- Email: [email protected]
Densities of unoccupied states in HOPG, C6Li and C8K studied by photoyield and Bremsstrahlung isochromat spectroscopy
Synthetic Metals, 8 ( 1 9 8 3 ) 131 - 137
131
DENSITIES OF UNOCCUPIED STATES IN HOPG, C6Li AND CsK STUDIED BY P H O T O Y I E L D AND B R E M S S T R A H L U N G ISOCHROMAT SPECTROSCOPY C. F. H A G U E
Laboratoire de Chimie Physique, Universitd P. et M. Curie, F - 75231 Paris C~dex 05 (France) G. I N D L E K O F E R , GIJNTHERODT
U.
M.
GUBLER,
V.
GEISER,
P.
OELHAFEN
and
H.-J.
Institut fiir Physik, Universita't Basel, CH-4056 Basel (Switzerland) J. S C H M I D T - M A Y a n d R. N Y H O L M
H. Institut fiir Experimentalphysik, Universitiit Hamburg, D-2500 Hamburg (F.R.G.) E. W U I L L O U D a n d Y. B A E R
lnstitut de Physique, Universit~ de Neuch~tel, CH-2000 Neuch~tel (Switzerland)
Summary The two techniques employed for studying the e m p t y conduction band states in stage-1 graphite intercalation compounds (here C6Li and CsK) provide complementary information. The photoyield experiments give the local (site decomposed) densities of unoccupied states and probe, in particular, the C ~r* band and the Li(2p) and K(3d)/(4s) states. The high energy Bremsstrahlung isochromat spectroscopy (BIS) data appear to enhance transitions to the C o* bands. The photoyield experiments show clearly the presence of C 7r* states at E F b u t indicate negligible contributions to the densities of states at E F from Li(2p) states and K(3d) states. The shift of E F into the conduction band is n o t completely rigid band-like, b u t the main features are observed ~ 1 eV closer to E F in CsK and ~ 2 eV closer in C6Li. The strong dependence of the intensity of observed peaks on the excitation polarization direction should contribute to the identification of their band origins if such effects are taken into account in band structure calculations.
1. Introduction A recent review paper [1] shows that most of the standard techniques appropriate to an investigation of the electronic properties of materials have been applied to the alkali-metal-graphite intercalation compounds. With the help of band structure calculations specially developed to deal with the difficult problem of layered materials, it seemed that a complete 0379-6779/83/$3.00
© Elsevier S e q u o i a / P r i n t e d in T h e N e t h e r l a n d s
132 understanding of the data was imminent. From a rigid band approach we would simply expect charge to be transferred from the alkali metal to the C 7r* antibonding states thus shifting the Fermi energy (EF) up into the graphite conduction band. In practice, however, even in the cases of C6Li and CsK (the most extensively studied materials), there are still difficulties in assessing the extent of hybridization between alkali-metal and graphite states. We m a y expect further information to be forthcoming from an analysis of the densities of unoccupied states. So far, electron energy-loss spectrosc o p y (EELS) [2, 3], secondary electron spectroscopy (SES) [4] and photoelectron yield spectroscopy (PYS) [5, 6] have been applied to these compounds, y e t an overall picture of the essential features of the e m p t y states is still n o t available. In this paper we compare detailed C6Li and CsK PYS data with the first Bremsstrahlung isochromat spectroscopy (BIS) measurements on these compounds.
2. Experimental procedure
2.1. Sample preparation As in earlier photoemission experiments [7, 8] the CsK samples were prepared by the two temperature m e t h o d in Pyrex vessels and C6Li was obtained b y a liquid phase reaction. Highly oriented pyrolytic graphite (HOPG) was used in both cases. The stage and homogeneity of the samples were verified b y X-ray diffraction. Transfer to the site of the experiments was ensured by placing the samples in sealed, evacuated ampoules. These were opened under an inert gas atmosphere in the measuring chambers. Some discolouration was observed during the p u m p - d o w n period but, in each experiment, fresh, uncontaminated surfaces were obtained by cleaving, once ultra-high vacuum conditions had been attained.
