X-ray photoelectron spectroscopy study of Sr and Ba compounds

Journal of Electron Spectroscopy and Related Phenomena, 56 (1991) 217-240

217

Elsevier Science Publishers B.V., Amsterdam

X-RAY PHOTOELECTRON Ba COMPOUNDS

SPECTROSCOPY

STUDY

OF Sr AND

R.P. VASQUEZ Center for Space Microelectronics %dmology, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 (USA)

(First received 17 September 1990; in final form 10 December 1990)

ABSTRACT X-ray photoelectron spectroscopy (XPS ) measurements of core level binding energies and kinetic energies of X-ray-induced Auger transitions for a comprehensive set of 25 Sr and Ba compounds are reported, including many for which there have been no previous XPS measurements. The majority of the Sr and Ba compounds measured here have cation core level binding energies which are lower than that of the metal, with the negative shifts largest for high temperature superconductors. These data are interpreted within a point charge model, which has previously been used to interpret XPS data from alkali halides and other ionic compounds. Data reported in the literature reveal that, rather than being anomalous, small or negative binding energy shifts are common to many ionic compounds, It is suggested that the large negative shifts observed for the high temperature superconductors are a conseyuence of large Madelung energies at the cation sites, and may be a measure of the hole distribution on the Cu-0 planes. Measurements of the energy loss regions from the core levels of the Sr compounds are also reported. A peak which has previously been assigned as a configuration interaction satellite of the Sr4s peak is shown to be a plasmon loss from the Sr4p peak.

INTRODUCTION

X-ray photoelectron spectroscopy (XPS) measurements of the oxidation of Ba metal reveal that a shift in Ba core level binding energies to values lower than that of the metal occurs [ 1-4 3. A systematic trend in the oxidation of alkaline earth metals is observed [ 21, with the heavier alkaline earths exhibiting progressively smaller binding energy shifts with oxidation, becoming negative for Ba. This unusual negative shift has been explained as resulting from increased occupation of Ba 5d orbitals, which are more compact than the 6s orbitals and hence provide more efficient screening, in Ba compounds relative to the metal, in either the initial state or the final state 151. The trend observed in the oxidation of alkaline earth metals is explained within this model by the progressively lower energy of the screening sd states for the heavier alkaline earths.

218

The above interpretation has been disputed, and an alternative explanation has been proposed based on the final state screening by 0 2p-derived states of the polarizable 02- anion being larger than metallic screening [4 ] . Within this model, the enhanced buildup of 02- states for the heavier alkaline earths explains the systematic trend previously noted in the oxidation of the metals. An earlier work also attributed the negative binding energy shift of BaO to final state relaxation effects [ 31. Most recently, it has been proposed [6] that the negative binding energy shifts are due to the position of the Fermi level in the oxide band gap. Apparent chemical shifts of + 0.5 eV to - 1.6 eV were observed for various Ba metal oxidation and oxide deposition conditions [ 6 ] _ A final model to be considered is the simple electrostatic point charge model [ 71, which has successfully been applied to ionic compounds such as the alkali halides [S] . This model has been found to work well for oxides with cations having low formal oxidation states and closed shell electronic configurations [ 91. Both of these conditions are satisfied for the compounds studied here. Of particular significance for this work, the point charge model has previously been used to explain negative core level binding energy shifts in Cu, Zn, Ag and Cd compounds [lo]. While this model has only been sparingly used in attempts to explain binding energy shifts in alkaline earth compounds, its qualitative success in predicting core level binding energies in other ionic compounds makes a more serious consideration worthwhile. In particular, this model will indicate whether the special screening conditions proposed in some models are necessary to explain the observed alkaline earth core level shifts. In addition to fundamental interest in the origin of the negative shifts, the solution of this problem may be of considerable practical importance in view of the fact that the reported alkaline earth core level binding energies of the high temperature superconductors [ ll-181are significantly lower than in other alkaline earth compounds. In this work, the core level binding energies of 25 Sr and Ba compounds are reported, including many which have not been previously measured. The majority of the Sr and Ba compounds measured here have core level binding energies which are lower than that of the metal. Where possible, these measurements are compared to previous results [l-4,11-27]. These measurements provide a comprehensive data set with which to compare predictions of the models. The present measurements are also compared with measurements on Ca and CaO reported here, and with previous works on alkali salts r&24,28,29] and the oxidation of alkali metals [4,30-321, to identify overall trends. It will be shown that the negative cation core level binding energy shifts observed for BaO are not anomalous, but are common to many ionic compounds. From the measured Ba core level binding energies and Auger kinetic energies, the extra-atomic relaxation energy is estimated and is found to increase linearly with increasing anion polarizability for atomic anions. However, an extra-atomic relaxation energy which is larger than that of the metal

219

is not necessary to account for the observed negative binding energy shifts. No correlation is found between the estimated extra-atomic relaxation energies and measured core level binding energies. It is suggested that the large negative shifts observed for the alkaline earth core levels of high temperature superconductors are a consequence of large Madelung energies at the cation sites, and may be a measure of the hole distribution on the superconducting Cu-0 planes. Measurements of the energy loss regions from the core levels of the Sr compounds are also reported. A high intensity satellite peak in the Sr 4s region [ 33,34 1, which has previously been assigned as a configuration interaction satellite [ 331, is shown to be a plasmon loss from the Sr4p peak. EXPERIMENTAL

