- Email: [email protected]
Cu (110)
Solid State Communications, Vol. 46, No. 9, pp. 713-716, 1983. Printed in Great Britain.
0038-1098/83 $3.00 + .00 Pergamon Press Ltd.
LAYER-DEPENDENT SHIFTS IN IONIZATION POTENTIAL AND AUGER ENERGIES FOR Kr/Cu (1 10) T. Mandel and G. Kaindl Institut ftir Atom- und Festk6rperphysik, Freie Universtit/it Berlin, D-1000 Berlin 33, West Germany and K. Horn, M. Iwan, H.U. Middelmann and C. Mariani Fritz-Haber-Institut der Max-Planck-Gesellschaft, D-1000 Berlin 33, West Germany
(Received 26 January 1983 by P.H. Dederichs)
Photoelectron spectra of the krypton 3d core-levels and MNN Auger spectra of krypton mono- and multilayers on Cu(1 10) have been recorded with synchrotron radiation. The Kr-3d line is found to shift to higher binding energies by 0.73 eV for first- and second-layer adsorption, respectively. This value is much larger than the work function decrease for Kr monolayer adsorption, A~ = - 0.29 eV. The shift in Auger line energies is found to be about three times larger than the 3d line shift. These observations can be readily explained in terms of image-charge screening of the hole states.
PHOTOEMISSION (PE) studies of monolayers of rare gases physisorbed on metal surfaces have attracted considerable interest in recent years [1-3]. Kaindl and Chiang have investigated multilayers of &r, Kr and Xe on various metal substrates, like Pd(100) [4, 5], AI(111) [6] and Pd(1 11) [7]. They found layerdependent shifts in the ionization potential of Xe--4d core-levels and Xe-5p, Kr-4p and &r-3p valence levels as compared to the free-atom values. In addition, layerdependent kinetic energy shifts of the Xe-100 Auger lines were observed and found to be by a factor of about three layer than the Xe-4d shifts. These observations led the authors to an interpretation of the observed effects in terms of image-charge screening of the hole states, where the magnitude of the shift is related to the distance between the rare-gas layer and the image plane of the metal substrate. The singly or doubly charged hole on the rare-gas atom is screened by conduction electrons of the metal, whereby the net screening energy is transferred to the emitted electron. This interpretation is supported by the work of Lang and Williams [8, 9]. On the other hand, Jacobi et al. [10] have recently proposed a different interpretation of these layerdependent shifts in terms of pure initial-state effects. Their arguments are solely based on PE studies of the valence levels of the rare gases physisorbed on various substrates. In particular, they identify the observed difference in ionization potential for 1st- and 2nd-layer rare gas atoms with the change in work function caused 713
by a monolayer of the rare gas for the substrate under discussion. In order to shed further light on the validities of these two rather contradictory interpretations, we have studied PE from Kr-3d levels and Kr-MNN Auger spectra from multilayers of Kr physisorbed on Cu(110). This system was chosen because of the relatively small change in work function upon adsorption of a monolayer of Kr (h~ = -- 0.29 eV) [11 ], and because layerdependent shifts of core-level ionization potentials and Auger lines of rare gases had been studied so far only for Xe [4]. The present results support the interpretation of the layer-dependent shifts in terms of image-charge screening of the hole states. In particular, we Fred that the Kr-3d ionization potential is by 0.73 eV higher in the second Kr layer as compared to the first layer. In agreement with the previous results for Xe multilayers on various metal substrates [4-7, 13] the Kr-MNN Auger lines exhibit layer-dependent shifts three times larger than the core-level PE lines. The experiments were performed in an angleintegrating photoelectron spectrometer (double-pass CMA from Pill Inc.), using synchroton radiation from the DORIS storage ring at DESY/Hamburg dispersed by the FLIPPER grazing-incidence monochromator. The Cu(110) single crystal was mounted on a special cryostat specimen holder [12] allowing temperatures as low as 20 K to be attained. The Cu(110) surface was cleaned in situ by cycles of &r-ion sputtering and annealing to 550 K.
