- Email: [email protected]
Auger photoelectron coincidence spectroscopy
Journal of Electron Spectroscopy and Related Phenomena 100 (1999) 161–165 www.elsevier.nl / locate / elspec
Auger photoelectron coincidence spectroscopy S.M. Thurgate* School of Mathematical and Physical Sciences, Murdoch University, Murdoch, Perth WA 6150, Australia Received 6 January 1999; accepted 19 April 1999
Abstract Auger photoelectron coincidence spectroscopy (APECS) is a technique that provides us with unique information and a chance to gain insight into the significance of processes in the Auger spectra of atoms in solids. Hence it is a great aid in our understanding of the Auger process in atoms where electron correlations are strong. Despite the first demonstration of the technique more than 20 years ago, there are still very few working experiments. The reasons why, and the ways forward are discussed. 1999 Elsevier Science B.V. All rights reserved. Keywords: Auger spectra; Electron correlations; Future prospects
1. Introduction The idea that we can learn more about an atomic process if we can collect all electrons coming from a particular event is not new. In atomic physics, (e,2e) experiments [1] have been used to investigate the details of atomic structure through capturing both the scattered electron and the ejected electron in electron-atom scattering. In such experiments the momenta of the incident, scattered and ejected electrons are either known or measured, so the momentum of the electron in the target atom can be found. In this way, the momentum of electrons in atoms and molecules has been mapped, and compared with theory. Early attempts to extend this type of experiment to solid state were unsuccessful [2]. The main problem was that the mean free path of electrons in solids is so small that both the ejected and scattered electrons *Tel.: 161-8-9360-2382; fax: 161-8-9310-1711. E-mail address: [email protected] (S.M. Thurgate)
quickly collided with other electrons, or ion cores, losing the information about the target state momentum. The other experimental problem that was immediately apparent was that the number of electrons in the inelastic background made discriminating between other random electrons and the electrons from the particular atomic event difficult. However, in recent years, there has been significant progress in this area first with the development of (e,2e) [3] experiments of self supporting thin films, and then (e,2e) experiments from surfaces using high efficiency detectors. An alternate coincidence experiment from solid surfaces was first successfully demonstrated by Sawatzky, and his student Haak [4,5]. They were able to measure the coincidence between an Auger electron and the photoelectron emitted during the ionisation process. They were able to show that it was possible to distinguish features in the L 3 VV Auger line from Cu due from either the photoionisation of the 2p 3 / 2 or the 2p 1 / 2 shells. The intensity they observed in the L 3 VV in coincidence
0368-2048 / 99 / $ – see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S0368-2048( 99 )00045-6
162
S.M. Thurgate / Journal of Electron Spectroscopy and Related Phenomena 100 (1999) 161 – 165
with the 2p 1 / 2 was due to the Coster–Kronig process L 2 L 3 V. They gave this new technique the acronym APECS for Auger photoelectron coincidence spectroscopy, and went on to investigate other lines in Cu and Ni. Much of the understanding of the APECS experiment has come from this early work of Hank and Sawatzky. Around the same time (1979), Ohno and Wendin published a many-body theory suitable for analysing APECS data [6]. Sawatzky [7] pointed out that it was possible to make coincidence measurements in reverse, fixing the energy of the Auger analyser and sweeping the photoelectron analyser through the line giving rise to the transition. If the lifetime of the intermediate state was short lived then Sawatzky argued that it should be possible to see features with energy widths less than the natural width of the photoelectron line. This attracted the attention of a group at Brookhaven National Laboratories, headed by Eric Jensen, who built an APECS instrument for use on the vacuum ultra violet (VUV) storage ring at the National Synchrotron Light Source (NSLS) [8,9]. The instrument took advantage of the time structure of the light output of the storage ring to get very good timing resolution while not degrading the energy resolution, or collection efficiency of the analyser. They used this instrument to look at the 3p photoemission peak in coincidence with the M 2,3 VV Auger line in Cu. As had been predicted, they were able to see features in the photoelectron spectrum that were less than the natural width of the line. They have gone on to use this instrument to look at a variety of other materials, though they have not been able to see further applications of the line narrowing they first observed in Cu [10,11]. Our interest in APECS came about through our interaction with the atomic physics group of J.F. Williams, also situated in Perth, Western Australia, and his gas phase coincidence experiments [12]. Our earliest attempts to find a coincidence signal failed as we struggled to see if we could measure an Auger electron in coincidence with a core electron loss feature from a solid surface [13]. This was an electron-stimulated experiment, where we tried to find out more about the physics of electron stimulated Auger. While we tried and failed in this experiment, we did produce an instrument with excellent time resolution [14]. We needed this in
order to distinguish the true coincidence electrons from the background electrons [15]. To convert this to an APECS system we simply needed to add an X-ray tube. The other invaluable benefit of the attempted coincidence experiment from electron stimulation was a deep appreciation of the requirements for a surface coincidence experiment. Unlike the surface (e,2e) experiment, the major experimental difficulty is to differentiate between the high density of random background electrons and the true coincident electrons. The two most significant factors are good timing resolution and good detection efficiency [16]. In the case of APECS, one does not need to determine the momentum of either electron, so it is possible to use electron analysers with large acceptance angles in order to improve the probability of detection. The other significant departure from (e,2e) experiments is that background count rates are high, so the bandwidth of the system is an important consideration.