2.2. PYS The photoyield experiments were performed at DESY using the Flipper I X-UV m o n o c h r o m a t o r facility at the DORIS storage ring [9]. The monochromatized synchrotron radiation was 100% polarized in the plane of the synchrotron orbit on the assumption of a point-source. The sample could either be placed with the J-axis (normal to the sample surface) perpendicular to the direction of polarization e', or b y rotating the sample holder to a position a b o u t 20 ° off the ~ II c" position. The basic process consists of exciting a core electron to an e m p t y state in the conduction band. The excitation is driven by the absorption of a p h o t o n of appropriate energy. The process is restricted to transitions towards e m p t y states on the site on which the core level is excited, as the radial part of the core level wavefunction does not extend b e y o n d its own atomic site. PYS is thus a local probe. The electron transition is subject to
133
the same matrix element dependence and dipole selection rules as in an X-ray absorption experiment. The variation in the bulk absorption coefficient is measured indirectly by recording the electron yield which results from the de-excitation of the core hole [10]. Further experimental details are given in ref. 6. The spectrum may be superimposed on a fairly complex background arising from ionization processes or other excitations. The curves presented have been corrected for variations in the incident p h o t o n flux only. 2.3. BIS
BIS is essentially the inverse of a photoemission experiment. The sample is b o m b a r d e d by a beam of monoenergetic electrons. As an electron decays to an unoccupied state in the conduction band it emits an X-ray photon. By monitoring the intensity of the X-ray emission at a fixed p h o t o n energy (here, 1486.6 eV) as a function of the energy of the incident electron beam, the densities of unoccupied states are probed. The experimental set-up has been described in detail previously [11] and the procedure is identical to that adopted for pure graphite [12]. By selecting 1486.6 eV photons the BIS experiment is exactly symmetrical to an X-ray photoelectron spectroscopy (XPS) measurement using monochromatized A1 Kc~ radiation. The wavevector of the high-energy monoenergetic electrons was set almost parallel to the J-axis. The spectra are superimposed on a background which increases with energy because of electron energy losses. The raw data are presented for the intercalation compounds.
3. Results and discussion The PYS [6] and high-energy BIS [12] data for HOPG are recalled in Fig. 1. A low-energy BIS measurement for a graphite-like film on a Pt(100) surface [13] is also presented for comparison. In the PYS experiment the main feature is a sharp peak (A) 2.1 eV above EF. The intensity drops in the 3 - 7 eV region and then rises again steeply towards 10 eV. The high energy BIS spectrum on the other hand, has a shoulder at 3 eV and the intensity then rises to a prominent peak (B) at 9.7 eV. A second peak (C) is situated at 14 eV. Peak A in the PYS experiment is due to the weakly dispersive 2pz derived It* bands at the M point on the Brillouin zone face. The origin of the structure in the BIS experiment can less easily be ascertained. By analogy with XPS we would expect the o* state cross-sections to be much larger than the 7r* cross-sections. Several band structure calculations place the weakly dispersing part of the o* bands ~ 7.5 eV above E F. This agrees well with SES results [4] and the constant initial state measurements performed along with the PYS experiment (inset to Fig. 1). Other calculations tend to place these bands further up [17, 18] and, in particular, ref. 17 situates non
134
GRAPHITE B\, I
,..C ",/
',\BIS
cIs i
uv-BIS z z
/
I
:'"
i
A
"
I
~ _ _ x _ _ l m o
10
lO ENERGY ABOVE E~(eV)
Fig. 1. P h o t o y i e l d (PYS) f r o m the C(ls) e x c i t a t i o n and high-energy Bremmstrahlung i s o c h r o m a t spectroscopy (BIS) for HOPG. Also shown, low-energy BIS (uv-BIS) for a graphite film on Pt(100) taken f r o m ref. 13. Inset shows position of the non-dispersive o* band as seen in the c o n s t a n t initial state m o d e (CIS). The intensities are arbitrary.
