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Solid state broadening effects in the Auger spectrum of xenon
Solid State Communications, Vol. 15, pp. 329—332, 1974.
Pergamon Press.
Printed in Great Britain
SOLID STATE BROADENING EFFECTS IN THE AUGER SPECTRUM OF XENON J.D. Nuttall and T.E. Gallon Department of Physics, University of York, Heslington, York, England (Received 31 January 1974 by C. W. McCombie)
The M4 5N4,5N4,5 Auger spectrum has been measured at high resolution from solid xenon. Comparison with the gas phase data shows that considerable broadening of the Auger lines occurs in the solid. Electron energy loss data is also reported for solid xenon.
THE USE of Auger Electron Spectroscopy for surface chemical analysis is now well established and the Auger spectra solids have been Most ofof thea large early number work onof solid surfa~ces underreported. clean conditions was performed with electron spectrometers of low resolving power, and the Auger lines appeared very broad. More recent work on clean solids using spectrometers of high resolving power’3 has detected the presence of considerable fine structure in the Auger spectra but the lines seem to be broader than those observed in high resolution studies of gases.
specifically solid state broadening process is phonon broadening, produced by fluctuations in atomic 5 While potential vibration of the lattice. increased caused lifetimebybroadening should only be signifi. cant in the case of transitions involving outer electron states, phonon broadening will affect all Auger transitions. Citrin6 has recently pointed out that interatomic Auger processes in solids may give rise to additional channels for the decay of an initial hole which are not available in the free atom. The effect of these new modes of decay will be to decrease the lifetime of the hole and hence lead to increased life. time broadening of the level.
If one or more of the electrons involved in the Auger transition lies in one of the energy bands of the solid, it might be expected that the Auger line wifi be broadened compared with the corresponding transition in the free atom. However, this should only lead to increased line breadths in solids for those transitions involving outer electrons and Auger processes involving core electrons should not be affected. Even in those cases where apparent band states are involved it appears that the Auger line, may not directly reflect the band structure of the solid.”2~3In addition to possible band effects other solid state broadening processes have been suggested. In an Auger transition involving a free atom the final two hole state may be stable or metastable, while in the solid, Coulomb
In an attempt to understand solid state line broadening processes a study has been undertaken of the Auger spectra of solid rare gases. The rare gas Auger spectra are well documented at high resolution in the gas phase and they contain many discrete, well resolved Auger lines.7 The major lines have been indexed and their energies are in good agreement with theoretical calculation, although understanding of relative intensities is not so complete. Thus there are high resolution, well understood gas phase spectra available for direct comparison with any data recorded from the solid phase. This note reports the results of measurements of the M 45 N4,5 N4,5 Auger spectrum of solid xenon.
interaction in the corresponding final state will force the holes apart onto neighbouring atoms. This means that the final state in the solid will be of shorter duration than in the gas and this may lead to increased 4 Another life-time broadening of the Auger lines.