2. APECS data The question one should ask then is why should one bother with an experiment that is at least twice as complex as regular electron spectroscopy, and one which has notoriously low count rates? The answer is that there is information available that is possibly not accessible by any other technique. The extent to which APECS is novel has been described recently by Ohno [17,18]. Photoemission spectroscopy is an excitation spectroscopy, in which the properties of the atom buried in a solid surface are revealed. Auger spectroscopy, on the other hand, is a deexcitation spectroscopy which reflects how an atom returns to a low energy configuration within the solid. Ohno points out that in many materials, electron correlation effects are strong, and manifest themselves in both Auger and photoemission spectroscopy. These many body effects often lead to serious discrepancies from the conventional, one electron picture. Ohno considered the circumstance where the initial core hole state could decay or relax by a variety of decay or relaxation mechanisms by using many-body theory. The one electron picture is a poor approximation in such cases. In such circumstances it is essential to pin down the correlations
S.M. Thurgate / Journal of Electron Spectroscopy and Related Phenomena 100 (1999) 161 – 165
between initial core hole states and Auger final states. One approach to this problem has been to use photoemission spectroscopy from synchrotron sources. In such experiments it is possible to vary the energy of the incident radiation and so to tune through thresholds, switching processes on and so identifying the origin of features in the excitation spectra with those in the de-excitation spectra. However, as the spectra are collected near threshold, the shake up / off features are suppressed, and so the effect of these processes cannot be investigated. On the other hand, APECS permits the experimenter the chance to directly test the origin of features in both spectra. In the APECS experiment, the energy of the incident photons are fixed, and in general, far from threshold, so shake up / off features are clearly seen. In practice, this means that it is possible to distinguish those features that are associated with loss in the initial state from those that occur in the final state. This can be seen clearly in the spectra from the L 3 VV from Cu in coincidence with 2p 3 / 2 as shown in Fig. 1. The solid line is the regular singles spectrum obtained by the same instrument at the same time. One of the nice features of APECS is that the spectrometer collects data as a function of the time in which electrons arrive at the spectrometer. If
Fig. 1. The L 3 VV Auger line of Cu in coincidence with the 2p 3 / 2 photoelectron. The solid line is the singles data from the same data set. Note the apparent improvement in energy resolution, the appearance of the 3 F term on the high energy side and the reduction in the background due to the absence of the 4.5 eV satellite from the L 2 L 3 V–L 3 VV process and the absence of the background contribution of the L 3 V–L 3 VV process.