dispersive out-of-plane o* bands at ~ 9.5 eV, in good agreement with the position of peak B. In the absence of theoretical densities of states which take into account the polarization effects, it is hazardous to attribute a definite identity to peaks B and C, b u t we tentatively assume they have mainly o character. The low energy BIS experiment gives rise to two peaks at 1.7 and 3.4 eV. The lower peak must have the same origin as A in the PYS experiment. The other peak coincides with the shoulder in the high-energy BIS curve. Very recently it has been suggested that such a structure may arise from transitions towards empty, free electron-like, interlayer states [18]. (Such states do n o t show-up in calculations which use minimal sets of basis functions.) Figure 2 shows the high-energy BIS data for C6Li and CsK compared with HOPG. Insets give the corresponding PYS data taken from ref. 6. The intensity in the 2 - 5 eV region is slightly attenuated in the Cs K BIS data and more markedly so in C6Li. The PYS data clearly show that E F has moved towards the ~* bands and that n o w lr* states contribute to the density of states at E F , as expected. No such effect is apparent in this energy region for the BIS data, which would tend to confirm that the 7r* cross-sections are, indeed, very low. The new positions of peaks A, B and C are tabulated (Table 1). The shifts relative to the position in the HOPG spectra are not rigorously rigid bandlike, i.e., E F does n o t shift towards each feature by exactly the same amount. Finally, the local densities of states for the alkali metal sites have been probed by PYS using the Li(ls) and K(2p) excitations (Fig. 3). Because of selection rules these excitations involve the e m p t y Li(2p) states and the
135
I
LiC6
o..? KC8~°°t /
} i
/-HOE
"° @
0
19
10
ENERGY ABOVE EF (eV)
Fig. 2. High energy BIS data for C6Li and CsK (raw data) and HOPG (smooth curve). Inset shows the PYS C(ls) excitations for the intercalation compounds.
TABLE 1 Positions, relative to EF, of the main features observed in PYS, BIS and SES [4] measurements on HOPG, C6Li and CsK (all values in eV)
HOPG CsK C6Li
A
AE A
D*
z~E D
B
AE B
C
AE C
2.1 0.9 0.6
1.2 1.5
7.6 6.0 5.6
1.6 2.0
9.7 8.8 7.4
0.9 2.3
14.0 12.9 12.0
1.1 2.0
*D is taken as the peak positions recorded by SES [4].
i
r
i
i
]
PYS - Lils exc ,
z
K 2p exc~
~ 3,2A
- K2pexcs.
/ \ ,'~
l 0
/
i
i
i
~/~@I
.cl .....
J
//
I
10 ENERGY A B O V E E~(eV)
Fig. 3. PYS data for alkali-metal core level excitations. The K(2p3/2 ) and (2pi/2) core levels give rise to two equivalent transitions. Inset shows effect of direction of polarization relative to c'-axis on the intensity of the C(ls) excitation and K(2p) excitation in CsK.
136
K(3d)/(4s) states. As discussed in ref. 6 the 2p -+ 3d transition is far more probable than the 2p-~ 4s transitions, so we interpret the K(2p) PYS data as indicating the presence of d states ~ 3 eV above E F. The inset to Fig. 3 shows that the intensity of the K(2p) excitation is not sensitive to the polarization direction of the incident p h o t o n excitation. The Li(ls) excitation gives rise to two features corresponding to empty Li(2p) states. The one at 7 eV coincides with peak B in the c o m p o u n d and must be fully hybridized with the graphite states. The peak at 2 eV lies a little above the C r* peak indicating that the Li(2p) states may be less hybridized with the C ~* bands. Neither the Li(2p) nor the K(3d) states contribute significantly to the density of states at E F . We conclude from the BIS data that these alkali metal states contribute little to the total density of states. However, the presence of alkali metal states with p or d s y m m e t r y indicates that extended basis sets must be included in calculations if the hybridization effects are to be understood. This has n o t been the case so far in the available band structure calculations .performed for C6Li and CsK [21]. As far as the graphite bands are concerned, the various features of the unoccupied states are certainly understood qualitatively, but it seems t h a t until polarization and transition matrix effects are included in density of states calculations it will be difficult to test the validity of calculations.
Acknowledgements We thank Professor C. Kunz and the staff of the Deutsches ElektronenSynchrotron staff for their help with the PYS measurements. It is a pleasure to acknowledge extremely stimulating conversations with M. Posternak and P. Pfluger. J.S.-M. is grateful to the Bundesminister fftr Forschung und Technologie for financial support. Thanks are also due for the financial support of the Swiss National Science Foundation. We thank Dr. A. W. Moore, Union Carbide, for providing the HOPG samples.
