- Email: [email protected]
Photoemission from single crystal xenon
~J
30, pp.545—549. Solid State Communications, Vol. Pergamon Press Ltd. 1979. Printed in Great Britain.
PH0TOE~4ISSIONFROM SINGLE CRYSTAL XENON K. Horn and A. M. Bradshaw Fritz—Haber—Institut der Max—Planck—Gesellschaft Faradayweg 14-6, 1000 Berlin 33, W. Germany Received February 19, 1979, by M. Cardona Normal exit photoemission spectra from the (iii) surface of a xenon single crystal indicate that the valence level emission is considerably broader than expected on the basis of band structure calculations. Similar observations from polycrystalline samples have lead Parrinello et al to propose an excitonic screening mechanism which would cause an apparent increase in the spin—orbit splitting at the r point. The details of the spectra reported here show, however, that the effect could simply be due to a dispersion of the bands stronger than that predicted theoretically. Anomalous intensity effects at ?lw = 21.2 eV are explained in terms of electron—electron scattering.
the position of the 5p1,’2 band at or near the F point can be located. We have also been able to effect a comparison between the valence states of solid xenon, physisorbed xenon and gas phase xenon. The measurements were performed using the rare gas resonance lines, NeI(16.8 cv), Hel (21.2 cv) and HeII(140.8 eV) in an angular re— solving uhv photoelectron spectrometer (ADES 1400, VG Scientific Ltd.). The Pd(100) crystal was mounted on a specially constructed manipu— lator, which enables sample temperatures as low as 10 K to be reached. The manipulator and cool— ing facility will be described elsewhere10. The crystal surface was cleaned with standard tech— niques and characterised with Auger electron spectroscopy. The establishment of a hexagonal physisorbed monolayer~’12’13 (as observed in LEED)
Photoemission investigations on polycristal— line rare gas solids by Schwentner ~ al1’2 and Nurriberger et al3 have revealed a significant discrepancy between experimental and calculated valence band widths. In the case of xenon, for example, the OPW band structure of Reilly ~ and the KEN band structure of Rö2ler5 both predict a valence band width of ~ eV, whereas experi— mentally a value of ~‘3 eV is obtained. A super— ficial glance at the data of Schwentner etal might indicate that this is due to increased spin—orbit splitting, but, as the authors point out, the latter must have the same value at the centre of the Brillouin zone as in the free atom (1 .31 eV). A more satisfactory explanation is that the band dispersion is larger than predic— ted theoretically. As Figure 2 of ref. 1 shows, the problem is then one of locating the position of the 5p 112—derived band at the F point. An 6. A valence is considered alternative explanation haselectron been offered by Parto be trapped rineno et al in the potential well of the loca— used hole. Both hole states are raised in ener— gy, the 5p~,’~state can shift more in energy than the 5p1,2 state, because of their relative positions. This so—called excitonic screening mechanism, which must be distinguished from polarisation effects or screening through plas— mon excitation, then leads to an apparent increase in the spin—orbit splitting. In order to throw some new light on this problem we have prepared xenon single crystals on palladium surfaces with the epitaxial rela— tionship (111)~ II[100]pd and studied them with normal exit pho~oemission. (The growth of single crystal xenon on Ir(100) had ~reviously been established by Ignatiev et al’.) This choice of electron exit angle in angular—resolved photo— emission leads to a considerable simplification of the spectrum: for a (111) surface only valence band states along the FL direction of the Bril—
was found to be a criterion for surface cleanli— growth. ness as well as a prerequisite for good epitaxial Condensation of xenon at 140 K followed by annealing at 55 K lead to epitaxial growth with the (lii) face of xenon parallel to the Pd(100) surface. The LEED pattern showed 12—fold sym— metry as in the case of xenon grown on Ir(100)7, indicating that two orthogonal domains were pre— sent. One such domain is shown in Figure 1, where the xenon [~io) direction is parallel to the pal— ladium [010]direction. Fortunately for the pho— toemission experiment, the one direction conanon to both domains is the [iii] direction normal to the surface. The normal exit photoemission EDC’s at the three energies are shown in Figure 2. The full Eel spectrum is shown again in Figure 3 together with spectra from gas phase and adsorbed xenon. The spectra from the crystal at the three ener— gies resemble the polycrystalhine data of Schwentner et al in that a distinction between the 5p~,. 2—derivedband at higher and the 5p3,’~~ derivea band at lower ionisation energy is ap— parent. The Eel spectrum is characterised by some sharp fine structure as well as by a high relative intensity of the highest ionisation
bum zone are sampled (see e.g. ref. 8 or, more recently, ref. 9). We find that the spectra can be interpreted in terms of a stronger dispersion of both the 5p1/2and 5p3,~—derivedbands and that 545
.
