x-ray photoelectron spectroscopy - theory...

X-ray photoelectron spectroscopy

Some Synchrotron Radiation Based Methods

Jesper Andersen

Division of Synchrotron Radiation Research, Lund University


h! typically

<1500 eV

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h! typically

<1500 eV


h! typically

<1500 eV

2


Binding Energy = h! -

Ekin

Note direction of x-axis


Binding Energy = h! -

Ekin

Note direction of x-axis

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Basics:A) Elemental analysis – what is in the sample (or rather on the sample) Different elements have different binding energies of the inner (core) levels.

B) Often, also the chemical state of the elements can be determined, eg. Al-metal can be distinguished from Al-oxide, O2 from O The exact binding energy of a core level depends on the chemical state.

Chemical shifts. ESCA (Electron Spectroscopy for Chemical Analysis)


Basics:A) Elemental analysis – what is in the sample (or rather on the sample) Different elements have different binding energies of the inner (core) levels.

B) Often, also the chemical state of the elements can be determined, eg. Al-metal can be distinguished from Al-oxide, O2 from O The exact binding energy of a core level depends on the chemical state.

Chemical shifts. ESCA (Electron Spectroscopy for Chemical Analysis)


! ! Advanced:

C) The surface geometry can be determined. Using diffraction effects and/or the chemical shifts of the binding energies

D) Often, also the mesoscopic scale can be addressed

E) Chemically sensitive microscopy is possible. (Note: Chemically, not just element specific)

Combine the above with either focusing of the incoming photons or magnifying electron optics.

Some 10 nanometers to micrometers are typical values for the spatial resolution.

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Elemental analysis: Photoionization cross-sectionsTo first approximation the intensity (area) of peak A from element Z is proportional to the amount of Z times the cross-section for photoionization of level A.

Crossections depend on Z and on what level we are looking at.



And on the photon energy.

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And on the photon energy.Mo valence (4d) overlaps

Yb 4f levels

Note:• Photons in – electrons out. Energy analyze electrons

• Element specific, concentrations, cross-sections depend on element, level, and photon energy

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Surface core level shifts

Rh 3d 5/2 level gives two peaks.

Surface and bulk


Attenuation length: "

I(x) = I0 e-x/"


Surface and bulk

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I(x) = I0 e-x/"

Using synchrotron radiation, we can vary h!

and thus the electron kinetic energy


Surface and bulk



I(x) = I0 e-x/"

XPS is surface sensitive

Using synchrotron radiation, we can vary h!

and thus the electron kinetic energy


Surface and bulk

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Changing the probe depth at fixed photon energy

Electron escape depth = µ cos #µ: attenuation lengthrather than inelastic mean free pathµ includes losses from elastic scattering

µ is around 0.9 x IMFP

Note:• Photons in – electrons out


• Surface sensitive, varies with photon energy and emission angle

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Surface core level shifts, why ?


d-band narrowing at the surface.

Assume binding energy equals the eigenvalue of the level.

Initial state model

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Initial state model

BINDING ENERGY (EB) :

The energy-cost to remove the core-electron

EB = Etotal(after) - Etotal(before)

EB = h! - Ekin




Initial state model




EB = h! - Ekin

Eb =

Eb

s =

SCLS = Eb – Eb

s =

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EB = h! - Ekin

Eb =

Eb

s =

SCLS =

Need to calculate total energy of a system

with an “impurity” placed in various sites.

In metallic systems the “impurity” is completely screened. i.e. the valence (and remaining core) electrons have relaxed to their ground state.

DFT-based supercell calculations can calculate the total energy di!erences needed.

In all-electron methods, a core electron can be explicitly removed. In pseudo-potential methods, a potential must be generated for the core-ionized state.

Note also that the “impurity” in many ways equals a Z+1 impurity, i.e. the next element in the periodic table.

Surface core level shifts and general chemical shifts, why ?




• The binding energy shifts can be calculated accurately by DFT, total energy di"erences

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Using the substrate core level shifts: CO/Rh(111)

Theory results

Total energies: almost degenerate for CO

in top and 3fold sites (which they should be!!)

Rh 3d shifts:

Clean: –500 meV

CO ind. (top): +450 meV (no buckling) +240 meV (+0.2Å buckling)

CO ind. (3-fold): –220 meV

Conclusion

CO in on-top sites on a buckled surface

Using the substrate core level shifts: CO/Rh(111)

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Stepped surfaces:

Rh(553), seeing the step atoms and following what happens when oxygen is adsorbed




• The binding energy shifts can be calculated accurately by DFT, total energy di!erences

• The shifts are local, i.e. mainly nearest neighbors

• The shifts can be large even though the initial state total energies are close

• Gives qualitative information on adsorption sites etc. Important input to e.g. DFT

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Using the adsorbate levels as fingerprints

• For adsorbed CO the C1s binding

energy provides a good fingerprint of

the adsorption site. Nearest neighbors.

• Ex. CO on Rh(111), pure CO, and co-

adsorbed with O and K. Electro-

positive or –negative does not matter.