References 1 P. Pfluger and H.-J. Giintherodt, in J. Treusch (ed.), Festk6rperprobleme (Advances in Solid State Physics), 21 (1981) 271. 2 J. J. Ritsko, Phys. Rev. B, 25 (1982) 6452. 3 J. J. Ritsko and C. F. Brucker, Solid State Commun., 44 (1982) 889. 4 J. Krieg, P. Oelhafen and H.-J. Giintherodt, Solid State Commun., 42 (1982) 831. 5 W. Eberhardt, I. T. McGovern, E. W. Plummer and J. E. Fischer, Phys. Rev. Lett., 44 (1980) 200. 6 C. F. Hague, G. Indlekofer, U. M. Gubler, P. Oelhafen, H.-J. Giintherodt and J. Schmidt-May, Solid State Commun., in press. 7 P. Oelhafen, P. Pfluger, E. Hauser and H.-J. Giintherodt, Phys. Rev. Lett., 44 (1980) 197.
137 8 U. M. Gubler, P. Oelhafen and H.-J. Giintherodt, Solid State Commun., 44 (1982) 1621. 9 F. Gerken, J. Barth, C. Kunz and J. Schmidt-May, Nucl. Instrum. Methods, 208 (1983) 307. 10 W. Gudat and C. Kunz, Phys. Rev. Lett., 29 (1972) 169. 11 J. K. Lang and Y. Baer, Rev. Sci. Instrum., 50 (1979) 221. 12 Y. Baer, J. Electron. Spectrosc. Relat. Phenom., 24 (1981) 95. 13 V. Dose, W. Rensing and H. Scheidt, Phys. Rev. B, 26 (1982) 984. 14 G. S. Painter and D. E. Ellis, Phys. Rev. B, 1 (1970) 4747. 15 L. Samuelson and I. P. Batra, J. Phys. C, 13 (1980) 5105. 16 R. C. Tatar and S. Rabii, Phys. Rev. B, 25 (1982) 4126. 17 C. P. Mallet, J. Phys. C, 14 (1981) L213. 18 N. A. W. Holzwarth, 8. G. Louie and S. Rabii, Phys. Rev. B, 26 (1982) 5382. 19 M. Posternak, A. Baldereschi, A. J. Freeman, E. Wimmer and M. Weinert, Phys. Rev. Lett., 50 (1980) 761; P. Posternak, A. Baldereschi, A. J. Freeman and E. Wimmer, Bull. Am. Phys. Soc., 28 (1983) 347. 20 N. A. W. Holzwarth, S. Rabii and L. A. Girifalco, Phys. Rev. B, 18 (1978) 5190. 21 D. P. DiVincenzo and S. Rabii, Phys. Rev. B, 25 (1982) 4110.
131
DENSITIES OF UNOCCUPIED STATES IN HOPG, C6Li AND CsK STUDIED BY P H O T O Y I E L D AND B R E M S S T R A H L U N G ISOCHROMAT SPECTROSCOPY C. F. H A G U E
Laboratoire de Chimie Physique, Universitd P. et M. Curie, F - 75231 Paris C~dex 05 (France) G. I N D L E K O F E R , GIJNTHERODT
U.
M.
GUBLER,
V.
GEISER,
P.
OELHAFEN
and
H.-J.
Institut fiir Physik, Universita't Basel, CH-4056 Basel (Switzerland) J. S C H M I D T - M A Y a n d R. N Y H O L M
H. Institut fiir Experimentalphysik, Universitiit Hamburg, D-2500 Hamburg (F.R.G.) E. W U I L L O U D a n d Y. B A E R
lnstitut de Physique, Universit~ de Neuch~tel, CH-2000 Neuch~tel (Switzerland)
Summary The two techniques employed for studying the e m p t y conduction band states in stage-1 graphite intercalation compounds (here C6Li and CsK) provide complementary information. The photoyield experiments give the local (site decomposed) densities of unoccupied states and probe, in particular, the C ~r* band and the Li(2p) and K(3d)/(4s) states. The high energy Bremsstrahlung isochromat spectroscopy (BIS) data appear to enhance transitions to the C o* bands. The photoyield experiments show clearly the presence of C 7r* states at E F b u t indicate negligible contributions to the densities of states at E F from Li(2p) states and K(3d) states. The shift of E F into the conduction band is n o t completely rigid band-like, b u t the main features are observed ~ 1 eV closer to E F in CsK and ~ 2 eV closer in C6Li. The strong dependence of the intensity of observed peaks on the excitation polarization direction should contribute to the identification of their band origins if such effects are taken into account in band structure calculations.
