equivalent as seen in ethylene oror s symmetry. benzene [78]. In this case there is an additional

We will in the following sections show how we symmetry selection rule operating that have beencan project the electronic structure in an atom-spe- studied in detail for free molecules and crystallinecific and orbital symmetry selective way for ad- solids [18,22]. In a simple picture we can regard thesorbed N , CO and glycine. In the case of N it is excitation process to an empty state with a particular2 2

necessary to selectively excite the two inequivalent symmetry as creating a delocalized core-hole statenitrogen atoms in the core-excitation process using with a symmetry following the dipole selection rule.the chemical shift in thep resonance. Furthermore, The excited electron is transferred to the substratein order to study CO in different adsorption sites we prior to the decay and the system is left with aneed to selectively excite the molecules in different delocalized core-hole state. The subsequent X-raysites using again the chemical shifts in thep transition will generate a final valence hole state withresonances. Similar, in glycine the two carboxylic a particular symmetry determined by the dipoleand aliphatic carbon atoms can also be selective selection rule. In the general case it is necessary toexcited for XES studies. In both systems we use the treat the process in a one-step description using achemical shifts in the XAS resonances described in scattering framework [18]. However, for the systemsSection 3.2. For all these cases we need to create the described in the following sections we can disregardcore-hole prior to the decay using core-excitation any of these effects and use a simple two-step pictureinstead of ionization. There is another specific reason considering the dipole selection rule only for atomicto use core-excitations close to threshold. If we valence projections.create a core-hole through an ionization process withhigh excess energy there is a large probability of 4 .2.1. Atom-specific probing of the surfacecreating multi-electron excitations in the core-hole chemical bond in adsorbed N2

state. These are normally seen as shake-up and CO and N adsorbed on the late transition metals2

shake-off satellites in XPS spectra (see left part of have become prototype systems regarding the gener-Fig. 8). The presence of additional initial states for al understanding of molecular adsorption. There has


been a large controversy over the years as to how to respectively [47]. Hence, by using different excita-describe the electronic structure and bonding in these tion energies, site-specific XE spectra can be re-systems. Most models have treated the molecule as a corded.unity and only considered the interaction of the The resulting XE spectra for the outer and inner NHOMO and LUMO orbitals with the metal. The atoms are shown in the left part of Fig. 29 [4,81].common description of the bonding is the so-called The spectra are plotted on a common binding energy‘Blyholder model’, with a s donation andp* scale relative to the Fermi level, obtained by sub-backdonation [79,80]. We can now with XES look in tracting the N1s core-level photoemission bindingan atom-specific way if these assumptions are energies of the two atoms from the emission ener-reasonable. What is the nature of the new orbitals gies. The figure is divided in an upper part, display-formed upon adsorption and are there any changes in ing states ofs symmetry (obtained by subtractingthe remaining molecular orbitals? from the grazing emission spectra the normal emis-

The upright adsorption geometry of the N mole- sion spectra scaled by 0.5) and a lower part, display-2

cule on Ni(100) in the on-top site leads to two ing states ofp symmetry (the normal emissionchemically inequivalent N atoms. If a separation spectra). From the symmetry and binding energies ofbetween the two atoms can be made it is an ideal the spectral features, it is straightforward to assigncase to study how the electronic states redistribute in all features above 5 eV binding energy in analogya homonuclear molecule upon adsorption. Further- with UPS measurements [82,83]. In order to facili-more, the adsorption energy is very weak, around 0.5 tate the comparison with the much studied COeV, and we therefore anticipate that orbitals will only molecule we shall useC symmetry notation for thev8

be slightly perturbed. We showed in Section 3.2.2, molecular orbitals.that the XAS spectrum for N on Ni(100) exhibits The novel information contained in Fig. 29 is the2

two 1s to 2p* resonances at 400.6 and 401.0 eV, large difference in the states located on the inner andcorresponding to the outer and inner N atoms, outer N atoms and the clearly resolved structures

Fig. 29. Experimental and theoretical XES spectra for N adsorbed on Ni(100). The upper panel displays states ofs symmetry and the2

lower panel states ofp symmetry.


within 5 eV binding energy, i.e., in the Ni d-band contribution, but not to the same extent. Further-region. All spectral peaks, representing the 2p atom- more, the polarization of the 1p orbital as well as theprojected molecular orbitals, exhibit different inten- appearance of the d state is also clearly seen in thep

sities or shapes for the inner and outer N atoms. calculations.Interesting findings are the localization of the 4s An important question was how thep system canstate to the inner N atom, with no visible spectral be bonding when the new substrate-induced orbitalintensity from the outer N atom, and the larger 5s essentially is a lone-pair orbital on the outer nitrogenlocalization to the outer N atom. Near the Fermi atom. It was found that the character of the resultinglevel we find the molecular states that are important molecular orbitals (MO) is much better described asfor the surface chemical bond. These states arise a three-orbital allylic interaction involving the forma-