LAYER-DEPENDENT SHIFTS IN Kr/Cu(1 10)
714
The preparation of monolayers, bilayers and trilayers of Kr was achieved by a suitable choice of gas exposure and sample temperature. The growth of separate layers could be monitored directly by the appearance of shifted peaks in the PE spectrum. PE spectra of the Kr-3d core-level of a monalayer, a bilayer and a trilayer of adsorbed Kr on Cu(1 10) are presented in Fig. 1. They were obtained at photon energies of 115 and 120 eV, respectively. The monalayer spectrum shows two well-resolved spin--orbit
Kr on Cu (110)
t~ jl %
Kr 3<:1
1 I
~ 3rd Layer ~ / 2 r i d Layer
(c) TriLayer
•
==
/~
/ 2 r i d Layer st Layer
.--. ~ _
~'~..~Z
.¢', ~ ", ~+Lo.~~ Y
~1~, "%. ~ "4
[ a ] MonoLayer
t
I
.I
~
~,i hu =J2OeV x~ '
t -%%~#- I ~'~ht,, =II 5aM
~ 95
,,,,~+.. •
I
"-d l
I ~
\ I
9~. 93 92 91 Binding energy reL t.o Evoc(eV)
I
90
Fig. 1. K r - 3 d PE spectra of a monolayer, a bilayer and a trilayer of Kr/Cu(1 10). The spectra were recorded at photon energies of 115 and 120 eV, respectively. The solid curve represents the result of a least-squares fit analysis. The subspectra represent contributions from the 1 st layer (dashed), the 2nd layer (dash-dotted), and the 3rd layer (dotted). split peaks separated by 1.21 eV. In the bilayer spectrum two sets o f shifted doublets from first- and second-layer Kr atoms can be clearly discerned, similar to, but less well-resolved than, in the case of Xe [4, 5, 7, 13 ]. In the trilayer spectrum, only two broad structures are resolved which are due to a superposition of
Vol. 46, No. 9
three shifted doublets. It is apparent, however, that the two peaks of the trilayer spectrum are asymmetric towards the low-binding energy side. In order to decompose the spectra into contributions from the separate layers, a least-squares fit procedure was applied, using doublets of asymmetric Gaussian lines with constant spin--orbit splitting. This resulted in the contributions from the three different Kr layers shown as subspectra in Fig. 1. The intensity ratios of these subspectra for the separate layers indicate that the topmost Kr layer was not completely filled in the bilayer and trilayer cases. Values of the K r - 3 d ionization potentials for the separate layers were obtained from results of the fit, but can also be evaluated directly from the spectra, at least in the bilayer case. For comparison with the free-atom values, a correct vacuum-level reference is needed. Following the arguments of Lang and Williams [9], who showed that the spatial region of the 4d core orbitals of even a first-layer adsorbed Xe atom lies outside of the dipole layer responsible for the work function change, we reference the experimental values to the vacuum level of a Kr-covered Cu(1 10) surface. This point is discussed in more detail below. The numerical values for the K r - 3 d peak shifts relative to the free atom value are given in Table 1, column 3. The shift between the 1st- and 2nd-layer peaks is 0.73 eV, considerably larger than the work function change of A¢ = -- 0.29 eV [11 ]. Note also that there is an additional shift in the K r - 3 d peak position between the 2nd and 3rd layer, although no further work function change occurs upon adsorption of the second layer. As mentioned above, Kaindl and Chiang [4-7] explained these shifts as being due to the image-charge screening of the final-state core-hole by the conduction electrons of the metal substrata. From energy conservation, the screening energy, which should decrease with increasing distance from the surface, is transferred to the emitted photoelectron, which therefore appears at a higher kinetic energy in the PE spectrum; this corresponds to a lower ionization potential. Following [4], we have calculated with a simple semi-classical image-charge screenhag model values for the expected shifts in the Kr core-level ionization potential, which are given in Table 1, column 5. The position of the image plane was taken from the results of local-density functional calculations by Lang and Williams [14], and assumed to be Xo = 0.79 ~ outside of the jellium edge of the Cu(1 10) substrata. The average distance d between the substrata surface and the incommensurate Kr overlayer [11 ] was calculated in a model of close-packed hard spheres assuming an atomic Kr radius of 2.0 A. When calculating 2nd- and 3rd-layer ionization
Vol. 46, No. 9
Table 1. Kr-3d binding energy shifts AEB(3d ) and Kr-MNN Auger kinetic energy shifts AE K relative to the ~as-phase values for Kr overlayers on Cu(1 10). A E ~ are theoretical values based on a pure imagecharge screening model (see text). All energies [in eV) are measured relative to the vacuum level o f the covered substrate. The error bars for the experimental results are estimated as + 0.10 eV Kr configuration
Layer
AEB(3d)
AEK/3
AE~8
Monolayer Bilayer
1 st 1st 2nd 1st 2nd 3rd
2.11 2.03 1.29 1.96 1.30 1.07
2.17 2.35 1.45 -
2.26 2.26 1.44 2.26 1.44 1.20
Trilayer
potential shifts, the influence of Kr underlayers was simulated by a homogeneous dielectric slab with the dielectric constant of solid Kr extending from the image plane to a distance d - x a from the metal surface [4]. As one can see from the values listed in Table 1, this simple model yields results which agree quite well with the experimental values, and does support the interpretation of our observed shifts in terms of imagecharge screening. Kr on Cu (110)Kr M4.5 NN-Aucjer
•
~.~.ID'~'~I,
=,,
,~
~--:~,. ,~,~.._ - ~ ..,o.,-,..--.~,..~ •~,_~. J
"-.
