15 Auger–Photoelectron Coincidence Spectroscopy Studiesfrom Surfaces

S.M. Thurgate, Z.-T. Jiang, G. van Riessen, and C. Creagh

15.1 Introduction

Auger photoelectron coincidence spectroscopy (APECS) from surfaces was first demonstratedin 1979 [1, 2]. Despite this early illustration of the utility of the technique, there have beenonly a small number of experiments following from this. Perhaps the most significant reasonfor this has been the perceived difficulty in acquiring data in a reasonable time [3]. Never-theless, progress has been made, with new instruments appearing on synchrotrons and a newgeneration of detectors have opened up opportunities for dramatically improved performance.

This chapter will review recent developments in APECS in our laboratory and the progressin understanding that has arisen from these experiments. It will highlight where APECS offersunique opportunities to study processes and point to exciting new directions that are becomingapparent as both the instrumentation and our understanding improve.

15.2 APECS Experiments

The fundamental idea of APECS is that an Auger electron emitted from a photo-ionizationprocess will only be recorded if the photoelectron emitted from the same process is also de-tected. This can result in significant reductions in the apparent complexity of Auger lines asoften the Auger line has a range of contributing ionization processes.

Processes that can produce a core hole that can be filled by an Auger emission, but wherethe resultant line-shape might be changed, include Auger cascade processes where the origi-nal ionization was of a deeper level, Auger final-state shake processes where more than oneelectron is emitted with the outgoing Auger electron, initial state shake processes where anelectron is emitted with the outgoing photoelectron in the creation of the initial state, and sec-ondary ionization from energetic electrons as they escape the solid. This last process is lesslikely for deep core levels.

The complexity that this system of possible events creates is enhanced by the dynamicsof how the resultant holes interact. If they hop away quickly, then the resultant Auger lineshape may well reflect, in part, the density of states of the material. If they stay localized, thenthe line shape will reflect the character of the atom. This distinction between band-like andatom-like Auger lines is well known. APECS provides an opportunity to study these manybody phenomena with clarity.

Correlation Spectroscopy of Surfaces, Thin Films, and Nanostructures. Edited by Jamal Berakdar, Jürgen Kirschner

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198 15 Auger–Photoelectron Coincidence Spectroscopy Studies from Surfaces

Figure 15.1: A simple Auger process. In an APECS experiment only the electrons that comefrom this ionization will be counted. Contributions from other ionization processes that lead toa hole in this core level are ignored.

Haak and Sawatzky [4] were the first to demonstrate the power of this approach. TheL3VV Auger spectrum of copper is known to contain a peak, some 2 eV below that of the main1G term, which is not accounted for as an atomic term. The feature is shown in Figure 15.2.Roberts et al. [5] correctly identified the source of this as due to a spectator hole that remainedlocal following the Coster–Kronig decay L2L3V. The additional electrostatic term slowed theoutgoing Auger electron resulting in the 2 eV shift. Haak and Sawatzky were able to showthat this peak appeared in coincidence with the 2p1/2 photoelectron and thus to confirm theorigin of the feature.

Figure 15.2: High resolution spectra of the LVV Auger line of Cu, showing the 1G satellite asa consequence of the Auger cascade L2VV-L3VV.

15.3 Applications 199

The Sawatzky experiment was based on commercial electron analyzers, though modifiedto increase collection angles. Several years later a group at the National Synchrotron LightSource (NSLS) at Brookhaven National Laboratories, NY developed an APECS system thattook advantage of the pulsed nature of the synchrotron light to achieve excellent timing reso-lution [6]. They have continued to examine a range of surfaces with this facility since then [7].Further APECS experiments have come on line at synchrotrons in the past few years. Theseinclude the multi-analyzer facility at Elettra in Trieste and a new dual coaxial CMA system atthe Photon Factory in Japan.

At Murdoch University we have designed and built several APECS systems. Our firstanalyzer used a slanted channel plates setup to reduce the degradation in timing resolutioncaused by differences in flight times through various possible flight paths through the analyzer[8]. We have subsequently improved on this apparatus through modifications to the electronanalyzers and the electron lenses. Our current system uses two 180◦ hemispherical electronanalyzers with five element lenses [9]. Each analyzer has a large separation between thehemispheres and the lens with a wide acceptance angle. A wider acceptance angle increasesthe probability of detection and so the rate of detection of coincidences [10].

15.3 Applications

15.3.1 Broadening of Cu 2p3/2

As well as the conventional APECS experiment, it is also possible to perform a “reverse”experiment, where the Auger analyzer is fixed on an Auger peak while the photoelectronanalyzer sweeps the correlated photoelectron line. In this way it is possible to isolate thesources of a particular feature in the Auger spectrum. We used this technique to look at theCu2p3/2 photoelectron line of Cu in coincidence with terms in the L3VV Auger line.

The L3VV Auger line of Cu is atom-like, with peaks resembling the atomic terms seenin spectra from Cu vapor. This phenomenon was first explained independently by Cini andSawatzky in 1977 [11, 12]. This means that it is possible to resolve the Cu L3VV spectra intospectroscopic terms. The main terms are labeled in Figure 15.2. We collected coincidencephotoelectron spectra of the 2p3/2 line with the 1G and 3F terms (Figures 15.3 and 15.4).

In coincidence with the 1G term, the line narrows on the low kinetic energy side, but thecentroid is unshifted. With the 3F term, the line shifts 0.15 eV towards lower kinetic energies.This was a very puzzling result at first. The simplest views of the Auger process suggest thatthe Auger emission is independent of the manner in which the core hole is produced. That isclearly not so in this case. However, a full account of this phenomenon was contingent on amore detailed description of the APECS line-shape. In the past few years Ohno has developeda many-body approach to explaining the coincidence line-shape [13, 14]. The approach hehas taken is to observe that the coincidence line-shape is a consequence of the decay of asingle core hole, and the dynamics of that process influence the shape of both the coincidencephotoelectron line and the coincidence Auger line. He has been able to show that the singlesspectrum is essentially proportional to the spectral function of the initial core hole, while thecoincidence line shapes depend also on the imaginary part of the initial core hole self energyand density of final states.