546
PHOTOEMISSION FROM SINGLE CRYSTAL XENON
Figure 1
Vol. 30, No. 9
[Dii]
[oil]
Model of the epitaxial relationship between the (111) plane of xenon and a Pd(100) single crystal surface. Only one domain of the xenon crystal is shown.
energy peak (A) associated with the 5p
1,~—de— rived band. The full extent of this intensity difference is seen more clearly in Figure 3. The effect is apparently caused by electron—electron scattering first occurring when the kinetic energy of the photoemitted electrons relative to the valence band edge equals the sum of the band gap energy and the energy of the first cx— citon (17.8 cv). A second onset for scattering will occur when an energy twice the band gap is reached (18.6 cv). The electrons in peak A lie just above the first threshold, the electrons 2 above and theboth depth of on— in the 5p3,2—derived bands thresholds. gin vary almost uniformly across Such sampled onsets will are not sharp the EDC. The electrons in peak A originate from a substantially thicker slab of material and the peak is correspondingly more intense. A similar effect has recentl~rbeen observed for the 14o level in solid CO1’. The Mel and Hel spectra both reveal a Se— cond peak (A’) at 0.7 eV lower ionisation ener— gy than peak A. Both peaks are present with the same separation at the two energies and with similar relative intensities. This favours an explanation in terms of emission from critical points in the initial state band to evanescent vacuum final states. It is thus reasonable to assume that the A’ peak is due to emission from the p~ 2 states at r and that the peak A repre— sents emission from the flat portion of the ~P1/2~ derived band near the L point. The Ia—space argu— merit concerning the vanishingly small number of states near r does not apply in this angular—re— solved experiment where only a single direction of the Brillouin zone is sampled. This would in— dicate that the dispersion of the p1,~—derived band is in fact ‘~0.7 eV and not ‘~.0.145 eV as in the KEN calculation. At the bottom of Figure 2 is shown an intuitive band structure along F—A—L, whereby the KEN bands of Rdssler have been “stretched” to give a r~—L~separation of 0.67 eV. The distances r~—r~,5and rg —rihave been increased proportionately to 0.27 eV and 0.97 eV respectively. The r~separa— tion is given by the atomic spin—orbit split—
—
ting of 1.31 eV as in Figure 3. The empirical band structure then gives reasonable agree— ment with experiment for the width of the p3,2—derived bands. The sharp peaked structure on the latter in the Hel spectrum is probably due to direct transitions into 7a conduction band states. The failure to observe the A’ peak at Hell energy appears to be due to the increased width of all spectral features. The operation of an excitonic relaxation mechanism is not entirely ruled out by the present results, but there is a strong indica— plained simply in terms of the tion that the experimental EDC band width structure. can be cx— The discrepancy between experiment and the cal— culated bands probably lies in the approximations used in the KEN calculation15.The constant part of the muffin tin potential in ref. 5 was chosen to bring the calculated band gap into line with the experimental band gap determined from optical measurements. A self—consistent calculation might give the correct degree of dispersion of the valence bands. The spectra of Figure 3 give us the unique opportunity of comparing the ionisation ener— gies of the same atom in three different phases. The spectra were all recorded under the same experimental conditions; for the gas phase measurement the Pd sample was raised several centimetres out of the ionisation region and the xenon pressure taken up to 5 x 10 6 Torr. The question to be asked is whether it is poss— ible to relate the gas phase ionisation energy to the corresponding ionisation energies in ad— sorbed and solid xenon. We could thereby gain some quantitative information on the additional screening mechanisms characteristic of condensed phases, which further lower the ionisation energy. The difference between the EF—related 5P1,2 10fl3 sation energy of the free atom in Figure 3 and the spectroscopically determined IP (13.14 cv) is 14.7 eV and represents the separation between EF and the vacuum level at the point of photoioni— sation. In the condensed or adsorbed phase we can still use the vacuum level as a common re— fererice by simply adding the work function of
Vol. 30, No. 9
PHOTOEMISSION FROM SINGLE CRYSTAL XENON
547
Hel In C
£1
.