The electron spectrometer used in these measurements was spectroscopy.8 a retarding hemispherical analyser for Auger This instrument wasdesigned operated 329
330
BROADENING EFFECTS IN THE AUGER SPECTRUM OF XENON
Vol. 15, No.2
N (E)
460
480
500
-
520
540 560 Kinetic Energy eV
FIG. I. M4,5 N4,5 N4,5 Auger spectrum of solid Xe at 100 K. The arrows indicate the halfwidth of the spectrometer transmission function. with a resolving power of 1200. A simple liquid helium cryostat was constructed to fit the spectrometer. The spectrometer was situated in a stainless steel UHV chamber, generally used without baking, and the background pressure was usually 10-8 torr. B.O.C. X grade Xebase was admitted into the and condensed on the of the cryostat at achamber temperature of 10°K. Contamination from the residual atmosphere was carefully monitored and when the presence of an impurity peak was observed fresh Xe was deposited. The Auger spectra were excited with a 2 keV, 5zA beam of electrons. Figure 1 shows the M4,5 N4,5 N4,5 Auger spectrum of solid Xe at 10°K; the arrows indicate the halfwidth of the peaks instrumental function in this energypeak range. Three can be readily distinguished, a may be identified as the M 4N4,5 N4,5 transition, and peak b is the M5N4,5 N4,5 transition. The separation between peak a and b is 12 eV in good agreement with the spin-orbit splitting of the M4 and M5 levels7 indicates in Xe. Comparison with the data of Werme et al. clearly considerable broadening occurredin the solidthat spectra. The peak labelled bhas is clearly resolved in the gas phase into three major peaks and two smaller peaks which may be indexed in terms of exchange splitting in the final state. While there is some indication of fine structure in the solid the distinct peaks found by Werme et al. have been broadened and peak b represents their envelope. The M 4N4,5 N4,5 data of Werme er al. indicate that this group consists mainly of an unresolved ‘D, ‘G peak with the other L—S multiplets being very much less intense and most
lying at high energies. Thus, a measure of the halfwidth of the ‘D, ‘G peak in the solid may be made by extrapolating the low energy side of peak a and taking the halfwidth as twice the width of the low energy side at1D, half‘Gheight. This suggests theat line which has a fullthat width M4N4,5 N4,5 half maximum of just over 1 eV in the gas phase has been broadened in the solid to a halfwidth of nearly 4 eV. The shape of the M 45 N4,5 N4,5 solid Xe spectrum shown in Fig. 1 is very similar to that of solid In and Ag’ even though, in the latter case, the N45 electrons lie in the conduction band. There appears to be a slight shift of energy in the Auger lines of the solid Xe, as peak a is about 3—4 eV higher in energy than the M4N4,5 N4,5 ‘D ‘G peak in the9 gas phase. This may be due to polarisation but more careful measurements need to be effects made to eliminate the possibility of charging before a definite statement can be made.
—
Peak c in Fig. 1 might correspond to the M5 N4,5 4,5 ‘S and ionisation observed inwith the 7 ordouble it might be a losspeaks peak associated gas phase either peak a or peak b. To examine this latter possibility the electron loss spectrum for 500 eV primary electrons was recorded and it is shown in Fig. 2. Twenty peaks could be plainly identified in the range 500 eV—350 eV. The main loss peaks are quite broad and are centred around 14 eV (peak 4). N
—
This suggests that peak c in Fig. 1, displaced by 8 eV from peak b at 19 eV from peak a, is not a loss associated with either peak. The separation of —6 eV between peak c and the leading portion of peak b is
Vol. 15, No.2
BROADENING EFFECTS IN THE AUGER SPECTRUM OF XENON
331
N(E)
Energp Lois eV
FIG. 2. The electron energy loss spectrum from solid Xe at 10°K. in good agreement with the separation for the multi7 and suggests that peak c is the broadened plet M splitting 5 N4,5 N4,5 ‘S peak. There are considerable loss electrons present which are not observed in the gas phase, as may be seen by the considerable low energy tail stretching to more than 50 eV below the main peaks, but any discrete losses present are smeared and do not clearly stand out. The loss spectrum of Fig. 2 shows two interesting sharp peaks with losses of —65 eV and —67 eV (lines 12 and 13). These are probably ionisation losses corresponding to ionisation of the 4d512 and 4d312 levels, possibly shifted by polarisation effects. However, they are quite narrow and seem to correspond to peaks in the N(E) curve rather than absorption edges. When the primary beam energy was raised to I keV those peaks were still present in the loss spectrum but with reduced intensity which is consistent with the behaviour of ionisation 1° loss peaks observed by Gerlach and DuCharm.