200 15 Auger–Photoelectron Coincidence Spectroscopy Studies from Surfaces

Figure 15.3: Cu 2p3/2 in coincidence with the L3VV 1G term. The solid line shows the singlesdata while the dashed line shows the best fit to the shifted coincidence data.

Figure 15.4: Cu 2p3/2 in coincidence with the L3VV 3F term. The solid line shows the singlesdata while the dashed line shows the best fit to the shifted coincidence data.

The case he has concentrated on is one where the initial core hole can decay either directlyor indirectly through the excitation of a shake up/off electron. He points out that in such a casean APECS measurement of the photoelectron line will contain contributions from both thedirect transition and the indirect one via the shakeup/down process and under these conditionsthe coincidence photoelectron line can move and change width, as observed in the case ofCu L3VV.

15.3 Applications 201

15.3.2 Broadening of Ag 3d5/2

The test of this proposal was to see if the Ag 3d5/2 photoelectron line moved relative to thesingles line in coincidence with different terms of the M4,5VV Auger line. The shakeup/downsatellite of the Ag 3d5/2 is known to be small compared to that of Cu 2p3/2. Ohno’s theorypredicted therefore that the photoelectron line would not shift in coincidence with differentterms of the Ag M4,5VV line. A high resolution XPS spectrum of Ag M4,5VV, with termsmarked, is shown in Figure 15.5.

Figure 15.5: High resolution XPS spectrum of the Ag M4,5VV Auger line.

The coincidence photoelectron spectra from the 3d5/2 in coincidence with the 1G and 3Fterms are shown in Figures 15.6 and 15.7 respectively. It can be seen that the shift in thesepeaks is very much less than in the corresponding case for Cu 2p3/2. The shifts for each lineare shown in Table 15.1.

Material Auger Line Shift

Cu 1G L3VV +0.253F L3VV −0.15

Ag 1G M4VV +0.043F M4VV −0.03

Table 15.1: Measured shifts in the coincidencephotoelectron peaks of Cu and Ag with differentAuger lines.

15.3.3 Disorder Broadening

It has been known for some time that the photoelectron lines of compositionally disorderedalloys can demonstrate a broadening due to the range of chemically different sites that atomsof each component can find themselves in. It was proposed [15] that we measure the M4,5VVAuger line of Ag in Ag0.5Pd0.5 alloy in coincidence with the 3d5/2 line. Ag0.5Pd0.5 is known

202 15 Auger–Photoelectron Coincidence Spectroscopy Studies from Surfaces

Figure 15.6: Ag 3d5/2 Photoelectron line in coincidence with the 1G term of the M5VV Augerline. The solid line is the singles data, while the best fit of the singles lineshape to the coinci-dence is shown as a dashed line.

Figure 15.7: Ag 3d5/2 Photoelectron line in coincidence with the 3F term of the M5VV Augerline. The solid line is the singles data, while the best fit of the singles lineshape to the coinci-dence is shown as a dashed line.

to be compositionally disordered with broadened photoemission lines. We then shifted thecentral position of the photoelectron analyzer within the broadened width to see if there wasa corresponding movement of the Auger line. The data from the experiment is shown inFigures 15.8, 15.9 and 15.10, and is summarized in Table 15.2.

15.3 Applications 203

Figure 15.8: Ag M5VV with 3d5/2 at the center of the photoemission line. There is no shift ofthe coincidence Auger line relative to the singles.

Figure 15.9: Ag M5VV with 3d5/2 0.8 eV above the center of the photoemission line. Thecoincidence Auger line has shifted −0.39 eV.

The incident radiation was Mg-Kα (not monchromatized) with a line-width of 700 meV.Thus the movement of the Auger line is due to the selection by the photoelectron analyzerof ionization events where the binding energy is either greater than or less than the meanenergy. The data show convincingly that when the binding energy is less than the mean, theAuger energy is greater than the mean, and vice versa. This confirms the picture that this alloyis compositionally disordered with atoms placed in a range of locations of different bindingenergy.

204 15 Auger–Photoelectron Coincidence Spectroscopy Studies from Surfaces

Figure 15.10: Ag M5VV with 3d5/2 0.8 eV below the center of the photoemission line. Thecoincidence Auger line has shifted 0.47 eV.

Photoemission AnalyzerSetting Relative to Cen-ter of Singles / eV

Measured Movement ofM5VV Auger Line Rela-tive to Singles / eV

0.0 0.0+0.8 −0.39−0.8 +0.47

Table 15.2: Changesin the photoelectronanalyzer energy settingsand the correspondingmeasured movement ofthe Auger line.

15.4 Conclusions

APECS is finding application in understanding a wide range of problems. In this chapter wehave shown how it has been used to investigate differences in decay mechanisms in Cu and Agand how it can be used to probe the nature of disordered broadening. There has been progressin understanding that the coincidence line-shape is not the simple sum of the component parts,but rather it can reflect the dynamics of the core hole decay processes.

References

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[2] H.W. Haak, University of Gronigen, PhD Thesis. (1983).

[3] S.M. Thurgate, J. Electron Spectrosc. Relat. Phenom. 100 (1999) 161.

[4] G.A. Sawatzky, Treatise Mater. Sci. Technol. 30 (1988) 167.

[5] E.D. Roberts, P. Weightman, and C.E. Johnson, J. Phys. C 8 (1975) 301.

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