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o I— .~
.
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,~ ..~
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Figure 2
2
1 0 Energy (eV)
EDC’s from a xenon (lii) crystal at normal emission using the Mel, Eel and Hell discharge lines. The radiation was unpolarised und incident at 60°. Experimental resolution ~0.2 eV. Below: An intuitive band structure along FL obtained by “stretching” the KEN bands in ref. 5.
the sample, which in turn can be obtained from the width of the EDC. Thus the 13.14 eV IP of the gas phase is to be compared with 11.1 eV for the monolayer at r and 10.8 eV for the crystal at r. The real problem is, however, one of accounting for the initial state shift which occurs, even in the absence of a chemical interact~n17when the atom is adsorbed or forms a solid . Under these conditions the energy levels are affected by the change in electrostatic potential which an atom encounters on traversing the intrinsic surface dipole layer. Unfortunately the extent of this “matrix” effect, which increas~ the ionisa—
tion energy is known neither for Pd metal nor for solid xenon. It is thus difficult to gain access quantitatively to the relaxation effects associa— ted with the solid state. Image charge screening will constitute an important relaxation mechanism for the positive hole on an adsorbed atom. In so— lid xenon, where banding is more important, a hole in the valence band will be smeared out and less effectively screened. From Figure 3 we can merely conclude that the shift of the valence levels to lower ionisation energy in both the solid and adsorbed states reflects the dominance of relaxation over initial state “matrix” effects.
548
PHOTOEMISSION FROM SINGLE CRYSTAL XENON
Vol. 30, No. 9
U)
a‘I >.~ U) C
Xe crystal
Xe monolayer__~~/
~
Xe gas
~
~
~
I
I
I
I
I
I
I
I
10
9
8
7
6
5
4
3
lonisation energy (eV) ~‘igure3
12’13 and the EDC’s phase xenon, adsorbed xenon xenon from singlegascrystal at normal emission. Hel. The spectra are a11 referenced to the Fermi level of the Pd sample.
Acknowledgements— We acknowledge useful discussions
with F.—J. Himpsel, U. RBsslar and M. Scheffler.
References 1. 2. 3. 14. 5.
6.
7. 8. 9. 10. 11. 12. 13. 114.
SCHWENPNER, N., HIMPSEL, F.-J., SAlLE, V., SKIBOWSKI, M., STEINMANN, W. and KOCH,E. E., Phys. Rev. Letters 314 528 (1975). SCHWENTNER, N., HIMPSEL, F.—J., KOCH, E. E. and SKIBOWSKI, M. in Vacuum UV Radiation Physics Pergamon—Vieweg, Braunschweig, p. 355 (19714). NURNBERGER, H., HIMPSEL, F.—J., KOCH, E. F. and SCHWENTNER, N., phys. stat. solidi (b) 81 503 (1977). REILLY, N. H., J. Them. Phys. Solids 28 2067 (1967). R~5SSLER,U., phys. stat. solidi 142 3145 (1970). PARRINELLO, N., TOSATTI, E., MARCH, H. and TOSI, M. P., Lett. Nuovo Cimento 18 3141 (1977). IGNATIEV, A., JONES, A. V. and RHODIN, T. N., Surface Sd. 30 573 (1972). FEUEBBACHER, B. and CHRISTENSEN, N. E., Phys. Rev. B1O 2373 (19714). GRANDKE, T., LEY, L., CARDONA, M. and PEElER, H., Solid. State Commun. 21i 287(1977). UNWIN, B., HORN, K. and GENG, P., to be published. PALMBEHG, P. W., Surface Sci. 25 598 (1971). HORN, K., SCHEFFLER, M. and BRADSHAW, A. K., Phys. Rev. Letters 141 822 (1978). SCHEFFLER, N., HORN, r~.,BRADSIIAW, A. M. and KANBE., K., Surface Sci. 80 69 (1979). BRAUN, W. and NEUMMN, N., private communication.
Vol. 30, No. 9 15.
16. 17.
PHOTOEMISSION FROM SINGLE CRYSTAL XENON
RöSSLER, U., private communication. CITHIN, P. H. and HANANN, D. H., Phys. Rev. BIO 149148 (19714). M~ZEL,D. in Photoemission and the Electronic Properties of Surfaces (Eds. Feuerbacher, Fitton & Willis) Wiley, Chichester (1978).