.
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2rid Loyer
/
~,~ _ / / I s t /
Table 2. Comparison between experimental differences in core-level binding energies for the 1st and 2nd layer o f a rare-gas bilayer. AEB(1,2) and work function changes AC for monolayer coverages {in eV)
,/"367 "~.
• .:..~ ~
eo %
~
he = 140 eV
(0) MonoLoyer
Further support to this interpretation can be obtained from the Kr-MNN Auger spectra of the different layer configurations presented in Fig. 2. The Auger spectrum of a Kr monolayer shows three peaks which correspond to the most intense lines in the Auger spectrum of gas-phase Kr [15 ]. Because of a smaller splitting the structures are less well-resolved than in the case of the Xe-NOO Auger spectra [4-7, 13]. The Kr-MNN Auger spectrum obtained from a bilayer of Kr/Cu(1 10) is shown in Fig. 2(b). Both spectra were least-squares fitted to a superposition of seven Lorentzian lines for each Kr layer (plus background due to the Cu-3p photoemission lines which overlapped at this photon energy) with energy separations as in the gas-phase spectrum. In order to obtain optimum fits the relative intensities of the seven individual lines had to be varied by a small amount (typically less than 5%) from the numbers applicable to gas-phase Kr. Despite the lower signalto-noise ratio of the Auger spectra, rather accurate values for the energy positions were obtained. The resulting shifts in Auger kinetic energies with respect to the gas-phase values divided by three, AEK/3, are given in Table 1, column 4. AEK/3 is very close to the shifts in K r - 3 d binding energy, supporting strongly the explanation of these shifts in terms of imagecharge screening. As pointed out in [4] this is because the screening potential is given by e2/4(d-xo), and the initial state in the Auger process contains one hole, whereas the final state contains two holes.
teyer
"~'~. ~''d~'J'~ X h~=t41eV "'.... " \ / ~ %
8
715
LAYER-DEPENDENT SHIFTS IN Kr/Cu(1 10)
System
AEB (1,2)
A~
Kr/Cu(1 10) Xe/AI(111) Xe/Pd(100) Xe/Pd(111) Xe/Gd(000 I)
-- 0.74 a -- 0.56 e -- 0.72 e --0.77 e -- 0.70 f
-- 0.29 b -- 0.29 d -- 0.68 e --0.81 e - 0.60 f
aThis work; b [11]; e [13]; d [6];
. . . . . . . /. \ ~ % ~ l ' e ~
.... ~ .....~.~ I
50
I
52
I
54
I
56
I
I
I
I
58 60 62 64 Kinetic energy (eV)
I
66
I
68
Fig. 2. Kr-MNN Auger spectra of a monolayer and a bilayer of Kr/Cu(110) taken at a photon energy of 140 and 141 eV. The solid line represents the least-squares fit result, and the single.layer contributions are indicated by dashed (1 st layer) and dash-dotted (2nd layer) subspectra.