163
one wants to display the coincidence spectrum, one simply displays the number of counts that occur within the known coincidence time. If one displays the total number of counts, regardless of when they arrive, then one is displaying the regular, singles spectrum. Hence one is able to compare the regular spectrum with the coincidence spectrum, with certainty that the data were collected under exactly the same conditions. There are a number of interesting differences between the singles spectrum and the coincident spectrum. There is an apparent improvement in energy resolution in the coincident spectrum, with the 3 F term clearly resolved in coincidence, while it is obscured in the singles spectrum. Haak [5] had observed a similar effect in his data and attributed it to an improvement in resolution as the initial state is better known in the coincidence experiment, defined by the setting of the Auger analyser to the 2p 3 / 2 photoemission line in this circumstance. More relevant to the current discussion however is the fact that the background underlying the L 3 VV line is suppressed in the coincidence experiment compared to the singles experiment. This is due to the fact that initial state shake up / off features are not present in the coincidence spectrum. In such events, the photoelectron loses energy to the shake electron, and so is shifted out of the range of the photoelectron spectrometer. The background underlying the L 3 VV peak is not in coincidence with the 2p 3 / 2 emission peak. We conclude that it is due to the Auger emission from a core hole state which has excited a shake up / off electron. It is clear from Fig. 1 that significant intensity is associated with this feature, and that it cannot be ignored in any attempt to model the intensity in this line. APECS can be used to determine the significance of these effects and so to better understand their role [19]. APECS has been used to study a range of other effects. In Sawatzky’s initial measurements he was able to show that the satellite 4.5 eV from the peak of the L 3 VV Auger line in Cu was due to the presence of a spectator hole from the Coster–Kronig process L 2 L 3 V. These Auger cascades can be investigated directly with APECS. We have measured the contribution of the cascade L 2 –L 3 M 5 –M 5 VV in the M 5 VV Auger line in Ag and In [20,21]. APECS can be used to simplify spectra and to remove interfering peaks. We have used it to remove
164
S.M. Thurgate / Journal of Electron Spectroscopy and Related Phenomena 100 (1999) 161 – 165
the intensity from the L 3 VV Auger line of Ga in GaAs in order to observe the plasmon underlying this Auger line from the L 2 VV Auger line [22].
3. Instrumental considerations The fundamental requirement of an APECS system is that it must be capable of measuring both the photoelectron and the Auger electron from a single atom [14,16]. To do this, the instrument must be able to distinguish between electrons that arise from random events, and those that are present as a consequence of the ionisation under investigation. Thus an APECS system must have at least two electron spectrometers that can be set independently, and fast timing circuits that can determine if the electrons arose from the same event. In practice the variation in flight times through the electron spectrometer is the major reason for loss of timing information. A variety of techniques have been used to overcome this. With the synchrotron-based experiments, the high timing resolution is a consequence of the bunch structure in the photon source. Typically bunches are separated in time by several nanoseconds, providing a convenient marker for determining if electrons came from the same event. At Murdoch, we built electron analysers with very good time resolution using the ideas of Volkel and Sandner [23]. They suggested using a multi-channel plate as a detector and angling it in such a way as to provide compensation for the variety of flight paths through the analyser. (e,2e) experiments have been performed using a scheme for providing flight time corrections depending on where on the channel plate a particular electron was detected. Whatever the technique used, the practical requirement is that a timing resolution of better than 5 ns is needed. APECS experiments have notoriously low count rates. Our initial experimental configuration produced data at very low rates. The Cu L 3 VV line in coincidence with the 2p 3 / 2 took around 10 days of data collection, running 24 h / day. However, we have been able to improve this so that we can collect equivalent data in 12 h [24]. Almost all this improvement has come through increasing the acceptance angle of the analysers. Our original experiment was configured so that we could look for coincidences
with the electron stimulated Auger. We needed the best possible timing resolution for this. In general, the wider the acceptance angle, the poorer the timing resolution. So we chose a very narrow acceptance angle. We subsequently re-built the analysers with greater acceptance angles and consequently greater count rates. (e,2e) experiments have taken advantage of parallel detection methods to increase count rates. The APECS experiment is not as easily amenable to these techniques. The large acceptance angles and the high electron backgrounds mean that actual count rates are high in both channels. This means that it is not possible to use resistive anode systems which require analysis of each pulse as it arrives. Count rates of 10 6 counts per second in each channel are typical. One possibility is to use multiple detectors in each electron analyser. This would require separate timing circuits for each identified pair of detectors, but significant savings in scan time should be achievable. Developments in the past 5 years or so in commercial XPS machines may also be useful in extending the performance of APECS systems. A number of companies have carefully characterised the flight times through their analysers so that they are able to use them for time-of-flight SIMS. A second small electron analyser could be located in the same chamber, and the system used for APECS. There would be no need to compromise the main analyser, so the system could be used routinely for XPS, and, when the need was clear, a second analyser could be switched in to provide the fast timing pulse for coincidence studies.