549
30, pp.545—549. Solid State Communications, Vol. Pergamon Press Ltd. 1979. Printed in Great Britain.
PH0TOE~4ISSIONFROM SINGLE CRYSTAL XENON K. Horn and A. M. Bradshaw Fritz—Haber—Institut der Max—Planck—Gesellschaft Faradayweg 14-6, 1000 Berlin 33, W. Germany Received February 19, 1979, by M. Cardona Normal exit photoemission spectra from the (iii) surface of a xenon single crystal indicate that the valence level emission is considerably broader than expected on the basis of band structure calculations. Similar observations from polycrystalline samples have lead Parrinello et al to propose an excitonic screening mechanism which would cause an apparent increase in the spin—orbit splitting at the r point. The details of the spectra reported here show, however, that the effect could simply be due to a dispersion of the bands stronger than that predicted theoretically. Anomalous intensity effects at ?lw = 21.2 eV are explained in terms of electron—electron scattering.
the position of the 5p1,’2 band at or near the F point can be located. We have also been able to effect a comparison between the valence states of solid xenon, physisorbed xenon and gas phase xenon. The measurements were performed using the rare gas resonance lines, NeI(16.8 cv), Hel (21.2 cv) and HeII(140.8 eV) in an angular re— solving uhv photoelectron spectrometer (ADES 1400, VG Scientific Ltd.). The Pd(100) crystal was mounted on a specially constructed manipu— lator, which enables sample temperatures as low as 10 K to be reached. The manipulator and cool— ing facility will be described elsewhere10. The crystal surface was cleaned with standard tech— niques and characterised with Auger electron spectroscopy. The establishment of a hexagonal physisorbed monolayer~’12’13 (as observed in LEED)
Photoemission investigations on polycristal— line rare gas solids by Schwentner ~ al1’2 and Nurriberger et al3 have revealed a significant discrepancy between experimental and calculated valence band widths. In the case of xenon, for example, the OPW band structure of Reilly ~ and the KEN band structure of Rö2ler5 both predict a valence band width of ~ eV, whereas experi— mentally a value of ~‘3 eV is obtained. A super— ficial glance at the data of Schwentner etal might indicate that this is due to increased spin—orbit splitting, but, as the authors point out, the latter must have the same value at the centre of the Brillouin zone as in the free atom (1 .31 eV). A more satisfactory explanation is that the band dispersion is larger than predic— ted theoretically. As Figure 2 of ref. 1 shows, the problem is then one of locating the position of the 5p 112—derived band at the F point. An 6. A valence is considered alternative explanation haselectron been offered by Parto be trapped rineno et al in the potential well of the loca— used hole. Both hole states are raised in ener— gy, the 5p~,’~state can shift more in energy than the 5p1,2 state, because of their relative positions. This so—called excitonic screening mechanism, which must be distinguished from polarisation effects or screening through plas— mon excitation, then leads to an apparent increase in the spin—orbit splitting. In order to throw some new light on this problem we have prepared xenon single crystals on palladium surfaces with the epitaxial rela— tionship (111)~ II[100]pd and studied them with normal exit pho~oemission. (The growth of single crystal xenon on Ir(100) had ~reviously been established by Ignatiev et al’.) This choice of electron exit angle in angular—resolved photo— emission leads to a considerable simplification of the spectrum: for a (111) surface only valence band states along the FL direction of the Bril—
was found to be a criterion for surface cleanli— growth. ness as well as a prerequisite for good epitaxial Condensation of xenon at 140 K followed by annealing at 55 K lead to epitaxial growth with the (lii) face of xenon parallel to the Pd(100) surface. The LEED pattern showed 12—fold sym— metry as in the case of xenon grown on Ir(100)7, indicating that two orthogonal domains were pre— sent. One such domain is shown in Figure 1, where the xenon [~io) direction is parallel to the pal— ladium [010]direction. Fortunately for the pho— toemission experiment, the one direction conanon to both domains is the [iii] direction normal to the surface. The normal exit photoemission EDC’s at the three energies are shown in Figure 2. The full Eel spectrum is shown again in Figure 3 together with spectra from gas phase and adsorbed xenon. The spectra from the crystal at the three ener— gies resemble the polycrystalhine data of Schwentner et al in that a distinction between the 5p~,. 2—derivedband at higher and the 5p3,’~~ derivea band at lower ionisation energy is ap— parent. The Eel spectrum is characterised by some sharp fine structure as well as by a high relative intensity of the highest ionisation
bum zone are sampled (see e.g. ref. 8 or, more recently, ref. 9). We find that the spectra can be interpreted in terms of a stronger dispersion of both the 5p1/2and 5p3,~—derivedbands and that 545
.