Recently, Jacobi et al. [10] have proposed a different interpretation of these layer-dependent shifts in terms of pure initial-state effects. In particular, they argued from the similarity of the work function change A~ for monolayer coverage of Xe/Pd(100) and the difference in the core-level ionization potentials for the first and second Xe layers, AEB(I, 2), that the shift is entirely due to different vacuum levels for the first and second layer. If this were correct, there should be a
LAYER-DEPENDENT SHIFTS IN Kr/Cu(1 10)
716
general agreement between AEB(1,2) and the work function change A~ due to adsorption of a rare-gas monolayer on the clean surface. In Table 2, we have listed the known values of AEB(1, 2) for Xe-4d and K r - 3 d core-levels as well as the A~ values [4--6, 13, 14]. It is obvious that the work function changes A~ do not track the values for AEs(1, 2), which clearly contradicts the basic assumption of Jacobi et al. Another argument of Jacobi et al. in support of their interpretation (also given by other authors [2, 3] ) is a constancy of the Xe-5p valence-electron ionization potential relative to the vacuum level of the clean substrate, observed for a large variety of clean [2, 3] as well as adsorbate-covered surfaces (i.e. adsorbate plus xenon layer on top) [10]. These observations are not thought to contradict our image charge screening interpretation for the following reasons: (1) l_ang and Williams have shown [8, 9] that the Xe-5p shell is sufficiently extended to partly overlap with the electrostatic dipole layer, which can lead to a modified vacuum level for emission from 5p states as compared to core-levels. This overlap is expected to depend on substrate geometry and electronic structure, causing a variation of the ionization potential. (2) The experimental ionization potential of Xe-5p electrons is expected to be influenced by band dispersion effects.
interpretation and does give strong support to our model. Finally, it should be mentioned that the magnitude of the Auger kinetic energy shifts, which are about three times as large as the core-level ionization potential shifts, cannot easily be explained in a model considering only initial-state effects. While the present data give clear evidence for the basic correctness of the image-charge screening-model, it will be useful, however, to obtain further data, especially in core-level photoemission from rare gases adsorbed on different substrates.
Acknowledgements - The authors acknowledge the hospitality of Prof. C. Kunz and his staff at HASYLAB. The work was supported by the Bundesministerium ffir Forschung und Technologic, grants 05-127KA and 05-144HR, and by the Sonderforschungsbereich-6 of the Deutsche Forschungsgemeinschaft. REFERENCES 1. 2. 3. 4. 5.
Table 3. Kr-3ds/2 and Xe-4ds/2 core-level ionization potentials (in eV) relative to the Fermi level, E~, and vacuum level of the covered substrate, Ev. Also given is AE a, the shift in E v relative to the gas-phase values System
E~
Ceov
EBv
z~EB
Kr/Cu(1 10) Xe/AI(111) Xe/Pd(100) Xe/Gd(0001)
87.46 a 61.43 e 60.53 e 62.8 f
4.19 b 4.01 d 4.83 e 2.70 f
91.65 65.44 65.34 65.5
2.11 2.06 2.16 2.0
Vol. 46, No. 9
6. 7. 8. 9. 10. 11.
aThis work; b [11]; e [13]; a [6]; e[4]; f[16].
12.
These problems are avoided if core-level ionization potentials (i.e. Xe-4d or Kr-3d) are solely considered as in the present work. If we compare the ionization potentials of the core levels as done in Table 3, it is found that there is in fact a constancy if we reference the data to the vacuum level of the adsorbate-covered surface. This is in contradiction to the initial-state
13. 14. 15. 16.