4. Conclusion APECS has been used to study more than 15 different materials to date. There has not yet been a study of chemisorbed systems, and very little work has been done on compound and alloy materials. The principal explanation of why so little has been done in the area has been the lack of apparatus. Currently some APECS capability is under construction as part of the instrumentation for the ALOISA beamline at the Elettra Synchrotron in Trieste, Italy [25]. Another has been constructed for use with an X-ray tube in Hungary [26]. However, the potential for use in
S.M. Thurgate / Journal of Electron Spectroscopy and Related Phenomena 100 (1999) 161 – 165
regular, laboratory-based experiments is clear. The possibility that APECS could be an option on commercial systems is yet to be seriously explored. In time, with improvements in equipment, APECS is likely to become widely used as it provides information that is simply not accessible by more conventional techniques. The challenge is to develop instruments that will allow us to tap this potential.
References [1] [2] [3] [4] [5]
[6] [7] [8] [9] [10]
I.E. McCarthy, E. Weigold, Rep. Prog. Phys. 54 (1991) 789. P.E. Best, H. Zhu, Rev. Sci. Instrum. 56 (1985) 389. M. Voss, I.E. McCarthy, Rev. Mod. Phys. 67 (1995) 713. H.W. Haak, G.A. Sawatzky, T.D. Thomas, Phys. Rev. Lett. 41 (1978) 1825. H.W. Haak, Auger Photoelectron Coincidence Spectroscopy — A Study of Correlation Effects in Solids, University of Groningen, Groningen, 1983. X. Ohno, X. Wendin, J. Phys. B 12 (1979) 1395. G.A. Sawatzky, Treatise Mater. Sci. Technol. 30 (1988) 167. E. Jensen, R.A. Bartynski, S.L. Hulbert, E.D. Johnson, R. Garrett, Phys. Rev. Lett. 62 (1989) 71. E. Jensen, R.A. Bartynski, S.L. Hulbert, E.D. Johnson, Rev. Sci. Instrum. 63 (1992) 3013. E. Jensen, R.A. Bartynski, M. Weinert, S.L. Hulbert, E.D. Johnson, R.F. Garrett, Phys. Rev. B 41 (1990) 12468.
165
[11] E. Jensen, R.A. Bartynski, R.F. Garrett, S.L. Hulbert, E.D. Johnson, C.-C. Kao, Phys. Rev. B 45 (1992) 13636. [12] M.A.B.P. Hayes, J. Flexman, J.F. Williams, Rev. Sci. Instrum. 50 (1988) 2445. [13] W. Todd, Electron excited coincidence Auger electron spectroscopy off solid surfaces, in: School of MPS, Murdoch University, Perth, WA, 1989. [14] S.M. Thurgate, B. Todd, B. Lohmann, A. Stelbovics, Rev. Sci. Instrum. 61 (1990) 3733. [15] A.T. Stelbovics, B.D. Todd, S.M. Thurgate, B. Lohmann, Surf. Sci. 278 (1992) 193. [16] S.M. Thurgate, J. Elec. Spec. Relat. Phenom. 81 (1996) 1. [17] M. Ohno, Phys. Rev. B 58 (1998) 12795. [18] M. Ohno, J. Elec. Spec. Relat. Phenom. (1999), in press. [19] C.P. Lund, S.M. Thurgate, A.B. Wedding, Phys. Rev. B 49 (1994) 11352. [20] C.P. Lund, S.M. Thurgate, Surf Sci. Lett. 376 (1997) L403. [21] S.M. Thurgate, C.P. Lund, Surf. Interface Anal. 25 (1997) 10. [22] C.P. Lund, S.M. Thurgate, Phys. Rev. B 50 (1994) 17166. [23] M. Volkel, W. Sandner, J. Phys. E.: Sci. Instrum. 16 (1983) 456. [24] S.M. Thurgate, C. Creagh, R. Craig, J. Elec. Spec. Relat. Phenom. 93 (1998) 209. [25] F. Tommasini, see http: / / has.area.trieste.it / tasc / has / aloisa / aloisa.html (1998). [26] L. Kover, Institute of Nuclear Research of the Hungarian Academy of Sciences (ATOMKI), private communication (1998).