The XPS spectra are accumulated on a Surface Science Instruments SSX501 spectrometer with monochromatized Al Ka! X-rays (1486.6 eV). For the core level measurements, the electron energy analyzer is set such that a peak fill width at half maximum (FWHM) of 0.85 eV is measured for the AuLif,,, peak from an Au film evaporated onto Si (loo), and for the measurements of the Auger and energy loss regions the analyzer is set such that the Au4fTiz FWHM is 1.5 eV. When necessary, sample charging is neutralized with a low energy electron flood gun in conjunction with a 90% transmitting fine mesh proximity screen, and the spectra are referenced to the Cls line at 284.6 eV. The flood gun energy and current are set to minimize photoelectron peak linewidth and asymmetry. XPS spectra of Ca, Sr and Ba @e obtained from bulk metal samples which have been freshly ion etched. The metal surfaces are found to oxidize to a detectable degree within a few hours in the vacuum chamber (base pressure 4 x 10-1o Torr ) . Mass spectra of the background gases show little or no detectable 02, but HZ0 is a significant background gas. The metal surfaces were therefore ion etched every few minutes to minimize the presence of oxide species in the photoemission spectra of the metals. XPS spectra of reference compounds are accumulated on high purity powders, typically obtained packed in argon, pressed onto In metal. The sample containers are opened in the ultrahigh purity Nz atmosphere of a glove box which encloses the sample introduction area of the XPS spectrometer, where all sample preparation is also done. The Ba ( ClO, ) 2 reduces to Ba ( ClO, ) 2 and BaCl, over a period of several hours. A C12p spectrum is measured at the end of data accumulation to verify that reduction is minimal. Similar precaution is taken with Ba(C103),*H20. Reduction of the Ba and Sr nitrates to the nitrites is also observed, and is monitored by measuring the Nls spectrum at the end of data accumulation. The SrTiO, ( 100 ) is a polished single crystal which has been exposed to air and, other than degreasing, is measured as received from the supplier.

220 RESULTS AND DISCUSSION

Core level and Auger measurements Alkaline earth compounds are known to react readily with air to form hydroxides and carbonates. Surface contamination is therefore a serious problem to be considered. In this work, a Cls peak corresponding to carbonates is not detected on any of the Ba or Sr compounds other than SrTiO,, which was air exposed, and the bulk carbonates themselves. However, in spite of the precautions to avoid air exposure, a small 01s signal is detected from compounds which nominally contain no 0 and can be seen in the wide scan spectra from the Sr halides shown in Fig. 1. Ion etching reduces, but does not eliminate, the

Sr 3d 1

-LA L

SrF2

I

SrEir2

1100 BINDING

ENERGY(eV)

Fig. 1. Wide scan spectra from the Sr halides, showing low levels of 0 contamination. The CIs peak is not detectable on this scale.

221

01s signal, so a low level contaminant phase appears to be present within the materials. Nevertheless, it is clear that contaminant phases are minor components and the spectra are representative of the materials being studied. Some oxygen containing materials also show evidence of some contamination, exhibited as asymmetric or multiple 01s peaks, as shown in Fig. 2. Even in these cases the contaminant phases are minor and are not evident in the Sr3d spectra, shown in Fig. 3. The Sr3d spectra from SrTiO, and SrO, shown in Figs. 3 (b) and 3 (c) respectively, can each be least-squares fit with a single doublet with a symmetric line shape in spite of the evidence for contamination in the 01s spectra. The lack of evidence for contaminants in the Sr3d signals may result from less surface sensitivity (higher photoelectron kinetic energy) relative to the 01s signals, or from contaminant phases having Sr3d binding energies which are fortuitously close to that of the compound being studied [ 211.A symmetric 01s peak is obtained for SrO by ion etching, but the binding energy is the same as that determined without ion etching. In this work, differential sample charging is found to have a larger effect on photoelectron line shapes than contamination. This is exhibited as asymmetry on the low binding energy side of all core levels, and is most evident in the Sr3d spectra of Figs. 3 (d) and 3 (k) from SrS and SrClz respectively. This problem could usually be eliminated or minimized by adjustment of the electron flood gun energy and filament current. Even for those compounds which still exhibited asymmetric core levels, the flood gun affects only the line shape and not the determined binding energy. The core level binding energies measured from Sr compounds in this work

539

536

533 BlNDlNG

530 ENERGY

527

524

(eV)

Fig. 2.01s spectra from (a) single crystal SrTiOB (100) and (b) SrO powder.

I

r

I

I

A

SrF2

.2

,/+(\

/

139

Sr 3d

136 BINDING

133

(0)

130

12

ENERGY &A’)

Fig. 3. Sr3d core level spectra measured from (a) a J3i,Sr,Ca,Cu,O,,+, polycrystiline film etched in a Br/ethanol solution (from ref. 11) , (b ) single crystal SrTiOB ( loo), (c) SrO powder, (d) SrS

powder, (e) SrC03powder, (f) Sr(OH)p-8H20 powder, (g) SrF, powder, (h) SrSO,powder, (i) Sr(N03)2 powder, (j) bulk Sr metal, (k) SrClz powder, (1) SrBr, powder and (m) Sri, powder.

are summarized together with data from the literature in Table 1. To the author’s knowledge, Sr core level binding energies have not previously been reported for SrCl,, SrBr,, S& or Sr (OH)2=8Hz0. Table 2 summarizes the core level binding energies and Auger kinetic energies measured from Ba compounds, primarily from measurements previously made in this laboratory [ 17,271. To the author’s knowledge, measurements on Ba( C104) 2 and Ba ( C103),*Hz0 have not previously been reported. For those Ba compounds which have been previously measured, comparison with data from the literature can be found elsewhere [ 17,271. Table 2 also includes new measurements

223 TABLE

1

Summary of the measured core level binding energies in electronvolts for Sr metal and Sr compounds. Numbers in parentheses are the measured full widths at half-maximum Compound

Sr3ds,z!