In conclusion the M4,5 N4,5 N4,5 Auger spectrum of solid Xe shows broadening compared with the gas phase spectrum. Since the levels involved all lie below 60 eV it is unlikely that two hole lifetime broadening contributes to the increase in breadth. The most probable mechanism is that of phonon broadening. However, the possibility of interatomic Auger processes leading to increased lifetime broadening cannot be ruled out, although it might be expected that such an effect in a solid rare gas would be small since the electrons are highly localised. Study of the temperature dependence of the line breadths should give further insight into the broadening mechanism since only the phonon broadening process is temperature dependant. Modifications of the cryostat to allow measurement over a range of temperatures are at pre~ntin hand.
Acknowledgements The for Authors to thank Dr. J.A.D. Matthew manywould usefullike discussions. —
REFERENCES 1.
BASSETF P.J., GALLON T.E.~MATTHEW J.A.D. and PRUTTON M.,Surface Science. 35,63(1973).
2. 3.
YIN L., TANG T., ADLER I. and YELLIN E., J. AppL Phys. 43,3464(1972). POWELL C.J. and MANDL A., Phys. Rev. Letr. 29, 1153 (1972).
4.
MATTHEW J.A.D.,Phys. Lett. 32A, 261 (1970).
5.
MATTHEW J.A.D., Surface Science 20, 183 (1970).
6.
CITRIN P.H.,Phys. Rev. Lert. 31, 1164(1973).
332
BROADENING EFFECTS IN THE AUGER SPECTRUM OF XENON
7.
WERME L.O., BERGMARK T. and SIEGBAHN K.,Physica Scripta. 6, 141 (1972).
8.
BASSETT P.J., GALLON T.E. and PRUTTON M.,J. Phys. E. 5, 1008 (1972).
9.
MATTHEW J.A.D., Surface Science 40,451(1973).
10.
GERLACH R.L. and DUCHARM A.R.,Phys. Rev. A 6, 1892 (1972).
Das M4,5N4,5 N4,5 Augerspektrum von festem Xenon wurde mit hoher Auflosung gemessen. Em Vergleich mit Daten aus der Gasphase zeigt eine beträchtliche Verbreitemng der Augerlinien fin Festkorper. Ferner werden Elektronenenergie-Verlustpeaks für festes Xenon angegeben.
Vol. 15, No.2
Pergamon Press.
Printed in Great Britain
SOLID STATE BROADENING EFFECTS IN THE AUGER SPECTRUM OF XENON J.D. Nuttall and T.E. Gallon Department of Physics, University of York, Heslington, York, England (Received 31 January 1974 by C. W. McCombie)
The M4 5N4,5N4,5 Auger spectrum has been measured at high resolution from solid xenon. Comparison with the gas phase data shows that considerable broadening of the Auger lines occurs in the solid. Electron energy loss data is also reported for solid xenon.
THE USE of Auger Electron Spectroscopy for surface chemical analysis is now well established and the Auger spectra solids have been Most ofof thea large early number work onof solid surfa~ces underreported. clean conditions was performed with electron spectrometers of low resolving power, and the Auger lines appeared very broad. More recent work on clean solids using spectrometers of high resolving power’3 has detected the presence of considerable fine structure in the Auger spectra but the lines seem to be broader than those observed in high resolution studies of gases.
specifically solid state broadening process is phonon broadening, produced by fluctuations in atomic 5 While potential vibration of the lattice. increased caused lifetimebybroadening should only be signifi. cant in the case of transitions involving outer electron states, phonon broadening will affect all Auger transitions. Citrin6 has recently pointed out that interatomic Auger processes in solids may give rise to additional channels for the decay of an initial hole which are not available in the free atom. The effect of these new modes of decay will be to decrease the lifetime of the hole and hence lead to increased life. time broadening of the level.