Auger photoelectron coincidence spectroscopy S.M. Thurgate* School of Mathematical and Physical Sciences, Murdoch University, Murdoch, Perth WA 6150, Australia Received 6 January 1999; accepted 19 April 1999
Abstract Auger photoelectron coincidence spectroscopy (APECS) is a technique that provides us with unique information and a chance to gain insight into the significance of processes in the Auger spectra of atoms in solids. Hence it is a great aid in our understanding of the Auger process in atoms where electron correlations are strong. Despite the first demonstration of the technique more than 20 years ago, there are still very few working experiments. The reasons why, and the ways forward are discussed. 1999 Elsevier Science B.V. All rights reserved. Keywords: Auger spectra; Electron correlations; Future prospects
1. Introduction The idea that we can learn more about an atomic process if we can collect all electrons coming from a particular event is not new. In atomic physics, (e,2e) experiments [1] have been used to investigate the details of atomic structure through capturing both the scattered electron and the ejected electron in electron-atom scattering. In such experiments the momenta of the incident, scattered and ejected electrons are either known or measured, so the momentum of the electron in the target atom can be found. In this way, the momentum of electrons in atoms and molecules has been mapped, and compared with theory. Early attempts to extend this type of experiment to solid state were unsuccessful [2]. The main problem was that the mean free path of electrons in solids is so small that both the ejected and scattered electrons *Tel.: 161-8-9360-2382; fax: 161-8-9310-1711. E-mail address: [email protected] (S.M. Thurgate)
quickly collided with other electrons, or ion cores, losing the information about the target state momentum. The other experimental problem that was immediately apparent was that the number of electrons in the inelastic background made discriminating between other random electrons and the electrons from the particular atomic event difficult. However, in recent years, there has been significant progress in this area first with the development of (e,2e) [3] experiments of self supporting thin films, and then (e,2e) experiments from surfaces using high efficiency detectors. An alternate coincidence experiment from solid surfaces was first successfully demonstrated by Sawatzky, and his student Haak [4,5]. They were able to measure the coincidence between an Auger electron and the photoelectron emitted during the ionisation process. They were able to show that it was possible to distinguish features in the L 3 VV Auger line from Cu due from either the photoionisation of the 2p 3 / 2 or the 2p 1 / 2 shells. The intensity they observed in the L 3 VV in coincidence
0368-2048 / 99 / $ – see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S0368-2048( 99 )00045-6
162
S.M. Thurgate / Journal of Electron Spectroscopy and Related Phenomena 100 (1999) 161 – 165
with the 2p 1 / 2 was due to the Coster–Kronig process L 2 L 3 V. They gave this new technique the acronym APECS for Auger photoelectron coincidence spectroscopy, and went on to investigate other lines in Cu and Ni. Much of the understanding of the APECS experiment has come from this early work of Hank and Sawatzky. Around the same time (1979), Ohno and Wendin published a many-body theory suitable for analysing APECS data [6]. Sawatzky [7] pointed out that it was possible to make coincidence measurements in reverse, fixing the energy of the Auger analyser and sweeping the photoelectron analyser through the line giving rise to the transition. If the lifetime of the intermediate state was short lived then Sawatzky argued that it should be possible to see features with energy widths less than the natural width of the photoelectron line. This attracted the attention of a group at Brookhaven National Laboratories, headed by Eric Jensen, who built an APECS instrument for use on the vacuum ultra violet (VUV) storage ring at the National Synchrotron Light Source (NSLS) [8,9]. The instrument took advantage of the time structure of the light output of the storage ring to get very good timing resolution while not degrading the energy resolution, or collection efficiency of the analyser. They used this instrument to look at the 3p photoemission peak in coincidence with the M 2,3 VV Auger line in Cu. As had been predicted, they were able to see features in the photoelectron spectrum that were less than the natural width of the line. They have gone on to use this instrument to look at a variety of other materials, though they have not been able to see further applications of the line narrowing they first observed in Cu [10,11]. Our interest in APECS came about through our interaction with the atomic physics group of J.F. Williams, also situated in Perth, Western Australia, and his gas phase coincidence experiments [12]. Our earliest attempts to find a coincidence signal failed as we struggled to see if we could measure an Auger electron in coincidence with a core electron loss feature from a solid surface [13]. This was an electron-stimulated experiment, where we tried to find out more about the physics of electron stimulated Auger. While we tried and failed in this experiment, we did produce an instrument with excellent time resolution [14]. We needed this in
order to distinguish the true coincidence electrons from the background electrons [15]. To convert this to an APECS system we simply needed to add an X-ray tube. The other invaluable benefit of the attempted coincidence experiment from electron stimulation was a deep appreciation of the requirements for a surface coincidence experiment. Unlike the surface (e,2e) experiment, the major experimental difficulty is to differentiate between the high density of random background electrons and the true coincident electrons. The two most significant factors are good timing resolution and good detection efficiency [16]. In the case of APECS, one does not need to determine the momentum of either electron, so it is possible to use electron analysers with large acceptance angles in order to improve the probability of detection. The other significant departure from (e,2e) experiments is that background count rates are high, so the bandwidth of the system is an important consideration.