K. Horn, M. Scheffler & A.M. Bradshaw,Phys. Rev. Lett. 41,822 (1978). J. Kiippers, K. Wandelt & G. Erthl, Phys. Rev. Lett. 43,928 (1979). J. Hiilse, J. Kiippers, K. Wandelt & G. Ertl, Appl. Surf. Sci. 6,453 (1980). G. Kaindl, T.-C. Chiang, D.E. Eastman & F.J. Himpsel, Phys. Rev. Lett. 45, 1808 (1980). G. Kaindl, T.-C. Chiang, F.J. Himpsel & D.E. Eastman, Ordering in Two Dimensions, (Edited by S.K. Sinha), p. 99. North-Holland (1980). T.-C. Chiang, G. Kaindl & D.E. Eastman,Solid State Commun. 41,661 (1982). G. Kaindl, T.-C. Chiang & D.E. Eastman, Phys. Rev. B25, 7846 (1982). N.D. Lang, Phys. Rev. Lett. 46,842 (1981). N.D. Lang & A.R. Williams, Phys. Rev. B25, 2940 (1982). K. Jacobi & H.H. Rotermund, Surf. Sci. 116, 435 (1982). K. Horn, C. Mariani & L. Cramer, Surf. Sci. 117, 376 (1982). R. Unwin, K. Horn & P. Geng, Vakuum-Technik 29, 149 (1980). G. Kaindl, T.-C. Chiang & T. Mandel (to be published). N.D. Land & A.R. Williams, Phys. Rev. B16, 2408 (1977). L.O. Werme, T. Bergmark & K. Siegbahn,Physica Scripta 6, 141 (1972). N.D. Lang, A.R. Williams, F.J. Himpsel, B. Reihl & D.E. Eastman,Phys. Rev. B26, 1728 (I982).
0038-1098/83 $3.00 + .00 Pergamon Press Ltd.
LAYER-DEPENDENT SHIFTS IN IONIZATION POTENTIAL AND AUGER ENERGIES FOR Kr/Cu (1 10) T. Mandel and G. Kaindl Institut ftir Atom- und Festk6rperphysik, Freie Universtit/it Berlin, D-1000 Berlin 33, West Germany and K. Horn, M. Iwan, H.U. Middelmann and C. Mariani Fritz-Haber-Institut der Max-Planck-Gesellschaft, D-1000 Berlin 33, West Germany
(Received 26 January 1983 by P.H. Dederichs)
Photoelectron spectra of the krypton 3d core-levels and MNN Auger spectra of krypton mono- and multilayers on Cu(1 10) have been recorded with synchrotron radiation. The Kr-3d line is found to shift to higher binding energies by 0.73 eV for first- and second-layer adsorption, respectively. This value is much larger than the work function decrease for Kr monolayer adsorption, A~ = - 0.29 eV. The shift in Auger line energies is found to be about three times larger than the 3d line shift. These observations can be readily explained in terms of image-charge screening of the hole states.
PHOTOEMISSION (PE) studies of monolayers of rare gases physisorbed on metal surfaces have attracted considerable interest in recent years [1-3]. Kaindl and Chiang have investigated multilayers of &r, Kr and Xe on various metal substrates, like Pd(100) [4, 5], AI(111) [6] and Pd(1 11) [7]. They found layerdependent shifts in the ionization potential of Xe--4d core-levels and Xe-5p, Kr-4p and &r-3p valence levels as compared to the free-atom values. In addition, layerdependent kinetic energy shifts of the Xe-100 Auger lines were observed and found to be by a factor of about three layer than the Xe-4d shifts. These observations led the authors to an interpretation of the observed effects in terms of image-charge screening of the hole states, where the magnitude of the shift is related to the distance between the rare-gas layer and the image plane of the metal substrate. The singly or doubly charged hole on the rare-gas atom is screened by conduction electrons of the metal, whereby the net screening energy is transferred to the emitted electron. This interpretation is supported by the work of Lang and Williams [8, 9]. On the other hand, Jacobi et al. [10] have recently proposed a different interpretation of these layerdependent shifts in terms of pure initial-state effects. Their arguments are solely based on PE studies of the valence levels of the rare gases physisorbed on various substrates. In particular, they identify the observed difference in ionization potential for 1st- and 2nd-layer rare gas atoms with the change in work function caused 713
by a monolayer of the rare gas for the substrate under discussion. In order to shed further light on the validities of these two rather contradictory interpretations, we have studied PE from Kr-3d levels and Kr-MNN Auger spectra from multilayers of Kr physisorbed on Cu(110). This system was chosen because of the relatively small change in work function upon adsorption of a monolayer of Kr (h~ = -- 0.29 eV) [11 ], and because layerdependent shifts of core-level ionization potentials and Auger lines of rare gases had been studied so far only for Xe [4]. The present results support the interpretation of the layer-dependent shifts in terms of image-charge screening of the hole states. In particular, we Fred that the Kr-3d ionization potential is by 0.73 eV higher in the second Kr layer as compared to the first layer. In agreement with the previous results for Xe multilayers on various metal substrates [4-7, 13] the Kr-MNN Auger lines exhibit layer-dependent shifts three times larger than the core-level PE lines. The experiments were performed in an angleintegrating photoelectron spectrometer (double-pass CMA from Pill Inc.), using synchroton radiation from the DORIS storage ring at DESY/Hamburg dispersed by the FLIPPER grazing-incidence monochromator. The Cu(110) single crystal was mounted on a special cryostat specimen holder [12] allowing temperatures as low as 20 K to be attained. The Cu(110) surface was cleaned in situ by cycles of &r-ion sputtering and annealing to 550 K.