1. Introduction A recent review paper [1] shows that most of the standard techniques appropriate to an investigation of the electronic properties of materials have been applied to the alkali-metal-graphite intercalation compounds. With the help of band structure calculations specially developed to deal with the difficult problem of layered materials, it seemed that a complete 0379-6779/83/$3.00
© Elsevier S e q u o i a / P r i n t e d in T h e N e t h e r l a n d s
132 understanding of the data was imminent. From a rigid band approach we would simply expect charge to be transferred from the alkali metal to the C 7r* antibonding states thus shifting the Fermi energy (EF) up into the graphite conduction band. In practice, however, even in the cases of C6Li and CsK (the most extensively studied materials), there are still difficulties in assessing the extent of hybridization between alkali-metal and graphite states. We m a y expect further information to be forthcoming from an analysis of the densities of unoccupied states. So far, electron energy-loss spectrosc o p y (EELS) [2, 3], secondary electron spectroscopy (SES) [4] and photoelectron yield spectroscopy (PYS) [5, 6] have been applied to these compounds, y e t an overall picture of the essential features of the e m p t y states is still n o t available. In this paper we compare detailed C6Li and CsK PYS data with the first Bremsstrahlung isochromat spectroscopy (BIS) measurements on these compounds.
2. Experimental procedure
2.1. Sample preparation As in earlier photoemission experiments [7, 8] the CsK samples were prepared by the two temperature m e t h o d in Pyrex vessels and C6Li was obtained b y a liquid phase reaction. Highly oriented pyrolytic graphite (HOPG) was used in both cases. The stage and homogeneity of the samples were verified b y X-ray diffraction. Transfer to the site of the experiments was ensured by placing the samples in sealed, evacuated ampoules. These were opened under an inert gas atmosphere in the measuring chambers. Some discolouration was observed during the p u m p - d o w n period but, in each experiment, fresh, uncontaminated surfaces were obtained by cleaving, once ultra-high vacuum conditions had been attained.
2.2. PYS The photoyield experiments were performed at DESY using the Flipper I X-UV m o n o c h r o m a t o r facility at the DORIS storage ring [9]. The monochromatized synchrotron radiation was 100% polarized in the plane of the synchrotron orbit on the assumption of a point-source. The sample could either be placed with the J-axis (normal to the sample surface) perpendicular to the direction of polarization e', or b y rotating the sample holder to a position a b o u t 20 ° off the ~ II c" position. The basic process consists of exciting a core electron to an e m p t y state in the conduction band. The excitation is driven by the absorption of a p h o t o n of appropriate energy. The process is restricted to transitions towards e m p t y states on the site on which the core level is excited, as the radial part of the core level wavefunction does not extend b e y o n d its own atomic site. PYS is thus a local probe. The electron transition is subject to
133
the same matrix element dependence and dipole selection rules as in an X-ray absorption experiment. The variation in the bulk absorption coefficient is measured indirectly by recording the electron yield which results from the de-excitation of the core hole [10]. Further experimental details are given in ref. 6. The spectrum may be superimposed on a fairly complex background arising from ionization processes or other excitations. The curves presented have been corrected for variations in the incident p h o t o n flux only. 2.3. BIS
BIS is essentially the inverse of a photoemission experiment. The sample is b o m b a r d e d by a beam of monoenergetic electrons. As an electron decays to an unoccupied state in the conduction band it emits an X-ray photon. By monitoring the intensity of the X-ray emission at a fixed p h o t o n energy (here, 1486.6 eV) as a function of the energy of the incident electron beam, the densities of unoccupied states are probed. The experimental set-up has been described in detail previously [11] and the procedure is identical to that adopted for pure graphite [12]. By selecting 1486.6 eV photons the BIS experiment is exactly symmetrical to an X-ray photoelectron spectroscopy (XPS) measurement using monochromatized A1 Kc~ radiation. The wavevector of the high-energy monoenergetic electrons was set almost parallel to the J-axis. The spectra are superimposed on a background which increases with energy because of electron energy losses. The raw data are presented for the intercalation compounds.