If one or more of the electrons involved in the Auger transition lies in one of the energy bands of the solid, it might be expected that the Auger line wifi be broadened compared with the corresponding transition in the free atom. However, this should only lead to increased line breadths in solids for those transitions involving outer electrons and Auger processes involving core electrons should not be affected. Even in those cases where apparent band states are involved it appears that the Auger line, may not directly reflect the band structure of the solid.”2~3In addition to possible band effects other solid state broadening processes have been suggested. In an Auger transition involving a free atom the final two hole state may be stable or metastable, while in the solid, Coulomb
In an attempt to understand solid state line broadening processes a study has been undertaken of the Auger spectra of solid rare gases. The rare gas Auger spectra are well documented at high resolution in the gas phase and they contain many discrete, well resolved Auger lines.7 The major lines have been indexed and their energies are in good agreement with theoretical calculation, although understanding of relative intensities is not so complete. Thus there are high resolution, well understood gas phase spectra available for direct comparison with any data recorded from the solid phase. This note reports the results of measurements of the M 45 N4,5 N4,5 Auger spectrum of solid xenon.
interaction in the corresponding final state will force the holes apart onto neighbouring atoms. This means that the final state in the solid will be of shorter duration than in the gas and this may lead to increased 4 Another life-time broadening of the Auger lines.
The electron spectrometer used in these measurements was spectroscopy.8 a retarding hemispherical analyser for Auger This instrument wasdesigned operated 329
330
BROADENING EFFECTS IN THE AUGER SPECTRUM OF XENON
Vol. 15, No.2
N (E)
460
480
500
-
520
540 560 Kinetic Energy eV
FIG. I. M4,5 N4,5 N4,5 Auger spectrum of solid Xe at 100 K. The arrows indicate the halfwidth of the spectrometer transmission function. with a resolving power of 1200. A simple liquid helium cryostat was constructed to fit the spectrometer. The spectrometer was situated in a stainless steel UHV chamber, generally used without baking, and the background pressure was usually 10-8 torr. B.O.C. X grade Xebase was admitted into the and condensed on the of the cryostat at achamber temperature of 10°K. Contamination from the residual atmosphere was carefully monitored and when the presence of an impurity peak was observed fresh Xe was deposited. The Auger spectra were excited with a 2 keV, 5zA beam of electrons. Figure 1 shows the M4,5 N4,5 N4,5 Auger spectrum of solid Xe at 10°K; the arrows indicate the halfwidth of the peaks instrumental function in this energypeak range. Three can be readily distinguished, a may be identified as the M 4N4,5 N4,5 transition, and peak b is the M5N4,5 N4,5 transition. The separation between peak a and b is 12 eV in good agreement with the spin-orbit splitting of the M4 and M5 levels7 indicates in Xe. Comparison with the data of Werme et al. clearly considerable broadening occurredin the solidthat spectra. The peak labelled bhas is clearly resolved in the gas phase into three major peaks and two smaller peaks which may be indexed in terms of exchange splitting in the final state. While there is some indication of fine structure in the solid the distinct peaks found by Werme et al. have been broadened and peak b represents their envelope. The M 4N4,5 N4,5 data of Werme er al. indicate that this group consists mainly of an unresolved ‘D, ‘G peak with the other L—S multiplets being very much less intense and most
lying at high energies. Thus, a measure of the halfwidth of the ‘D, ‘G peak in the solid may be made by extrapolating the low energy side of peak a and taking the halfwidth as twice the width of the low energy side at1D, half‘Gheight. This suggests theat line which has a fullthat width M4N4,5 N4,5 half maximum of just over 1 eV in the gas phase has been broadened in the solid to a halfwidth of nearly 4 eV. The shape of the M 45 N4,5 N4,5 solid Xe spectrum shown in Fig. 1 is very similar to that of solid In and Ag’ even though, in the latter case, the N45 electrons lie in the conduction band. There appears to be a slight shift of energy in the Auger lines of the solid Xe, as peak a is about 3—4 eV higher in energy than the M4N4,5 N4,5 ‘D ‘G peak in the9 gas phase. This may be due to polarisation but more careful measurements need to be effects made to eliminate the possibility of charging before a definite statement can be made.