LAYER-DEPENDENT SHIFTS IN Kr/Cu(1 10)
714
The preparation of monolayers, bilayers and trilayers of Kr was achieved by a suitable choice of gas exposure and sample temperature. The growth of separate layers could be monitored directly by the appearance of shifted peaks in the PE spectrum. PE spectra of the Kr-3d core-level of a monalayer, a bilayer and a trilayer of adsorbed Kr on Cu(1 10) are presented in Fig. 1. They were obtained at photon energies of 115 and 120 eV, respectively. The monalayer spectrum shows two well-resolved spin--orbit
Kr on Cu (110)
t~ jl %
Kr 3<:1
1 I
~ 3rd Layer ~ / 2 r i d Layer
(c) TriLayer
•
==
/~
/ 2 r i d Layer st Layer
.--. ~ _
~'~..~Z
.¢', ~ ", ~+Lo.~~ Y
~1~, "%. ~ "4
[ a ] MonoLayer
t
I
.I
~
~,i hu =J2OeV x~ '
t -%%~#- I ~'~ht,, =II 5aM
~ 95
,,,,~+.. •
I
"-d l
I ~
\ I
9~. 93 92 91 Binding energy reL t.o Evoc(eV)
I
90
Fig. 1. K r - 3 d PE spectra of a monolayer, a bilayer and a trilayer of Kr/Cu(1 10). The spectra were recorded at photon energies of 115 and 120 eV, respectively. The solid curve represents the result of a least-squares fit analysis. The subspectra represent contributions from the 1 st layer (dashed), the 2nd layer (dash-dotted), and the 3rd layer (dotted). split peaks separated by 1.21 eV. In the bilayer spectrum two sets o f shifted doublets from first- and second-layer Kr atoms can be clearly discerned, similar to, but less well-resolved than, in the case of Xe [4, 5, 7, 13 ]. In the trilayer spectrum, only two broad structures are resolved which are due to a superposition of
Vol. 46, No. 9
three shifted doublets. It is apparent, however, that the two peaks of the trilayer spectrum are asymmetric towards the low-binding energy side. In order to decompose the spectra into contributions from the separate layers, a least-squares fit procedure was applied, using doublets of asymmetric Gaussian lines with constant spin--orbit splitting. This resulted in the contributions from the three different Kr layers shown as subspectra in Fig. 1. The intensity ratios of these subspectra for the separate layers indicate that the topmost Kr layer was not completely filled in the bilayer and trilayer cases. Values of the K r - 3 d ionization potentials for the separate layers were obtained from results of the fit, but can also be evaluated directly from the spectra, at least in the bilayer case. For comparison with the free-atom values, a correct vacuum-level reference is needed. Following the arguments of Lang and Williams [9], who showed that the spatial region of the 4d core orbitals of even a first-layer adsorbed Xe atom lies outside of the dipole layer responsible for the work function change, we reference the experimental values to the vacuum level of a Kr-covered Cu(1 10) surface. This point is discussed in more detail below. The numerical values for the K r - 3 d peak shifts relative to the free atom value are given in Table 1, column 3. The shift between the 1st- and 2nd-layer peaks is 0.73 eV, considerably larger than the work function change of A¢ = -- 0.29 eV [11 ]. Note also that there is an additional shift in the K r - 3 d peak position between the 2nd and 3rd layer, although no further work function change occurs upon adsorption of the second layer. As mentioned above, Kaindl and Chiang [4-7] explained these shifts as being due to the image-charge screening of the final-state core-hole by the conduction electrons of the metal substrata. From energy conservation, the screening energy, which should decrease with increasing distance from the surface, is transferred to the emitted photoelectron, which therefore appears at a higher kinetic energy in the PE spectrum; this