546
PHOTOEMISSION FROM SINGLE CRYSTAL XENON
Figure 1
Vol. 30, No. 9
[Dii]
[oil]
Model of the epitaxial relationship between the (111) plane of xenon and a Pd(100) single crystal surface. Only one domain of the xenon crystal is shown.
energy peak (A) associated with the 5p
1,~—de— rived band. The full extent of this intensity difference is seen more clearly in Figure 3. The effect is apparently caused by electron—electron scattering first occurring when the kinetic energy of the photoemitted electrons relative to the valence band edge equals the sum of the band gap energy and the energy of the first cx— citon (17.8 cv). A second onset for scattering will occur when an energy twice the band gap is reached (18.6 cv). The electrons in peak A lie just above the first threshold, the electrons 2 above and theboth depth of on— in the 5p3,2—derived bands thresholds. gin vary almost uniformly across Such sampled onsets will are not sharp the EDC. The electrons in peak A originate from a substantially thicker slab of material and the peak is correspondingly more intense. A similar effect has recentl~rbeen observed for the 14o level in solid CO1’. The Mel and Hel spectra both reveal a Se— cond peak (A’) at 0.7 eV lower ionisation ener— gy than peak A. Both peaks are present with the same separation at the two energies and with similar relative intensities. This favours an explanation in terms of emission from critical points in the initial state band to evanescent vacuum final states. It is thus reasonable to assume that the A’ peak is due to emission from the p~ 2 states at r and that the peak A repre— sents emission from the flat portion of the ~P1/2~ derived band near the L point. The Ia—space argu— merit concerning the vanishingly small number of states near r does not apply in this angular—re— solved experiment where only a single direction of the Brillouin zone is sampled. This would in— dicate that the dispersion of the p1,~—derived band is in fact ‘~0.7 eV and not ‘~.0.145 eV as in the KEN calculation. At the bottom of Figure 2 is shown an intuitive band structure along F—A—L, whereby the KEN bands of Rdssler have been “stretched” to give a r~—L~separation of 0.67 eV. The distances r~—r~,5and rg —rihave been increased proportionately to 0.27 eV and 0.97 eV respectively. The r~separa— tion is given by the atomic spin—orbit split—
—
ting of 1.31 eV as in Figure 3. The empirical band structure then gives reasonable agree— ment with experiment for the width of the p3,2—derived bands. The sharp peaked structure on the latter in the Hel spectrum is probably due to direct transitions into 7a conduction band states. The failure to observe the A’ peak at Hell energy appears to be due to the increased width of all spectral features. The operation of an excitonic relaxation mechanism is not entirely ruled out by the present results, but there is a strong indica— plained simply in terms of the tion that the experimental EDC band width structure. can be cx— The discrepancy between experiment and the cal— culated bands probably lies in the approximations used in the KEN calculation15.The constant part of the muffin tin potential in ref. 5 was chosen to bring the calculated band gap into line with the experimental band gap determined from optical measurements. A self—consistent calculation might give the correct degree of dispersion of the valence bands. The spectra of Figure 3 give us the unique opportunity of comparing the ionisation ener— gies of the same atom in three different phases. The spectra were all recorded under the same experimental conditions; for the gas phase measurement the Pd sample was raised several centimetres out of the ionisation region and the xenon pressure taken up to 5 x 10 6 Torr. The question to be asked is whether it is poss— ible to relate the gas phase ionisation energy to the corresponding ionisation energies in ad— sorbed and solid xenon. We could thereby gain some quantitative information on the additional screening mechanisms characteristic of condensed phases, which further lower the ionisation energy. The difference between the EF—related 5P1,2 10fl3 sation energy of the free atom in Figure 3 and the spectroscopically determined IP (13.14 cv) is 14.7 eV and represents the separation between EF and the vacuum level at the point of photoioni— sation. In the condensed or adsorbed phase we can still use the vacuum level as a common re— fererice by simply adding the work function of
Vol. 30, No. 9
PHOTOEMISSION FROM SINGLE CRYSTAL XENON
547
Hel In C
£1
.
..%~ ..
o I— .~
.