—
Peak c in Fig. 1 might correspond to the M5 N4,5 4,5 ‘S and ionisation observed inwith the 7 ordouble it might be a losspeaks peak associated gas phase either peak a or peak b. To examine this latter possibility the electron loss spectrum for 500 eV primary electrons was recorded and it is shown in Fig. 2. Twenty peaks could be plainly identified in the range 500 eV—350 eV. The main loss peaks are quite broad and are centred around 14 eV (peak 4). N
—
This suggests that peak c in Fig. 1, displaced by 8 eV from peak b at 19 eV from peak a, is not a loss associated with either peak. The separation of —6 eV between peak c and the leading portion of peak b is
Vol. 15, No.2
BROADENING EFFECTS IN THE AUGER SPECTRUM OF XENON
331
N(E)
Energp Lois eV
FIG. 2. The electron energy loss spectrum from solid Xe at 10°K. in good agreement with the separation for the multi7 and suggests that peak c is the broadened plet M splitting 5 N4,5 N4,5 ‘S peak. There are considerable loss electrons present which are not observed in the gas phase, as may be seen by the considerable low energy tail stretching to more than 50 eV below the main peaks, but any discrete losses present are smeared and do not clearly stand out. The loss spectrum of Fig. 2 shows two interesting sharp peaks with losses of —65 eV and —67 eV (lines 12 and 13). These are probably ionisation losses corresponding to ionisation of the 4d512 and 4d312 levels, possibly shifted by polarisation effects. However, they are quite narrow and seem to correspond to peaks in the N(E) curve rather than absorption edges. When the primary beam energy was raised to I keV those peaks were still present in the loss spectrum but with reduced intensity which is consistent with the behaviour of ionisation 1° loss peaks observed by Gerlach and DuCharm.
In conclusion the M4,5 N4,5 N4,5 Auger spectrum of solid Xe shows broadening compared with the gas phase spectrum. Since the levels involved all lie below 60 eV it is unlikely that two hole lifetime broadening contributes to the increase in breadth. The most probable mechanism is that of phonon broadening. However, the possibility of interatomic Auger processes leading to increased lifetime broadening cannot be ruled out, although it might be expected that such an effect in a solid rare gas would be small since the electrons are highly localised. Study of the temperature dependence of the line breadths should give further insight into the broadening mechanism since only the phonon broadening process is temperature dependant. Modifications of the cryostat to allow measurement over a range of temperatures are at pre~ntin hand.
Acknowledgements The for Authors to thank Dr. J.A.D. Matthew manywould usefullike discussions. —
REFERENCES 1.
BASSETF P.J., GALLON T.E.~MATTHEW J.A.D. and PRUTTON M.,Surface Science. 35,63(1973).
2. 3.
YIN L., TANG T., ADLER I. and YELLIN E., J. AppL Phys. 43,3464(1972). POWELL C.J. and MANDL A., Phys. Rev. Letr. 29, 1153 (1972).
4.
MATTHEW J.A.D.,Phys. Lett. 32A, 261 (1970).
5.
MATTHEW J.A.D., Surface Science 20, 183 (1970).
6.
CITRIN P.H.,Phys. Rev. Lert. 31, 1164(1973).
332
BROADENING EFFECTS IN THE AUGER SPECTRUM OF XENON
7.
WERME L.O., BERGMARK T. and SIEGBAHN K.,Physica Scripta. 6, 141 (1972).
8.
BASSETT P.J., GALLON T.E. and PRUTTON M.,J. Phys. E. 5, 1008 (1972).
9.
MATTHEW J.A.D., Surface Science 40,451(1973).
10.
GERLACH R.L. and DUCHARM A.R.,Phys. Rev. A 6, 1892 (1972).
Das M4,5N4,5 N4,5 Augerspektrum von festem Xenon wurde mit hoher Auflosung gemessen. Em Vergleich mit Daten aus der Gasphase zeigt eine beträchtliche Verbreitemng der Augerlinien fin Festkorper. Ferner werden Elektronenenergie-Verlustpeaks für festes Xenon angegeben.
Vol. 15, No.2