• Large shifts even when ground-state

total energies are almost degenerate

• General rule: The C 1s binding energy

decreases as the coordination to the

substrate increases

Using the adsorbate levels as fingerprints

• For adsorbed CO the C1s binding

energy provides a good fingerprint of

the adsorption site. Nearest neighbors.

• Ex. CO on Rh(111), pure CO, and co-

adsorbed with O and K. Electro-

positive or –negative does not matter.

• Large shifts even when ground-state

total energies are almost degenerate

• General rule: The C 1s binding energy

decreases as the coordination to the

substrate increases

• Another rule

Watch out for diffraction !!!

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Partial vs total coverages

Making use of the site-sensitive fingerprint, ex. CO on Rh(111)

Partial coverages vs T and P

Partial vs total coverages

Making use of the site-sensitive fingerprint, ex. CO on Rh(111)

Partial coverages vs T and P

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Pt(332)

Pt(332)

Pt(332) - CO

Rh(553) - CO - H

Pt(332) - O

Fingerprints also on a more mesoscopic length scale








• Adsorbate levels are often good fingerprints, sites, step-adsorption, dissociation

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PdO(101) on top

of Pd(100)

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• Core level photoemission and DFT (and STM) is an excellent combination

Monitoring an ongoing reduction of a surface oxide

CO on the Rh(111)-(9x9) surface oxide

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In-situ reduction of (2x1) and (9x9) by CO: 1x10-8 mbar and 100 C


2x1

9x9

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• MECHANISM for 9x9 reduction by CO: Reduction nucleates (at defects?), reduced areas grow and chemisorbed oxygen

appears on these areas and reacts with CO.

Surface oxide functions as oxygen source

2x1

9x9


• MECHANISM for 9x9 reduction by CO: Reduction nucleates (at defects?), reduced areas grow and chemisorbed oxygen

appears on these areas and reacts with CO.

Surface oxide functions as oxygen source

2x1

9x9

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• Time development can be monitored in-situ (but watch out for sample damage)

Vibrational splittings in Core Level Photoemission from adsorbed molecules

Not all shifts are chemical

J. N. Andersen, et al., Chem. Phys. Lett., 269, 371, 1997.

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Franck CondonDiatomic molecule

Ground state

Co

re i

on

ized

st

ate(

s)




For CHx radicals:

The number x of H atoms bonding to a particular C atom is proportional to the strength of the first vibrational component relative to the adiabatic peak.

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Ethanol on Rh surfaces at 300K


CO The C1s spectra and the fact that we never see any atomic O indicates that the CO group of the ethanol molecule stays intact

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But what about the peaks below 285 eV ?



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On Rh(111) C-H vibration Intensity à CH

On Rh(553) ????

Surface Site Eads (eV) CLS (eV)

Rh(111) Hcp -6.54 0.00

Fcc -6.34 -0.16

Rh(553) Fcc(Up) -6.58 -0.13

Hcp(Terrace) -6.55 0.00

Hcp(Up) -6.53 -0.03

Hcp(Low) -6.44 0.55

Fcc(Low) -6.39 0.04

Fcc(Terrace) -6.28 -0.14

Step(Bridge) -6.16 -0.12

Surface Site Eads (eV) CLS (eV)

Rh(111) Hcp -7.18 0.03

Rh(553) Hcp(Low) -7.54 0.46

Fcc(Up) -7.00 0.33

Hcp(Up) -7.26 0.23

Hcp(Terrace) -7.15 0.05

Fcc(Terrace) -6.75 0.01

Fcc(Low) -6.79 0.06

Bridge(Step) -6.80 0.08

Eads relative to CH in gas-phase

CLS relative CH in hcp site on Rh(111)

Eads relative to C in gas-phase

CLS relative CH in hcp site on Rh(111)

CH C

From the core level shifts, the dominant peak on Rh(553) could

be CH. However, the lack of vibrational structure excludes this.

" " Must be atomic Carbon at the step

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Adsorption energies (referenced to a CH radical in gas phase) for the initial state (left), the transition state (middle), and the final state (right) for the lowest energy dissociation paths on (a): Rh(111) (full) and (b): Rh(553) (dashed). Insert shows the initial (left) and final (right) state geometry for the two surfaces.

It fits:Activation energy for dissociation is lowered by 0.4 eV on Rh(553) and dissociation becomes exothermic











• Not all shifts are chemical. Vibrational shake-up. Electronic shake-ups also exist

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Things were much easier in the old days

Siegbahn, Nordling, Fahlman, Nordberg, Hamrin, Hedman, Johansson, Bergmark, Karlsson, Lindgren, Lindberg,

ESCA - Atomic, Molecular and Solid State Structure Studied by Means of Electron Spectroscopy,

Nova Acta Regiae Societatis Scientiarum Upsaliensis, Almqvist & Wiksell, Uppsala 1967

Conclusions• Photons in – electrons out










• Not all shifts are chemical. Vibrational shake-up. Electronic shake-ups also exist

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