˜from interaction of molecularp states, as discussed tion of a totally bonding (1p), a non-bonding (d )p

below, with the Ni d states. There is a state located and a totally antibonding orbital (2p*) as illustratedon the outer N atom centered at about 2.5 eV binding in the left part of Fig. 30 [72,81]. If we only considerenergy denoted d . In some sense we can label this the Ni atom that is directly involved in the bonding,p

state as a lone-pair orbital on the outer atom. Closer thep system will involve three atoms and threepto the Fermi level there is also intensity on the inner orbitals will thus be generated. The lowest orbitalnitrogen atom. will always be bonding between all three centers and

Density functional theory (DFT) calculations the highest orbital will be antibonding. The inter-simulating the radiative decay process using a Ni mediate orbital should be antibonding between the13

cluster have been performed in order to provide end-atoms, which for a symmetrical molecule resultsfurther understanding of the XE spectra, shown in in no contribution on the center atom and this orbitalthe right part of Fig. 29 [72]. The polarization of the can be denoted non bonding. The bonding orbital iss system is well reproduced by the calculations. By similar to the free molecule 1p orbital, but slightlyinspecting the calculated wave functions, we find polarized on the inner nitrogen atom with a small butthat the 2s contribution (which is not probed in the significant contribution from the Ni3d orbital. Sinceexperiment) also polarizes in the same way as the 2p the 1p population is smaller compared with the free

Fig. 30. Schematic illustration of the orbital interactions in the N –Ni chemisorption system in terms of the atomic N 2p and Ni 3d orbitals.2


molecule we have weakened the N–N bond and around 20 eV above the 2p* resonance [85] and toinstead formed a covalent Ni–N interaction. The mix with this orbital is very costly. This is anintermediate orbital is essentially non bonding with a essential difference compared with thep systemmain contribution from the metal. This orbital has no where the orbital mixing involved a low-lying un-correspondent on the free molecule and may be occupied orbital. In order to minimize the Pauli

˜viewed as a lone-pair orbital resulting from breaking repulsion the system will depopulate some of the ds

(partially) one internal bond in the N molecule and orbitals through a polarization of the 4sp density2

transferring this into bonding to the surface and into from the central Ni atom out towards the surroundingthe formation of a lone-pair. The antibonding orbital metal centers. However, the energy gain is notis only weakly occupied on the N molecule and the enough to overcome the Pauli repulsion including the2

molecular character is similar to the 2p* orbital of costs for 6s* orbital mixing and metal polarization.the free molecule with major contributions from the The resulting picture is a repulsives system. At aNi3d and 4p orbitals. This orbital is probed in the longer molecule–metal bond distance the Pauli re-XAS measurements described in Section 3.2.3. The pulsion could become smaller and thes system netbonding to the metal substrate is accompanied by a bonding. However, the energy gain throughp bond-weakening of the internal N–N bond through the ing is large enough to overcome thes repulsion atmixing of the 1p and 2p* orbitals. In a valence bond the equilibrium bond distance. It is interesting topicture, we can see this as a partial breaking of the note that this picture is rather different comparedinternal N p bond, partially compensated by the with the often considered 5s donation bonding2

surface bond. This increases the N–N bond length scheme.and decreases the internal stretch frequency. The From these results we can derive a model of the‘backdonation’ into the 2p* level is the involvement surface chemical bond that is different from theof this orbital in the formation of totally bonding, traditional picture of the Blyholder model in that itnon-bonding and antibonding orbitals in the allylic involves all molecular orbitals and where the re-three-level molecular orbital diagram. For N ad- sulting binding energy is obtained from a balance2

sorbed on Ni the overall change in totalp occupation between repulsion in thes system and bondingis small, while for CO chemisorbed on Ni somewhat based on thep orbitals. Instead of bonding throughmore 2p* is added than internally bonding 1p is s donation the main effect from thes system is thuslost; this is the connection to the normalp* bac- a repulsive interaction. The balance between attrac-kbonding picture. tivep interaction and repulsives interaction ex-

The s system can be summarized in a molecular plains why the adsorption energy is weak evenorbital diagram, similar to the allylic configuration of though the adsorbate electronic structure is the resultthe p system. However, for symmetry reasons more of a complete rehybridization. Likewise, the attrac-orbitals are involved (N2s, N2p and Ni bands), tive adsorbate–metal interaction in thep channelmaking the situation more complicated. We can in a leads to a weakening of the internal molecular bond,

˜way picture some of the highest orbitals as the 4s which is countered by a strengthening of the internal˜ ˜and 5s with mainly adsorbate character and the ds molecular bond through the repulsive molecule–

orbitals with mainly metal character, where we use, metal interaction in thes channel.˜e.g., the 4s to indicate the 4s-derived orbital re- .