corresponds to a lower ionization potential. Following [4], we have calculated with a simple semi-classical image-charge screenhag model values for the expected shifts in the Kr core-level ionization potential, which are given in Table 1, column 5. The position of the image plane was taken from the results of local-density functional calculations by Lang and Williams [14], and assumed to be Xo = 0.79 ~ outside of the jellium edge of the Cu(1 10) substrata. The average distance d between the substrata surface and the incommensurate Kr overlayer [11 ] was calculated in a model of close-packed hard spheres assuming an atomic Kr radius of 2.0 A. When calculating 2nd- and 3rd-layer ionization
Vol. 46, No. 9
Table 1. Kr-3d binding energy shifts AEB(3d ) and Kr-MNN Auger kinetic energy shifts AE K relative to the ~as-phase values for Kr overlayers on Cu(1 10). A E ~ are theoretical values based on a pure imagecharge screening model (see text). All energies [in eV) are measured relative to the vacuum level o f the covered substrate. The error bars for the experimental results are estimated as + 0.10 eV Kr configuration
Layer
AEB(3d)
AEK/3
AE~8
Monolayer Bilayer
1 st 1st 2nd 1st 2nd 3rd
2.11 2.03 1.29 1.96 1.30 1.07
2.17 2.35 1.45 -
2.26 2.26 1.44 2.26 1.44 1.20
Trilayer
potential shifts, the influence of Kr underlayers was simulated by a homogeneous dielectric slab with the dielectric constant of solid Kr extending from the image plane to a distance d - x a from the metal surface [4]. As one can see from the values listed in Table 1, this simple model yields results which agree quite well with the experimental values, and does support the interpretation of our observed shifts in terms of imagecharge screening. Kr on Cu (110)Kr M4.5 NN-Aucjer
•
~.~.ID'~'~I,
=,,
,~
~--:~,. ,~,~.._ - ~ ..,o.,-,..--.~,..~ •~,_~. J
"-.
.
%
2rid Loyer
/
~,~ _ / / I s t /
Table 2. Comparison between experimental differences in core-level binding energies for the 1st and 2nd layer o f a rare-gas bilayer. AEB(1,2) and work function changes AC for monolayer coverages {in eV)
,/"367 "~.
• .:..~ ~
eo %
~
he = 140 eV
(0) MonoLoyer
Further support to this interpretation can be obtained from the Kr-MNN Auger spectra of the different layer configurations presented in Fig. 2. The Auger spectrum of a Kr monolayer shows three peaks which correspond to the most intense lines in the Auger spectrum of gas-phase Kr [15 ]. Because of a smaller splitting the structures are less well-resolved than in the case of the Xe-NOO Auger spectra [4-7, 13]. The Kr-MNN Auger spectrum obtained from a bilayer of Kr/Cu(1 10) is shown in Fig. 2(b). Both spectra were least-squares fitted to a superposition of seven Lorentzian lines for each Kr layer (plus background due to the Cu-3p photoemission lines which overlapped at this photon energy) with energy separations as in the gas-phase spectrum. In order to obtain optimum fits the relative intensities of the seven individual lines had to be varied by a small amount (typically less than 5%) from the numbers applicable to gas-phase Kr. Despite the lower signalto-noise ratio of the Auger spectra, rather accurate values for the energy positions were obtained. The resulting shifts in Auger kinetic energies with respect to the gas-phase values divided by three, AEK/3, are given in Table 1, column 4. AEK/3 is very close to the shifts in K r - 3 d binding energy, supporting strongly the explanation of these shifts in terms of imagecharge screening. As pointed out in [4] this is because the screening potential is given by e2/4(d-xo), and the initial state in the Auger process contains one hole, whereas the final state contains two holes.