3. Results and discussion The PYS [6] and high-energy BIS [12] data for HOPG are recalled in Fig. 1. A low-energy BIS measurement for a graphite-like film on a Pt(100) surface [13] is also presented for comparison. In the PYS experiment the main feature is a sharp peak (A) 2.1 eV above EF. The intensity drops in the 3 - 7 eV region and then rises again steeply towards 10 eV. The high energy BIS spectrum on the other hand, has a shoulder at 3 eV and the intensity then rises to a prominent peak (B) at 9.7 eV. A second peak (C) is situated at 14 eV. Peak A in the PYS experiment is due to the weakly dispersive 2pz derived It* bands at the M point on the Brillouin zone face. The origin of the structure in the BIS experiment can less easily be ascertained. By analogy with XPS we would expect the o* state cross-sections to be much larger than the 7r* cross-sections. Several band structure calculations place the weakly dispersing part of the o* bands ~ 7.5 eV above E F. This agrees well with SES results [4] and the constant initial state measurements performed along with the PYS experiment (inset to Fig. 1). Other calculations tend to place these bands further up [17, 18] and, in particular, ref. 17 situates non
134
GRAPHITE B\, I
,..C ",/
',\BIS
cIs i
uv-BIS z z
/
I
:'"
i
A
"
I
~ _ _ x _ _ l m o
10
lO ENERGY ABOVE E~(eV)
Fig. 1. P h o t o y i e l d (PYS) f r o m the C(ls) e x c i t a t i o n and high-energy Bremmstrahlung i s o c h r o m a t spectroscopy (BIS) for HOPG. Also shown, low-energy BIS (uv-BIS) for a graphite film on Pt(100) taken f r o m ref. 13. Inset shows position of the non-dispersive o* band as seen in the c o n s t a n t initial state m o d e (CIS). The intensities are arbitrary.
dispersive out-of-plane o* bands at ~ 9.5 eV, in good agreement with the position of peak B. In the absence of theoretical densities of states which take into account the polarization effects, it is hazardous to attribute a definite identity to peaks B and C, b u t we tentatively assume they have mainly o character. The low energy BIS experiment gives rise to two peaks at 1.7 and 3.4 eV. The lower peak must have the same origin as A in the PYS experiment. The other peak coincides with the shoulder in the high-energy BIS curve. Very recently it has been suggested that such a structure may arise from transitions towards empty, free electron-like, interlayer states [18]. (Such states do n o t show-up in calculations which use minimal sets of basis functions.) Figure 2 shows the high-energy BIS data for C6Li and CsK compared with HOPG. Insets give the corresponding PYS data taken from ref. 6. The intensity in the 2 - 5 eV region is slightly attenuated in the Cs K BIS data and more markedly so in C6Li. The PYS data clearly show that E F has moved towards the ~* bands and that n o w lr* states contribute to the density of states at E F , as expected. No such effect is apparent in this energy region for the BIS data, which would tend to confirm that the 7r* cross-sections are, indeed, very low. The new positions of peaks A, B and C are tabulated (Table 1). The shifts relative to the position in the HOPG spectra are not rigorously rigid bandlike, i.e., E F does n o t shift towards each feature by exactly the same amount. Finally, the local densities of states for the alkali metal sites have been probed by PYS using the Li(ls) and K(2p) excitations (Fig. 3). Because of selection rules these excitations involve the e m p t y Li(2p) states and the
135
I
LiC6
o..? KC8~°°t /
} i
/-HOE
"° @
0
19
10
ENERGY ABOVE EF (eV)
Fig. 2. High energy BIS data for C6Li and CsK (raw data) and HOPG (smooth curve). Inset shows the PYS C(ls) excitations for the intercalation compounds.
TABLE 1 Positions, relative to EF, of the main features observed in PYS, BIS and SES [4] measurements on HOPG, C6Li and CsK (all values in eV)
HOPG CsK C6Li
A
AE A
D*
z~E D
B
AE B
C
AE C
2.1 0.9 0.6
1.2 1.5
7.6 6.0 5.6
1.6 2.0
9.7 8.8 7.4
0.9 2.3
14.0 12.9 12.0
1.1 2.0
*D is taken as the peak positions recorded by SES [4].
i
r
i
i
]
PYS - Lils exc ,
z
K 2p exc~
~ 3,2A
- K2pexcs.
/ \ ,'~
l 0
/
i
i
i
~/~@I
.cl .....
J
//
I
10 ENERGY A B O V E E~(eV)
Fig. 3. PYS data for alkali-metal core level excitations. The K(2p3/2 ) and (2pi/2) core levels give rise to two equivalent transitions. Inset shows effect of direction of polarization relative to c'-axis on the intensity of the C(ls) excitation and K(2p) excitation in CsK.