Sr

(1.2)

SrCl, SrBr, Sri,

134.0 134.4 134.2 134.25” 132.6 135.3 133.7” 132.2 132.8 132.7 132.2 133.0 132.7 133.5” 133.35” 133.85 134.2 134.0 133.9 134.5 133.85 133.55 134.5 134.5 134.8

SrTiOs (100)

132.5

(0.95)

SrHz sro

Sr (OH),-BH,O srco,

srs S&e SrSO, Sr(NO&

SrFz

01s (1.5)

(2.3) (1.6) (Sr3d) (2.1)

530.0 (1.5) 530.5 530.7 530.6 (3.2) 530.9 (1.9) 531.1 531.3 -

(1.7) (Sr3d) ( 1.6) (Sr3d) (1.65 )

(1.8) (1.75) (1.8)

531.6 (2.0) 532.1 532.8 (2.0) 532.9 533.6 -

Other 269.55 270.0 270.0 269.8 -

(SAp,,,) (Sr3p3,2) (Sr3p,,,) (Sr3p,,,)

269.25 -

(Sr3p,,,)

132.7 132.9 133.15 132.8

(I

M4N4.bN4.d

529.0(1.1)457.9 (1.0, Ti2p3i2) 529.5 528.6 529.2 528.8 528.7 529.0

This work

PI [I91 WI

This work

288.8 (1.9, Cls) 289.1 (Cls) 289.3 (Cls) 160.0 (2.0, S~P~,~) 160.75 @2p,,,) 159.1 (Se3p,,z) 168.55 (1.7, S2p,,,) 168.9 (S2p) 407.1 (1.7, Nls) 406.9 (Nls) 407.9 (Nls) 684.6 (1.8, Fls) 684.8 (Fls) 198.9 (1.6, CI~P~,~) 68.9 (1.8, Br3&,,) 619.4 (2.0,13d,,,) 516.6

133.9 131.9, 131.7, 132.2 131.8, 132.2 131.7,

Source

458.6 458.4 158.2 158.0 158.5

(Ti2p,,,) (TiSp,& (Bi4f,,,) (Bi4f,,*) (Bilf,,,)

158.5 158.4

(Bi4f,,,) (Bi4f,,,)

[21 PO1 WI

This work This work

WI WI

This work [20,23 1 [26,23 3 This work

1221

This work

[211 WI

This work

[241

This This This This This

work work work work work

[251 [26jb [ill

iI21

1131 1141 iI51 1161

“Deduced from the reported Sr3pa,z binding energy. bSemiconducting phase, obtained by heating the crystal in vacuum.

on BaO powder packed in argon, which differ slightly from previously reported results [ 17 1. Where appropriate, the data from the literature in Tables 1 and 2 have been restated so that the binding energies are referenced to Cls at 284.6 eV or Au4f,,, at 64.0 eV.

224 TABLE 2 Summary of the measured core level binding energies and Ba MJV,,alv,,s kinetic energies in electronvolts for Ba and Ba compounds. Numbers in parenthew are the measured full widths at half maximum Compound

Ba3&,,

Ba KGJr~.a

Ba BaHz BaO

( 1.7)

596.5 -

(2*0) (1.9) (2.6) (2.0) (2.3) (2.0)

598.0

BaIrr BaCl, BaBq2H20 BaIem2HsO

789.4 782.9 779.2 779.2 779.3 779.1 779.8 780.1 786.5 780.7 789.0 779.8 789.2 786.2 789.4

(2.0) (2.1) (2.4) (1.7) (1.6) (1.5)

595.5 596.5 596.0 596.7 597.0 597.5

YBa&u,O,_, T12BazCa&ua010

777.7 (1.6) 778.3

606.0 -

Ba(OH)s BaCOa BaS BaSO, Ba(NG)z

BaW104)2 Ba(ClO&-HrO

597.4 597.3

Other

01s

90.0 (1.35, Bati& 530.0 (2.0) 530.3 (2.4) 530.8 (2.4)

599.4

597.1 596.3

Source

531.5 532.7 532.8 532.5 531.7

(2.3) (2.0) (2.0) (2.0)

528.3 (1.8) 529

289.0 169.8 168.3 407.1 407.3 208.0 205.7 683.7 198.2 68.2 618.9 518.0 933.3

1171 ml This work

(2.7, Cls) (1.6, S2p,,,) (1.8, S2p,,,) (1.6, Nls) (Nls) (1.5, C12p,,,) (1.8, C12p,,,) (2.2, Fls) (1.3, C12p,,*) (1.2, Br3&,*) (1.4, I3&,,) (I M&&%s ) (Cu2p&

[I71 [I71 1271 1271 This work

P21 This work This work

r171 [I71 1171 [I71 1171 t171 IIf31

It is apparent from Tables 1 and 2 that most Sr and Ba compounds have alkaline earth core level binding energies lower than the corresponding metal. This is evident in measurements made in this laboratory and elsewhere for most of the compounds. However, the values for both alkaline earth and 01s core levels reported for BaO (measurements tabulated in [ 171) and SrO (Table 1) have greater variance and must be considered controversial. Note that the high temperature superconductors have Sr or Ba core level binding energies which are significantly lower than other Sr or Ba compounds. Possible charging-related energy referencing problems for the superconductors, which are normal state metals, can be ruled out, while the core level binding energies of other atoms in the insulating materials agree well with values reported by other researchers on similar compounds and exhibit “normal” positive chemical shifts. For example, the Cls carbonate peak at w 289 eV is within the range normally observed for carbonates [ 17,241. Furthermore, these measurements have high sample-to-sample reproducibility, usually to + 0.1 eV. Experimental uncertainties related to charging therefore appear to be minimal. Core level chemical shifts in XPS are often interpreted as a measure of the charge on an atom. Initial state charge transfer to the anions in a compound increases the electrostatic potential at the cation site, and thus should increase the cation core level binding energies. Final state relaxation, which is larger in