:
•; C
i I
I
,~ ..~
U
~
r~ p1,21
r~
/7/
A
p31211
(,~ ~fL;~ 3
Figure 2
2
1 0 Energy (eV)
EDC’s from a xenon (lii) crystal at normal emission using the Mel, Eel and Hell discharge lines. The radiation was unpolarised und incident at 60°. Experimental resolution ~0.2 eV. Below: An intuitive band structure along FL obtained by “stretching” the KEN bands in ref. 5.
the sample, which in turn can be obtained from the width of the EDC. Thus the 13.14 eV IP of the gas phase is to be compared with 11.1 eV for the monolayer at r and 10.8 eV for the crystal at r. The real problem is, however, one of accounting for the initial state shift which occurs, even in the absence of a chemical interact~n17when the atom is adsorbed or forms a solid . Under these conditions the energy levels are affected by the change in electrostatic potential which an atom encounters on traversing the intrinsic surface dipole layer. Unfortunately the extent of this “matrix” effect, which increas~ the ionisa—
tion energy is known neither for Pd metal nor for solid xenon. It is thus difficult to gain access quantitatively to the relaxation effects associa— ted with the solid state. Image charge screening will constitute an important relaxation mechanism for the positive hole on an adsorbed atom. In so— lid xenon, where banding is more important, a hole in the valence band will be smeared out and less effectively screened. From Figure 3 we can merely conclude that the shift of the valence levels to lower ionisation energy in both the solid and adsorbed states reflects the dominance of relaxation over initial state “matrix” effects.
548
PHOTOEMISSION FROM SINGLE CRYSTAL XENON
Vol. 30, No. 9
U)
a‘I >.~ U) C
Xe crystal
Xe monolayer__~~/
~
Xe gas
~
~
~
I
I
I
I
I
I
I
I
10
9
8
7
6
5
4
3
lonisation energy (eV) ~‘igure3
12’13 and the EDC’s phase xenon, adsorbed xenon xenon from singlegascrystal at normal emission. Hel. The spectra are a11 referenced to the Fermi level of the Pd sample.
Acknowledgements— We acknowledge useful discussions
with F.—J. Himpsel, U. RBsslar and M. Scheffler.
References 1. 2. 3. 14. 5.
6.
7. 8. 9. 10. 11. 12. 13. 114.
SCHWENPNER, N., HIMPSEL, F.-J., SAlLE, V., SKIBOWSKI, M., STEINMANN, W. and KOCH,E. E., Phys. Rev. Letters 314 528 (1975). SCHWENTNER, N., HIMPSEL, F.—J., KOCH, E. E. and SKIBOWSKI, M. in Vacuum UV Radiation Physics Pergamon—Vieweg, Braunschweig, p. 355 (19714). NURNBERGER, H., HIMPSEL, F.—J., KOCH, E. F. and SCHWENTNER, N., phys. stat. solidi (b) 81 503 (1977). REILLY, N. H., J. Them. Phys. Solids 28 2067 (1967). R~5SSLER,U., phys. stat. solidi 142 3145 (1970). PARRINELLO, N., TOSATTI, E., MARCH, H. and TOSI, M. P., Lett. Nuovo Cimento 18 3141 (1977). IGNATIEV, A., JONES, A. V. and RHODIN, T. N., Surface Sd. 30 573 (1972). FEUEBBACHER, B. and CHRISTENSEN, N. E., Phys. Rev. B1O 2373 (19714). GRANDKE, T., LEY, L., CARDONA, M. and PEElER, H., Solid. State Commun. 21i 287(1977). UNWIN, B., HORN, K. and GENG, P., to be published. PALMBEHG, P. W., Surface Sci. 25 598 (1971). HORN, K., SCHEFFLER, M. and BRADSHAW, A. K., Phys. Rev. Letters 141 822 (1978). SCHEFFLER, N., HORN, r~.,BRADSIIAW, A. M. and KANBE., K., Surface Sci. 80 69 (1979). BRAUN, W. and NEUMMN, N., private communication.
Vol. 30, No. 9 15.
16. 17.
PHOTOEMISSION FROM SINGLE CRYSTAL XENON
RöSSLER, U., private communication. CITHIN, P. H. and HANANN, D. H., Phys. Rev. BIO 149148 (19714). M~ZEL,D. in Photoemission and the Electronic Properties of Surfaces (Eds. Feuerbacher, Fitton & Willis) Wiley, Chichester (1978).
549