2. APECS data The question one should ask then is why should one bother with an experiment that is at least twice as complex as regular electron spectroscopy, and one which has notoriously low count rates? The answer is that there is information available that is possibly not accessible by any other technique. The extent to which APECS is novel has been described recently by Ohno [17,18]. Photoemission spectroscopy is an excitation spectroscopy, in which the properties of the atom buried in a solid surface are revealed. Auger spectroscopy, on the other hand, is a deexcitation spectroscopy which reflects how an atom returns to a low energy configuration within the solid. Ohno points out that in many materials, electron correlation effects are strong, and manifest themselves in both Auger and photoemission spectroscopy. These many body effects often lead to serious discrepancies from the conventional, one electron picture. Ohno considered the circumstance where the initial core hole state could decay or relax by a variety of decay or relaxation mechanisms by using many-body theory. The one electron picture is a poor approximation in such cases. In such circumstances it is essential to pin down the correlations
S.M. Thurgate / Journal of Electron Spectroscopy and Related Phenomena 100 (1999) 161 – 165
between initial core hole states and Auger final states. One approach to this problem has been to use photoemission spectroscopy from synchrotron sources. In such experiments it is possible to vary the energy of the incident radiation and so to tune through thresholds, switching processes on and so identifying the origin of features in the excitation spectra with those in the de-excitation spectra. However, as the spectra are collected near threshold, the shake up / off features are suppressed, and so the effect of these processes cannot be investigated. On the other hand, APECS permits the experimenter the chance to directly test the origin of features in both spectra. In the APECS experiment, the energy of the incident photons are fixed, and in general, far from threshold, so shake up / off features are clearly seen. In practice, this means that it is possible to distinguish those features that are associated with loss in the initial state from those that occur in the final state. This can be seen clearly in the spectra from the L 3 VV from Cu in coincidence with 2p 3 / 2 as shown in Fig. 1. The solid line is the regular singles spectrum obtained by the same instrument at the same time. One of the nice features of APECS is that the spectrometer collects data as a function of the time in which electrons arrive at the spectrometer. If
Fig. 1. The L 3 VV Auger line of Cu in coincidence with the 2p 3 / 2 photoelectron. The solid line is the singles data from the same data set. Note the apparent improvement in energy resolution, the appearance of the 3 F term on the high energy side and the reduction in the background due to the absence of the 4.5 eV satellite from the L 2 L 3 V–L 3 VV process and the absence of the background contribution of the L 3 V–L 3 VV process.