225

the metal owing to the more efficient metallic screening compared to the polarization screening in compounds, is an additional contribution to a positive chemical shift. These considerations prompted proposals [ 4,5] for special screening conditions in BaO to account for its observed negative binding energy shift. These two models will now be considered. Effects of initial or find state d orbital occupancy

Increased initial stats d orbital occupancy relative to the metal is supported by both theoretical [ 35 ] and experimental [ 36 ] results for one material, BaC,. However, band structure calculations for alkaline earth fluorides [ 37-39) and Sr chalcogenides [ 40,411 indicate that the filled states at the top of the valence band are primarily derived from the anion p orbitals, and the cation s and d orbit& form empty conduction band states. These results are consistent with photoemission measurements [ 41 which show no occupied d states for oxidized Ba. Increased initial state d orbital occupancy has also been proposed [42] to explain the Ba core level binding energies in high temperature superconductors, which show the largest negative binding energy shifts. However, band structure calculations (e.g. 143] ) and inverse photoemission measurements (e.g. [44] ) agree that the alkaline earth d states are empty in the initial state in high temperature superconductors. Finally, the 5d states in Ba metal do appear to be partially occupied in the initial state [4,45-481. Increased initial state occupancy of d orbitals in alkaline earth compounds, and in particular in high temperature superconductors, compared to the metal, can therefore in most cases be ruled out. In fact, the opposite case seems to be more common, i.e. the d orbitals are occupied in the initial state of the metals, but not in compounds. Final state screening by d electrons may contribute to the observed negative binding energy shifts. In the simplest approximation, the screening electron can be considered to have the character of the states at the bottom of the conduction band, although using initial state energy levels to estimate final state occupancies is not necessarily valid. Band structure calculations for the rocksalt structure Sr chalcogenides [40,41] show that the conduction band minimum is at the X point and is primarily derived from Sr4d states. Cation core holes in these compounds can therefore be considered to be screened by d electrons, consistent with the commonly used equivalent core (or 2 + 1) approximation. Band structure calculations for the fluorite structure alkaline earth fluorides [ 37-391 show that the conduction band minimum is at the r point and the states are derived primarily from cation s states. Although cation core holes in these compounds may be screened by s rather than d electrons, negative binding energy shifts are still observed (see Tables 1 and 2 ) . Regardless of the screening state in the halides, the fact that d electrons also screen the core holes in the alkaline earth metals [4,45-48] makes it unlikely that d

electron screening is the primary factor responsible for the observed shifts. Although the Sr and Ba chalcogenides do have significantly lower core level binding energies than the halides (see Tables 1 and Z), consistent with the possible differences in final state screening, these differences do not originate from differences in extra-atomic relaxation energies, as shown later. A final argument against the observed negative binding energy shifts and trends in metal oxidation being a result of final state screening by d electrons is that similar observations are reported for other ionic compounds and for the oxidation of alkali metals, and final state screening by d electrons is not expected for many of these cases. The oxidation of Na is accompanied by a + 0.7 eV shift in the Nals binding energy [30], the shift in the K2p,,, line is < 0.2 eV when K is oxidized [ 311, and small negative binding energy shifts are observed when Cs is oxidized [4,32]. This is the same trend as that observed in the oxidation of the alkaline earth metals. In addition, the Nals binding energies reported [24,28] for Na compounds are close to, or slightly less than, that reported for Na metal [ 191, the K2p,,, binding energies reported [ 24,281 for K compounds are w l-2 eV lower than that reported for K metal [ 311, and the Cs3d,,, binding energies reported [24-281 for Cs compounds are z 2 eV lower than that reported for Cs metal [49]. While d-state screening is a possibility for K and Cs [ 29 3, it can certainly be ruled out for Na. Negative binding energy shifts are also often observed in rare earth compounds [ 24 ], but transition metal compounds with unfilled d shells exhibit “normal” positive chemical shifts, for reasons discussed elsewhere [lo]. However, small or negative binding energy shifts are observed for Cu+ compounds [ 10,17,24,27], as well as for Zn2+, Ag+ and Cd2+ compounds [ 10,241, all of which have filled d shells. Final state screening by additional d electrons in these compounds can therefore have no role in the observed negative binding energy shifts. Rather than being anomalous, the negative binding energy shift observed for BaO can thus be seen to be consistent with observations from many other ionic compounds. Polarization screening of the final state

Polarization screening as a mechanism responsible for the negative binding energy shifts can be tested using the data in Table 2. The difference in Auger parameters (the sum of the core level binding energy and the Auger kinetic energy) between two materials is, to a good approximation, twice the difference in extra-atomic relaxation energy if (i) the extra-atomic relaxation energies are the same for the core levels involved in the Auger transition, and (ii) if the intra-atomic relaxation energies are the same for the atom of interest in the different chemical environments of the two materials [50]. Equal extraatomic relaxation energies for the core levels is verified by the constant 690.4 eV difference in energy between the Ba3dSi2 and 4d5,, levels, which are involved in the Ba MNN Auger transition, observed in this work. The intra-