136
K(3d)/(4s) states. As discussed in ref. 6 the 2p -+ 3d transition is far more probable than the 2p-~ 4s transitions, so we interpret the K(2p) PYS data as indicating the presence of d states ~ 3 eV above E F. The inset to Fig. 3 shows that the intensity of the K(2p) excitation is not sensitive to the polarization direction of the incident p h o t o n excitation. The Li(ls) excitation gives rise to two features corresponding to empty Li(2p) states. The one at 7 eV coincides with peak B in the c o m p o u n d and must be fully hybridized with the graphite states. The peak at 2 eV lies a little above the C r* peak indicating that the Li(2p) states may be less hybridized with the C ~* bands. Neither the Li(2p) nor the K(3d) states contribute significantly to the density of states at E F . We conclude from the BIS data that these alkali metal states contribute little to the total density of states. However, the presence of alkali metal states with p or d s y m m e t r y indicates that extended basis sets must be included in calculations if the hybridization effects are to be understood. This has n o t been the case so far in the available band structure calculations .performed for C6Li and CsK [21]. As far as the graphite bands are concerned, the various features of the unoccupied states are certainly understood qualitatively, but it seems t h a t until polarization and transition matrix effects are included in density of states calculations it will be difficult to test the validity of calculations.
Acknowledgements We thank Professor C. Kunz and the staff of the Deutsches ElektronenSynchrotron staff for their help with the PYS measurements. It is a pleasure to acknowledge extremely stimulating conversations with M. Posternak and P. Pfluger. J.S.-M. is grateful to the Bundesminister fftr Forschung und Technologie for financial support. Thanks are also due for the financial support of the Swiss National Science Foundation. We thank Dr. A. W. Moore, Union Carbide, for providing the HOPG samples.
References 1 P. Pfluger and H.-J. Giintherodt, in J. Treusch (ed.), Festk6rperprobleme (Advances in Solid State Physics), 21 (1981) 271. 2 J. J. Ritsko, Phys. Rev. B, 25 (1982) 6452. 3 J. J. Ritsko and C. F. Brucker, Solid State Commun., 44 (1982) 889. 4 J. Krieg, P. Oelhafen and H.-J. Giintherodt, Solid State Commun., 42 (1982) 831. 5 W. Eberhardt, I. T. McGovern, E. W. Plummer and J. E. Fischer, Phys. Rev. Lett., 44 (1980) 200. 6 C. F. Hague, G. Indlekofer, U. M. Gubler, P. Oelhafen, H.-J. Giintherodt and J. Schmidt-May, Solid State Commun., in press. 7 P. Oelhafen, P. Pfluger, E. Hauser and H.-J. Giintherodt, Phys. Rev. Lett., 44 (1980) 197.
137 8 U. M. Gubler, P. Oelhafen and H.-J. Giintherodt, Solid State Commun., 44 (1982) 1621. 9 F. Gerken, J. Barth, C. Kunz and J. Schmidt-May, Nucl. Instrum. Methods, 208 (1983) 307. 10 W. Gudat and C. Kunz, Phys. Rev. Lett., 29 (1972) 169. 11 J. K. Lang and Y. Baer, Rev. Sci. Instrum., 50 (1979) 221. 12 Y. Baer, J. Electron. Spectrosc. Relat. Phenom., 24 (1981) 95. 13 V. Dose, W. Rensing and H. Scheidt, Phys. Rev. B, 26 (1982) 984. 14 G. S. Painter and D. E. Ellis, Phys. Rev. B, 1 (1970) 4747. 15 L. Samuelson and I. P. Batra, J. Phys. C, 13 (1980) 5105. 16 R. C. Tatar and S. Rabii, Phys. Rev. B, 25 (1982) 4126. 17 C. P. Mallet, J. Phys. C, 14 (1981) L213. 18 N. A. W. Holzwarth, 8. G. Louie and S. Rabii, Phys. Rev. B, 26 (1982) 5382. 19 M. Posternak, A. Baldereschi, A. J. Freeman, E. Wimmer and M. Weinert, Phys. Rev. Lett., 50 (1980) 761; P. Posternak, A. Baldereschi, A. J. Freeman and E. Wimmer, Bull. Am. Phys. Soc., 28 (1983) 347. 20 N. A. W. Holzwarth, S. Rabii and L. A. Girifalco, Phys. Rev. B, 18 (1978) 5190. 21 D. P. DiVincenzo and S. Rabii, Phys. Rev. B, 25 (1982) 4110.