163
one wants to display the coincidence spectrum, one simply displays the number of counts that occur within the known coincidence time. If one displays the total number of counts, regardless of when they arrive, then one is displaying the regular, singles spectrum. Hence one is able to compare the regular spectrum with the coincidence spectrum, with certainty that the data were collected under exactly the same conditions. There are a number of interesting differences between the singles spectrum and the coincident spectrum. There is an apparent improvement in energy resolution in the coincident spectrum, with the 3 F term clearly resolved in coincidence, while it is obscured in the singles spectrum. Haak [5] had observed a similar effect in his data and attributed it to an improvement in resolution as the initial state is better known in the coincidence experiment, defined by the setting of the Auger analyser to the 2p 3 / 2 photoemission line in this circumstance. More relevant to the current discussion however is the fact that the background underlying the L 3 VV line is suppressed in the coincidence experiment compared to the singles experiment. This is due to the fact that initial state shake up / off features are not present in the coincidence spectrum. In such events, the photoelectron loses energy to the shake electron, and so is shifted out of the range of the photoelectron spectrometer. The background underlying the L 3 VV peak is not in coincidence with the 2p 3 / 2 emission peak. We conclude that it is due to the Auger emission from a core hole state which has excited a shake up / off electron. It is clear from Fig. 1 that significant intensity is associated with this feature, and that it cannot be ignored in any attempt to model the intensity in this line. APECS can be used to determine the significance of these effects and so to better understand their role [19]. APECS has been used to study a range of other effects. In Sawatzky’s initial measurements he was able to show that the satellite 4.5 eV from the peak of the L 3 VV Auger line in Cu was due to the presence of a spectator hole from the Coster–Kronig process L 2 L 3 V. These Auger cascades can be investigated directly with APECS. We have measured the contribution of the cascade L 2 –L 3 M 5 –M 5 VV in the M 5 VV Auger line in Ag and In [20,21]. APECS can be used to simplify spectra and to remove interfering peaks. We have used it to remove
164
S.M. Thurgate / Journal of Electron Spectroscopy and Related Phenomena 100 (1999) 161 – 165
the intensity from the L 3 VV Auger line of Ga in GaAs in order to observe the plasmon underlying this Auger line from the L 2 VV Auger line [22].
3. Instrumental considerations The fundamental requirement of an APECS system is that it must be capable of measuring both the photoelectron and the Auger electron from a single atom [14,16]. To do this, the instrument must be able to distinguish between electrons that arise from random events, and those that are present as a consequence of the ionisation under investigation. Thus an APECS system must have at least two electron spectrometers that can be set independently, and fast timing circuits that can determine if the electrons arose from the same event. In practice the variation in flight times through the electron spectrometer is the major reason for loss of timing information. A variety of techniques have been used to overcome this. With the synchrotron-based experiments, the high timing resolution is a consequence of the bunch structure in the photon source. Typically bunches are separated in time by several nanoseconds, providing a convenient marker for determining if electrons came from the same event. At Murdoch, we built electron analysers with very good time resolution using the ideas of Volkel and Sandner [23]. They suggested using a multi-channel plate as a detector and angling it in such a way as to provide compensation for the variety of flight paths through the analyser. (e,2e) experiments have been performed using a scheme for providing flight time corrections depending on where on the channel plate a particular electron was detected. Whatever the technique used, the practical requirement is that a timing resolution of better than 5 ns is needed. APECS experiments have notoriously low count rates. Our initial experimental configuration produced data at very low rates. The Cu L 3 VV line in coincidence with the 2p 3 / 2 took around 10 days of data collection, running 24 h / day. However, we have been able to improve this so that we can collect equivalent data in 12 h [24]. Almost all this improvement has come through increasing the acceptance angle of the analysers. Our original experiment was configured so that we could look for coincidences
with the electron stimulated Auger. We needed the best possible timing resolution for this. In general, the wider the acceptance angle, the poorer the timing resolution. So we chose a very narrow acceptance angle. We subsequently re-built the analysers with greater acceptance angles and consequently greater count rates. (e,2e) experiments have taken advantage of parallel detection methods to increase count rates. The APECS experiment is not as easily amenable to these techniques. The large acceptance angles and the high electron backgrounds mean that actual count rates are high in both channels. This means that it is not possible to use resistive anode systems which require analysis of each pulse as it arrives. Count rates of 10 6 counts per second in each channel are typical. One possibility is to use multiple detectors in each electron analyser. This would require separate timing circuits for each identified pair of detectors, but significant savings in scan time should be achievable. Developments in the past 5 years or so in commercial XPS machines may also be useful in extending the performance of APECS systems. A number of companies have carefully characterised the flight times through their analysers so that they are able to use them for time-of-flight SIMS. A second small electron analyser could be located in the same chamber, and the system used for APECS. There would be no need to compromise the main analyser, so the system could be used routinely for XPS, and, when the need was clear, a second analyser could be switched in to provide the fast timing pulse for coincidence studies.