teyer
"~'~. ~''d~'J'~ X h~=t41eV "'.... " \ / ~ %
8
715
LAYER-DEPENDENT SHIFTS IN Kr/Cu(1 10)
System
AEB (1,2)
A~
Kr/Cu(1 10) Xe/AI(111) Xe/Pd(100) Xe/Pd(111) Xe/Gd(000 I)
-- 0.74 a -- 0.56 e -- 0.72 e --0.77 e -- 0.70 f
-- 0.29 b -- 0.29 d -- 0.68 e --0.81 e - 0.60 f
aThis work; b [11]; e [13]; d [6];
. . . . . . . /. \ ~ % ~ l ' e ~
.... ~ .....~.~ I
50
I
52
I
54
I
56
I
I
I
I
58 60 62 64 Kinetic energy (eV)
I
66
I
68
Fig. 2. Kr-MNN Auger spectra of a monolayer and a bilayer of Kr/Cu(110) taken at a photon energy of 140 and 141 eV. The solid line represents the least-squares fit result, and the single.layer contributions are indicated by dashed (1 st layer) and dash-dotted (2nd layer) subspectra.
Recently, Jacobi et al. [10] have proposed a different interpretation of these layer-dependent shifts in terms of pure initial-state effects. In particular, they argued from the similarity of the work function change A~ for monolayer coverage of Xe/Pd(100) and the difference in the core-level ionization potentials for the first and second Xe layers, AEB(I, 2), that the shift is entirely due to different vacuum levels for the first and second layer. If this were correct, there should be a
LAYER-DEPENDENT SHIFTS IN Kr/Cu(1 10)
716
general agreement between AEB(1,2) and the work function change A~ due to adsorption of a rare-gas monolayer on the clean surface. In Table 2, we have listed the known values of AEB(1, 2) for Xe-4d and K r - 3 d core-levels as well as the A~ values [4--6, 13, 14]. It is obvious that the work function changes A~ do not track the values for AEs(1, 2), which clearly contradicts the basic assumption of Jacobi et al. Another argument of Jacobi et al. in support of their interpretation (also given by other authors [2, 3] ) is a constancy of the Xe-5p valence-electron ionization potential relative to the vacuum level of the clean substrate, observed for a large variety of clean [2, 3] as well as adsorbate-covered surfaces (i.e. adsorbate plus xenon layer on top) [10]. These observations are not thought to contradict our image charge screening interpretation for the following reasons: (1) l_ang and Williams have shown [8, 9] that the Xe-5p shell is sufficiently extended to partly overlap with the electrostatic dipole layer, which can lead to a modified vacuum level for emission from 5p states as compared to core-levels. This overlap is expected to depend on substrate geometry and electronic structure, causing a variation of the ionization potential. (2) The experimental ionization potential of Xe-5p electrons is expected to be influenced by band dispersion effects.
interpretation and does give strong support to our model. Finally, it should be mentioned that the magnitude of the Auger kinetic energy shifts, which are about three times as large as the core-level ionization potential shifts, cannot easily be explained in a model considering only initial-state effects. While the present data give clear evidence for the basic correctness of the image-charge screening-model, it will be useful, however, to obtain further data, especially in core-level photoemission from rare gases adsorbed on different substrates.
Acknowledgements - The authors acknowledge the hospitality of Prof. C. Kunz and his staff at HASYLAB. The work was supported by the Bundesministerium ffir Forschung und Technologic, grants 05-127KA and 05-144HR, and by the Sonderforschungsbereich-6 of the Deutsche Forschungsgemeinschaft. REFERENCES 1. 2. 3. 4. 5.
Table 3. Kr-3ds/2 and Xe-4ds/2 core-level ionization potentials (in eV) relative to the Fermi level, E~, and vacuum level of the covered substrate, Ev. Also given is AE a, the shift in E v relative to the gas-phase values System
E~
Ceov
EBv
z~EB
Kr/Cu(1 10) Xe/AI(111) Xe/Pd(100) Xe/Gd(0001)
87.46 a 61.43 e 60.53 e 62.8 f
4.19 b 4.01 d 4.83 e 2.70 f
91.65 65.44 65.34 65.5
2.11 2.06 2.16 2.0
Vol. 46, No. 9
6. 7. 8. 9. 10. 11.
aThis work; b [11]; e [13]; a [6]; e[4]; f[16].
12.
These problems are avoided if core-level ionization potentials (i.e. Xe-4d or Kr-3d) are solely considered as in the present work. If we compare the ionization potentials of the core levels as done in Table 3, it is found that there is in fact a constancy if we reference the data to the vacuum level of the adsorbate-covered surface. This is in contradiction to the initial-state
13. 14. 15. 16.
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