227

atomic relaxation energies of the alkaline earth metals and the ions differ sig.nificantly 151J, but are likely to be the same in the compounds, which can be considered ideally ionic with closed shell electronic configurations, as discussed later. The simple relationship noted above between Auger parameters and extra-atomic relaxation energies should therefore be valid when comparisons are made among the dation core levels of the ionic compounds. Since the extra-atomic relaxation in ionic compounds is due to polarization of neighboring ions, the cation core level binding energies should decrease with increasing anion polarizability. Tabulations of ionic polarizabilities, calculated from optical data, are available in the literature [ 52,531. Polarizabilities of the ClO, , Cl03 and OH- ions are estimated in this work from the ClausiusMosotti relation [ 52 1, using available optical data for alkali and alkaline earth compounds [ 54], and molecular volumes estimated from the densities [54] _ Anisotropic materials are treated as isotropic with an index of refraction which is the average of the actual indices of refraction. The estimated polarizabilities are 1.73 A3 for OH- (from data for Ca(OH),), 4.25 A3 for ClO, (from data for Sr (ClO,),) and 4.82 A3 for ClO, (from data for CsClO,). Figure 4 shows the extra-atomic relaxation energies (dE,) estimated from the Auger parameters ((x,), relative to BaF2, plotted as a function of the anion polarizabilities ( ap) estimated here and from the literature [ 53 1. For the atomic anions, the linear relationship LlE, =

(1)

0.19ap

0.01+

is obtained from a least-squares fit, and is also shown in Fig. 4. The increase in extra-atomic relaxation energy with increasing anion polarizability is consistent with expectations. For the polyatomic anions the extra-atomic relaxation energy seems independent of polarizability. The nearly constant extra-

0 ATOMIC ANIONS 0 POLYATOMIC ANIONS

L

0

”

1

u

1

’

2 ANION

1

3

”

0

4

1 5

B

1 6

a

I 7

POLARIZABILITY

Fig. 4. Estimated extra-atomic relaxation energies as a function of the anion polarizabilities from ref. 53 and estimated in this work. The line is a least-squares fit to the atomic anion data only.

TABLE 3 Summary of the measured Auger parameters (a,) in electronvolts, anion polarizabilities carp1 in A3 and estimated extra-atomic reIaxation energies (d& 1 in electronvolts, relative to BaF2, for Ba compounds Compound

Bfl*

BaO BaCl, BaBr, BaS BaI, Ba(OHh Ba(N03)2 BaCO$ Ba(ClO& Ba (C10412 BaS04 YBa2Cu307’

aA"

eb

PER

1391.1 1392.5 1392.2 1392.5 1393.8 1393.2 1391.9 1391.7 1391.9 1391.8 1391.5 1392.2 1393.0

0.87 2.23 3.06 4.28 5.57 6.52 1.73 3.93 4.15 4.25 4.82 4.98

0 (reference) 0*7 0.55 0.7 1.35 1.05 0.4 0.3 0.4 0.35 0.2 0.55 0.95

“Ba3d,,* binding energy f M4ZV4JV4, kinetic energy. bFrom ref. 53 except ClO, , ClO, , and OH-, which are estimated here. “Not included in Fig. 2.

atomic relaxation energy in these compounds suggests that the nearest neighbor atoms (0 in all cases here), rather than the anion as a whole, are dominant in the polarization screening of the cation core hole. These data are summarized in Table 3. Since the extra-atomic relaxation energy lowers the binding energy, the ordering of Ba and Sr core level binding energies should be in the opposite order of the anion polarizabilities. For Ba and Sr halides, the reverse of this expected ordering is observed, and the chalcogenides have significantly lower cation core level binding energies. Furthermore, BaI, and YBa&u307, which have estimated extra-atomic relaxation energies which differ by only 0.1 eV, have Ba core level binding energies which differ by 2.7 eV. There appears to be no correlation between the Ba core level binding energies and the anion polarizabilities or estimated extra-atomic relaxation energies, and other factors must therefore play a significant role in determining the binding energies. The point charge model Within the electrostatic point charge model, the Fermi level-referenced core level binding energy Ez is given by

229

where EFreeis the binding energy for the free atom (for the metal) or free ion (for the compounds), E Mad is the Madelung energy and @w is the work function. The extra-atomic relaxation energy is thus only one of several factors which affect EB. It was previously found [lo] that a combination of these factors accounted for the negative binding energy shifts observed in Group Ib and IIb compounds. The individua1 contributions to the binding energy will now be considered. Measurements of the free atom binding energies and semi-empirical estimates of the free ion binding energies for the alkaline earth elements have been made [ 551. The Madelung energy at the cation sites is given by E Mad =

(3)

@Made2/R

where e is the electron charge and @M, is related to the Madelung constant and has been calculated [B] to be - 1.75q for the rocksalt structure (alkaline earth chalcogenides) and - 1.64~~for the fluorite structure (alkaline earth fluorides and chlorides), where q is the cation charge. It can be seen in Table 4 that the values of EhiLad are close to, or larger in magnitude than, the ionization energies, and will thus contribute to small or negative binding energy shifts, as previously noted for the alkali halides [B]. The values of Etid in Table 4 TABLE

4

Estimation

of E3a3A12 and Sr3d5i2 binding energies for selected halides and chalcogenides

Compound

Free atom’

Ba BaO BaS

788.1

BaE, BaClz Sr SrO SrS SrFz &Cl,

Free ionb

ENIadc

803.2 803.2 803.2 803.2

-

18.2 15.8 17.6 14.9

158.1 158.1 158.1 158.1

-

19.5 16.7 18.8 15.6

141.9 -

“Experimental values from ref. 55. bSemiempirical estimates from ref. CCalculated in this work. dR.elaxation energies for the metals and the work function, relaxation described in the text. “Metal work functions are from ref. 4. ‘[l?]. e[27]. hMeasured in this work.