4. Conclusion APECS has been used to study more than 15 different materials to date. There has not yet been a study of chemisorbed systems, and very little work has been done on compound and alloy materials. The principal explanation of why so little has been done in the area has been the lack of apparatus. Currently some APECS capability is under construction as part of the instrumentation for the ALOISA beamline at the Elettra Synchrotron in Trieste, Italy [25]. Another has been constructed for use with an X-ray tube in Hungary [26]. However, the potential for use in
S.M. Thurgate / Journal of Electron Spectroscopy and Related Phenomena 100 (1999) 161 – 165
regular, laboratory-based experiments is clear. The possibility that APECS could be an option on commercial systems is yet to be seriously explored. In time, with improvements in equipment, APECS is likely to become widely used as it provides information that is simply not accessible by more conventional techniques. The challenge is to develop instruments that will allow us to tap this potential.
References [1] [2] [3] [4] [5]
[6] [7] [8] [9] [10]
I.E. McCarthy, E. Weigold, Rep. Prog. Phys. 54 (1991) 789. P.E. Best, H. Zhu, Rev. Sci. Instrum. 56 (1985) 389. M. Voss, I.E. McCarthy, Rev. Mod. Phys. 67 (1995) 713. H.W. Haak, G.A. Sawatzky, T.D. Thomas, Phys. Rev. Lett. 41 (1978) 1825. H.W. Haak, Auger Photoelectron Coincidence Spectroscopy — A Study of Correlation Effects in Solids, University of Groningen, Groningen, 1983. X. Ohno, X. Wendin, J. Phys. B 12 (1979) 1395. G.A. Sawatzky, Treatise Mater. Sci. Technol. 30 (1988) 167. E. Jensen, R.A. Bartynski, S.L. Hulbert, E.D. Johnson, R. Garrett, Phys. Rev. Lett. 62 (1989) 71. E. Jensen, R.A. Bartynski, S.L. Hulbert, E.D. Johnson, Rev. Sci. Instrum. 63 (1992) 3013. E. Jensen, R.A. Bartynski, M. Weinert, S.L. Hulbert, E.D. Johnson, R.F. Garrett, Phys. Rev. B 41 (1990) 12468.
165
[11] E. Jensen, R.A. Bartynski, R.F. Garrett, S.L. Hulbert, E.D. Johnson, C.-C. Kao, Phys. Rev. B 45 (1992) 13636. [12] M.A.B.P. Hayes, J. Flexman, J.F. Williams, Rev. Sci. Instrum. 50 (1988) 2445. [13] W. Todd, Electron excited coincidence Auger electron spectroscopy off solid surfaces, in: School of MPS, Murdoch University, Perth, WA, 1989. [14] S.M. Thurgate, B. Todd, B. Lohmann, A. Stelbovics, Rev. Sci. Instrum. 61 (1990) 3733. [15] A.T. Stelbovics, B.D. Todd, S.M. Thurgate, B. Lohmann, Surf. Sci. 278 (1992) 193. [16] S.M. Thurgate, J. Elec. Spec. Relat. Phenom. 81 (1996) 1. [17] M. Ohno, Phys. Rev. B 58 (1998) 12795. [18] M. Ohno, J. Elec. Spec. Relat. Phenom. (1999), in press. [19] C.P. Lund, S.M. Thurgate, A.B. Wedding, Phys. Rev. B 49 (1994) 11352. [20] C.P. Lund, S.M. Thurgate, Surf Sci. Lett. 376 (1997) L403. [21] S.M. Thurgate, C.P. Lund, Surf. Interface Anal. 25 (1997) 10. [22] C.P. Lund, S.M. Thurgate, Phys. Rev. B 50 (1994) 17166. [23] M. Volkel, W. Sandner, J. Phys. E.: Sci. Instrum. 16 (1983) 456. [24] S.M. Thurgate, C. Creagh, R. Craig, J. Elec. Spec. Relat. Phenom. 93 (1998) 209. [25] F. Tommasini, see http: / / has.area.trieste.it / tasc / has / aloisa / aloisa.html (1998). [26] L. Kover, Institute of Nuclear Research of the Hungarian Academy of Sciences (ATOMKI), private communication (1998).