˜sulting from the interaction with the metal. The 4s˜and 5s are bonding orbitals with respect to the metal 4 .2.2. Adsorption site-dependent electronic

and will undergo a downwards shift in energy in structure of COcomparison with the free molecule while the essen- The interplay between the electronic and geomet-

˜tially metal ds is an antibonding orbital which will ric structure of adsorbates is of fundamental impor-therefore shift upwards in energy [84]. In this orbital tance for the understanding of many surface phenom-diagram all orbitals, both bonding and antibonding, ena. Molecular CO is known for its ability toare occupied which will lead to Pauli repulsion. The populate different adsorption sites, depending on theN 6s* orbital has been seen in XAS spectra of N metal, substrate structure, coverage, temperature and2 2


influence from coadsorbate species. These often from the calculations. Furthermore, the transitioncoexisting phases indicate only small energetic dif- probabilities of the 1p have been used to normalizeferences for different sites, which has been inter- the carbon and oxygen XES relative intensities.preted as an indication of rather similar bonding. The From these calculations, also orbital contour plotsnature of CO bonding in different sites has been have been generated of the most interesting valencestudied using the coadsorbate CO–H system on Ni, orbitals in CO gas and adsorbed on a Ni cluster in13

where CO can be populated in on-top, bridge and on-top and hollow sites, shown in Figs. 32 and 33 forhollow sites, described in Section 3.1.1. Selective p and s symmetry, respectively. On the basis ofexcitations at chemically shiftedp resonances have these studies the adsorbate orbitals can be discussedbeen used to separately probe the adsorbate in a in a conceptually simple way in terms of theirspecific site for mixed overlayers)see Sections 3.1.1 molecular or metallic character.and 3.2.2). We can note in principle the same changes in the

Fig. 31 shows C and O K-emission spectra of CO electronic structure upon adsorption as in the case ofin different sites [86]. On the left side spectra ofp the isoelectronic molecule N . The 5s orbital polar-2

symmetry are displayed and on the right side spectra izes towards the (outer) oxygen atom and the 4s

of s symmetry. All spectral features with a binding towards the (inner) carbon atom (mainly C2s charac-energy above 5 eV can be readily assigned in ter). The relative strength of the 4s and 5s states inaccordance to previous photoemission measurements the oxygen XES spectra, is a measure of the degree[30]. Theoretical spectra have been generated with of polarization upon adsorption. In the free COgood agreement to experiment. As XES at the C1s molecule the intensity of the 4s is five times largerand O1s edges probes selectively atomic 2p contribu- than the 5s in the oxygen spectrum. On Ni, this ratio

˜tions, the remaining contributions, s and d, are taken changes dramatically and the 4s is even weaker than˜the 5s. The degree of polarization increases with

increasing Ni coordination. In thep system we˜observe the dominant 1p state in both the carbon and

oxygen XES spectra. Towards lower binding energy,new states are observed, which differ in the oxygenand carbon spectra. In analogy with N we denote2

these spectral features as the d band. At the bottomp

of this band (higher binding energy), intensity isonly observed in the oxygen spectrum. This state isthe characteristic oxygen lone-pair state ofp symme-try. At the top of the band, close to the Fermi-level,intensity is present in both the carbon and oxygenXES data. Similar to adsorbed N the 1p forms a2

bonding combination to the metal d states, maximiz-ing overlap through internal polarization towards thecarbon end of the molecule. At the same time acharacteristic lone-pair state at the outer atom isformed with large Ni d character. Both the amount of1p polarization and adsorbate character in the lone-pair state increases with increasing Ni coordination.We can note that the 1p orbital for CO in the hollowsite has the same contribution on both atoms whichis rather different compared with the free molecule.