-ERd

-qiS,”

-5.0 -2.8 -3-5 -2.1 -2.7 -5.31 -3.0 -3.7 -2.3 -2.9

-2.7 - 5.1 -4.3 -6.9 -5.7 - 2.59 -5.4 -4.5 - 7.2 -66.0

EB.WIC

777.1 779.6 776.6 779.9 130.2 133.2 129.8 133.6

E B.Wp 780.4’ 779.2’ 779.19 779.8’ 780.2’ 134.0h 132.6h 132.7h 133.85h 134.5h

55. are assumed to be the difference between the atom-solid shift energies for the alkaline earth compounds are estimated as 60; work functions for the compounds are calculated from sqn.

230

were calculated assuming that the compounds are ideally ionic, and should thus be considered an upper limit. A significant,covalent character in the bonds would cause a decrease in the magnitude of EMad.However, for the Sr and Ba halides and chalcogenides the ionicities have been calculated to be x92-99% [56,57 3, and it has been found experimentally that metal oxides in which the cations have low formal oxidation states and closed shell electronic configurations are well-described as ionic within a Madelung model [ 9 J. The assumption that the alkaline earth compounds studied in this work are ideally ionic is therefore unlikely to introduce large errors to the estimates of EMa+ Note from Table 4 that the Madelung energy differences between the fluorides and chlorides lowers the estimated cation core level binding energy of the fluoride relative to the chloride. This ordering is observed for the Sr and Ba halides (see Tables 1 and 2), and often for the alkali halides 124,283, Since this ordering is the reverse of that expected from the charge transfer considerations used for more covalent compounds, it is tempting to conclude that the binding energies are predominantly determined by the Madelung energy. Even though the predicted ordering is observed, the binding energy differences are much smaller than predicted. Including an additional term in eqn. 2 to account for the repulsion energy between neighboring ions in the crystal reduces the discrepancy by a few tenths of an electronvolt [ 81, but the discrepancy remains significant. The same ordering is observed in XPS measurements of gas phase Cs halides, in which there is no Madelung energy contribution to the binding energies [ 581. It has also been noted that the cation core level binding energies of the Ca, Sr and Ba chalcogenides cannot be explained by the Madelung energy alone [ 201. The extra-atomic relaxation energies En in Table 4 for the metals are the atom-solid shifts. The validity of this approximation is evident, since the difference between the Ba3d,,, binding energy in the free atom [55] and the metal 1171 is 5.0 eV, while the calculated extra-atomic relaxation energy is 4.6 eV [ 451. For the alkaline earth compounds, the relative extra-atomic relaxation energies can be obtained from Table 3. As previously noted, the metal is not a vabd reference due to differing intra-atomic relaxation energies for the atom and the ion. The polarization energies for BaO and SrO are therefore calculated using the technique previously described for obtaining the polarization energy induced by a point charge in a continuous dielectric medium [8,59 ] and are used as a reference. The results are ER = 2.8 eV for BaO and 3.0 eV for SrO. The experimental estimates of the relative values of ER from Table 3 can then be combined with the calculated value for BaO or SrO to obtain the values for the other compounds in Table 4. For the Sr compounds, there are no experimental estimates of the relative values of ER, so these are assumed to be the same as for the Ba compounds. This is likely to be a valid assumption in view of the close agreement in the calculated values of ER obtained for the oxides.

231

The considerations above yield binding energy estimates relative to the vacuum level. Fermi level referencing is obtained by subtracting the work functions from these estimates. Experimental metal work functions are available [ 60 ] , and the intrinsic Fermi energies of the compounds relative to the vacuum level can be estimated [ 611 from l/N

#w= 2.86

(4)

where x is the electronegativity and the product is over the atoms in the formula unit. It can be seen in Table 4 that the metal work functions are a 2-3 eV, while those for the compounds are x 4-7 eV, and will thus be a significant contribution to core level binding energy shifts, i.e. the negative shifts are, in part, a natural consequence of Fermi level referencing. It is to be noted that this treatment neglects surface dipole effects, which can be significant. Although there are non-negligible quantitative discrepancies between the estimated and measured cation core level binding energies listed in Table 4, this simple model is useful in demonstrating that work function differences and lattice potential effects can qualitatively account for the negative cation core level binding energy shifts in the compounds considered here. The need to assume unusually large polarization energies or unrealistic d state occupancies is thus eliminated. The influence of the lattice potential on the binding energies may be significant in the high temperature superconductors, which exhibit the lowest alkaline earth core level binding energies. Several recent studies [62-651 have considered the high temperature superconductors as ionic compounds in some respects. While the Cu-0 bonding in the superconducting planes is covalent, the adjacent alkaline earth sites can be considered to be ionic. The Madelung energy of YBa&u30, _ x has been found to be sensitive to the hole distribution on the Cu-0 planes [ 62-631, with ordered hole distributions resulting in Madelung energies which are larger in magnitude. This suggests that the low alkaline earth core level binding energies may be a lattice potential effect, reflecting the distribution of holes on the adjacent Cu-0 planes. Although the validity of applying a Madelung model to a metallic material is open to question, the fact that the extra-atomic relaxation for the Ba site in YBazCu307_. is close to that in the oxide (see Table 3), and 2 eV less than that in Ba metal, does suggest that such an application is not unreasonable. The above analysis is based on referencing of the core levels to the intrinsic Fermi level. Surface states and defect levels can pin the Fermi level near a band edge, rather than at midgap, and this effect can, in principle, shift the measured core level positions by as much as half the bandgap. For BaO this effect has been observed and causes changes in apparent core level shifts relative to Ba metal over a range of 2 eV IS]. An estimate of the Fermi level position can be obtained by measuring the valence band maximum, which should be at half

232

the band gap energy for an intrinsic Fermi level. The valence regions are measured for only a few of the compounds studied here. The band maxima are estimated by extrapolating the leading edge of the band to a flat background, yielding estimates of 6.2,3.2, 1.9 and 1.9 eV ( 20.2 eV) for SrF2, SKY,, SrBr, and SrTiO, (100) respectively. Data from the literature indicate that the band gap is 11.25 eV for SrFz [66 ],7.5 eV for SrClz [67] and 3.2 eV for SrTiOs (100) [6], values which are reasonably close to twice the estimates for the valence band maxima. The Fermi levels in these materials are therefore close to midgap as expected for intrinsic materials. For many of the materials studied here, the measured negative alkaline earth core level binding energy shifts, or the estimates obtained for the intrinsic materials, are sufficiently large that a positive shift would be expected only in the special case of Fermi level pinning near the conduction band edge. While Fermi level pinning will affect the measured binding energies, Fermi level pinning as a general cause of the observed negative shifts therefore seems unlikely. Oxidation of Ca and Sr metals A Sr3d,,, binding energy shift of +0.9 eV is observed when Sr metal is oxidized [2 1, while a shift of - 1.4 eV is observed in this work between bulk SrO powder and Sr metal. Similarly, the CaBp,,, shift of + 1.4 eV observed [2 ] when Ca metal is oxidized differs significantly from the shift of -0.3 eV observed here between bulk CaO powder and Ca metal. Possible causes of the discrepancies are surface contaminants, charging effects, or Fermi level pinning. In this work, the SrO powder is obtained packed in argon, and is never exposed to air. Some surface contaminants are apparent as a high binding energy shoulder on the 01s peak (see Fig. 21, but the dominant peak is at a lower binding energy than the 01s peaks from the hydroxide or carbonate (see Table 11, and is consistent with the oxide. Ion etching removes the contaminants but induces no change in the core level binding energies. The CaO powder is never exposed to air, but is not obtained packed in argon. In this case, the dominant 01s peak at 531 eV is clearly associated with contaminants, and the surface stoichiometry is CaO,.o. After ion etching, the high binding energy 01s peak is almost completely removed, the dominant peak is at 529.4 eV, and the measured surface stoichiometry is CaO,.,. In spite of the significant change in the 01s spectra, no change is observed in the CaZp binding energy after ion etching. This observation is consistent with measurements in this laboratory and elsewhere [ 211 on Sr and Ba compounds, which show that the cation core level binding energies of the oxide, hydroxide, and carbonate are close to each other. It is thus unlikely that the oxide core level binding energies reported here are significantly affected by surface contaminants. In order to determine if charging effects may be significant when the metals are oxidized, oxidation of Ca and Sr by the background gases in the vacuum

233

chamber is allowed to occur. The results for Ca and Sr in Figs. 5 and 6 respectively, clearly show an additional species at higher binding energy than the corresponding metal. An electron flood gun induces no significant changes in the spectra even for maximum filament current and electron energy, eliminating charging as a contributor to the positive shifts observed in these experiments. An estimate of the binding energy of the oxide species is obtained by scaling the metal spectrum so that the high binding energy background matches that of the oxidized spectrum, and least-squares fitting a doublet to the difference spectrum. The results are a + 1.1 eV shift for oxidized Ca and +0.7 eV for oxidized Sr, in agreement with the earlier results [2] in spite of the very different oxidation conditions. It thus appears that the discrepancies observed between bulk oxides and oxidized metals are real, and not the result of experimental artifacts+

360

I

I

I

I

356

352

348

344

BINDING ENERGY

(a) 34

(et’)

Fig. 5. Ca2p spectra of (a) ion etched Ca metal, (b) Ca metal after 2 h in vacuum and (c) CaO powder.

Sr+5h VACUUM 143

137

134

BINDING ENERGY

(eV)

Fig. 6. Sr3d spectra of ion etched Sr metal (solid line) and Sr metal after 5 h in vacuum (broken line ) .

234

Another possibility for the different results obtained for the bulk oxides and the oxidized metals is that the metal surfaces may not be completely oxidized. Alkali and alkaline earth metals are known to diffuse to the surface of the oxide (or the oxygen diffuses into the bulk of the metal) to form metal-rich or subsurface oxides [4,32,68-761. An interesting observation is that when Sr is oxidized, the oxide forms below a surface metal layer which has a work function 0.9 eV lower than that of the bulk metal [ 691. Core levels which are referenced to the Fermi level would therefore be expected to have binding energies 0.9 eV higher than the bulk metal, which is observed [ 2 ] . Although suggestive, further work is needed. A final possibility is Fermi level pinning similar to that demonstrated for BaO [ 61. Energy losses from core levels in Sr compounds In addition to the primary photoelectron peak, additional weaker peaks at higher binding energies are also observed. As discussed in ref. 29, these can be classified as intrinsic, corresponding to other possible final states, or extrinsic, corresponding to discrete energy losses subsequent to photoemission. The energy loss structure from core levels in several of the Sr compounds were measured here, and can be compared to previously reported measurements from alkaline earth fluorides [ 33,341. Figure 7 shows the energy loss regions of the Sr3d core levels of the halides. The Sr3dsj2 peak is the reference, and the core levels are scaled to the same integrated intensity. The spectrum from SrFz in Fig. 7 (a) is similar to previously measured spectra [33,34], as is the energy loss region of the Fls core level. The peak at GZ18 eV has been attributed to plasmon excitation of valence electrons and the peak at x32 eV to plasmon excitation of Sr4p electrons

ENERGY

(eV)

Fig. 7. Energy loss structure associated with the Sr3d core level of (a) SrF*, (b) SK&., (c) SrBr, and (d) Sri,. The energy reference is the Sr3d5,2 peak position.