Both thep bonding and thes repulsion increasewith increasing Ni coordination in such a way thatFig. 31. Carbon and oxygen K-edge XES spectra of CO adsorbed

on Ni(100) in on-top, bridge and hollow sites. the resulting adsorption energy is rather similar


Fig. 32. Contour plots ofp orbitals for CO in the gas phase and adsorbed on Ni(100) in on-top and hollow sites. Solid and dashed linesindicates different phases of the wave function.

between on-top and bridge sites and slightly less each other. However, the interaction is very differentfavorable for hollow sites. The gain in increasingp between the sites causing a dramatic change in thebonding is lost due to the increaseds repulsion. In electronic structure, which increases with increasingthe same manner as for adsorbed N the energetics Ni coordination. Based on these findings we can2

of the two symmetry channels partly compensate understand the change in vibrational frequencies and


Fig. 33. Contour plots ofs orbitals for CO in the gas phase and adsorbed on Ni(100) in on-top and hollow sites. Solid and dashed linesindicates different phases of the wave function.

different reactivity for CO in the different adsorption emission and grazing emission along the [110] andsites. In adsorption systems where thep interaction [100] directions), we can project the electronicis weaker, such as CO on Cu thes repulsion will structure onto the 2p , 2p and 2p valence orbitals.x y z

become too dominant at higher coordination, leading The two chemically different carbon atoms in theto population of only on-top sites. COO and CH groups give rise to chemically shifted2

resonances in the XAS spectra. We may selectively4 .2.3. Glycine on Cu(110) excite each core-level separately. Fig. 34 shows XE

The potential of XES applied to more complicated spectra measured on four different atomic sitesmolecular adsorbates is well illustrated with glycine projected in three directions [53,87]. The picture thatadsorbed on Cu(110), described in Section 3.2.3. emerges is an experimental version of the LCAOThe Cu(110) surface has a 2-fold symmetry, where (linear combination of atomic orbitals) approach forthe surface Cu atoms are oriented into rows in the molecular orbital theory. We can study how molecu-[110] direction. The molecule adsorbs in an orienta- lar orbitals of a specific symmetry are distributedtion perpendicular to these rows. From measure- over different atomic sites in a complicated molecu-ments of XE spectra in three directions (normal lar complex. Combined with theoretical calculations,


Fig. 34. p , 2p and 2p symmetry resolved X-ray emission spectra for glycine adsorbed on Cu(110). Inserted is a structural model of thex y z

adsorption complex. The spectra are measured at the O atom, the C atom in the (COO) group, the C atom in the (CH ) group and at the N2

atom.

details in the chemical bond can be obtained from plementary and related to each other. Using X-raysuch studies. photoelectron spectroscopy (XPS or ESCA), the

ionization process is studied. Chemical shifts provide5 . Summary information about the structural and chemical state.

In X-ray absorption spectroscopy (XAS) the emptyCore-level spectroscopy provides a method to states are studied. The localized nature of the core-

locally study the electronic and geometric properties hole and the dipole selection rule make it possible tocentered around one atomic site. This is in particular probe the atomic population as well as the symmetryimportant when studying complex systems with of the various unoccupied states. In Auger decay, themany different atomic sites such as molecular adsor- core-hole state is filled from a valence level whilebates. The various constituents can selectively be another valence electron is emitted leading to aprobed both through the elemental specificity but double-hole final state. In X-ray emission spectros-also due to the chemical shift between atoms of the copy (XES) the core-hole state is de-excited by thesame element in different local surroundings. It is emission of X-ray photons. The final state of thethrough the combination of the various core-level X-ray emission process is simpler than the corre-spectroscopy methods that such studies become very sponding Auger final state since it involves only apowerful. The different methods are both com- single valence hole, as in valence photoemission.


¨ ¨[6] O. Bjorneholm, A. Nilsson, A. Sandell, B. Hernnas, N.The X-ray transition is governed by the same rules as˚Martensson, Phys. Rev. Lett. 68 (1992) 1892.in XAS.

¨ ¨[7] A. Fohlisch, N. Wassdahl, J. Hasselstrom, O. Karis, D.In the present review we have demonstrated how ˚Menzel, N. Martensson, A. Nilsson, Phys. Rev. Lett. 81

we can use core-level spectroscopies to obtain (1998) 1730–1733.[8] K. Siegbahn, C. Nordling, G. Johansson, J. Hedman, P.F.information about the chemical state, local geometric

Heden, K. Hamrin, U. Gelius, T. Bergmark, L.O. Werme, R.structure, nature of chemical bonding and dynamicsManne, Y. Baer, in: ESCA Applied to Free Molecules,

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[10] S. Hufner, in: Photoelectron Spectroscopy, Springer, Berlin,cesses the coupling to the substrate has to be muchHeidelberg, 1995.

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The present review paper is a brief summary of ˚[15] N. Martensson, A. Nilsson, J. Electron Spectrosc. Rel.Phenom. 72 (1995) 1.work involving a number of collaborators. In par-

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J.N. Andersen, R. Nyholm, N. Wassdahl, M. Weinelt, Heidelberg, 1995.¨T. Wiell, O. Karis, P. Bennich, J. Hasselstrom, A. [17] J. Nordgren, E.Z. Kurmaev, J. Electron Spectrosc. Rel.

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applications of core level spectroscopy to adsorbates

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