235

[33,34]. The peak at x 14 eV has been attributed to a shake-up transition within the cation [33], or to the band-to-band transition r& +r;2 (F2p--+Sr4d) [ 341. The primarily intrinsic nature of this feature is evident in the increased intensity in this region relative to that observed in either the electron energy loss spectra [ 331 or in the anion core level loss regions. Energy losses originating from extrinsic processes should result in similar features in all the core levels, though peak intensities and positions actually do depend somewhat on the core level [ 29 ] . The difference in the anion and cation core level loss regions is evident in Fig. 8, which compares the Sr3d and 14d loss regions of Sri,. The b/2 component of each spectrum is the energy reference, and the core levels are scaled to the same integrated intensity. These two core levels are particularly well-suited for direct comparison since the line shapes, spin-orbit splittings and branching ratios are the same, and the kinetic energies of the primary peaks differ by only a few percent. The shapes and intensities of the extrinsic loss features should therefore be very similar, as is observed. The enhanced intensity in the Sr3d loss region near = 10 eV, and to a lesser extent near = 20 eV, must therefore be due to intrinsic processes. Qualitatively similar results are apparent when the cation and anion core level loss regions of the other halides or SrO are compared. Similar findings have also been reported for Ca compounds, and the fact that the cation core level losses in several compounds all show this intrinsic loss at nearly the same energy prompted the assignment of a shake-up within the cation [ 32 1. However, among the Sr halides the position of this feature decreases by 4 eV on going from SrFp to Sri, (see Fig. 7), favoring the band interpretation [ 341. The reason for the difference in loss intensity in the cation and anion core levels in this case is uncertain. A high intensity configuration interaction satellite has been reported to be associated with the ns cation core level peak in compounds of the heavier alkali and alkaline earth elements, where n is the principal quantum number of the

I

I

I

I

I

I

40

30

20

10

0

ENERGY

(ev)

Fig. 8. Comparison of the Sr3d and I4d energy losses in S& is the energy reference in each case.

The position of the &,2 component

outermost filled shell in the initial state (see the discussions in refs. 29,33 and 71 and works cited therein). The cation ns satellite in K and Rb compounds has been attributed to interaction of the ns’np6 (“S) and the ns2np4nd (2S) final states [ 72,731 The corresponding cation ns satellite peaks in XPS spectra from compounds of the heavier alkaline earths have been similarly explained [ 33 J. Within this model, the absence of a satellite in the Na2s region in data from Na salts is simply due to the nonexistence of 2d states. A possible problem with this explanation is that the intensity of the satellite in solid alkali halides is far higher than in the isoelectronic rare gas atoms, contrary to expectations [ 74 1. The discrepancy has been attributed to state mixing in the conduction bands of the solids [74 J. A possible extrinsic origin for these satellites has also been considered [ 291, since the energy difference between the primary and satellite peaks is close to the plasmon excitation energy of valence electrons, though the intensity of this satellite in the cation ns region is much greater than that observed in the other core level loss regions. A possibility which has apparently not been considered is that a significant contribution to the satellite of the ns peak is an extrinsic loss from the np peak. Among the heavier alkali and alkaline earth elements, the energy splitting between the ns and np levels is GZ15-20 eV [19,24]. The plasmon excitation energy of the cation np electrons in alkali and alkaline earth halides is w 2030eV (see ref. 29 and Fig. 7), which would place this extrinsic loss peak at = 10 eV higher binding energy than that of the cation ns level. For Na, however, the 2s peak is at 33 eV higher binding energy than the 2p peak [ 19,24 ] , so that the 2p extrinsic loss peak would be at lower binding energy than the 2s peak and no high intensity satellite would be observed in the 2s energy loss region, as is the case. That the above explanation is probable can be seen in Fig. 9, which compares the energy loss regions of the Sr3d and Sr4s/F2s/Sr4p core levels from SrF2. l

ENERGY

(eV}

Fig. 9. Comparison of the energy loss structure in the Sr3d and Sr4p/F2s/Sr4s The energy references are the Sr3&,, and Sr4p,,, peak positions respectively.

regions of SrF,.

237

The spectra have been scaled so that the Sr3d and Sr4p peaks have the same integrated intensities, and shifted so that the positions of the 3d5,, and 4p3,2 peaks are the same. The peak position and intensity of the extrinsic loss peak can be seen to correspond closely to those of the 4s “satellite” peak. Similar results are obtained from the other Sr halides and from SrO, except for SrIz in which the I4d peak is in the Sr4s loss region. The fact that the Sr4s satelliteSr4p primary peak separation in these compounds tracks the 4p plasmon peakprimary peak separation in the Sr3d region is further evidence that this extrinsic loss from the Sr4p peak is the major contributor to the satellite peak in the Sr4s energy loss region. It is clear that any intensity from possible configuration interaction satellites is a minor contribution to the Sr4s satellite peak, and the same is likely to be true for the its satellites in compounds of the other heavier alkali and alkaline earth elements. CONCLUSIONS

XPS core level binding energies for a comprehensive set of Sr and Ba compounds and kinetic energies of X-ray-induced Auger transitions for Ba compounds have been measured, many for the first time. Most of these compounds have core level binding energies lower than that of the metal, with the negative shifts largest for high temperature superconductors. Negative binding energy shifts can qualitatively be accounted for within the point charge model previously used for other ionic compounds. Polarization screening which is larger in the compounds than the metallic screening in the elemental solids, or d orbital occupancy, either in the initial or final state, which is larger in the compounds thti in the metal, are not necessary to explain the negative binding energy shifts. The extra-atomic relaxation energy, estimated from the Auger parameters for the Ba compounds, increases linearly with anion polarizability far the atomic anions, but is not correlated with the core level binding energy. Data reported in the literature for alkali and other ionic compounds reveal that, rather than being anomalous, small or negative binding energy shifts are common to many of these materials. The large negative shifts observed for the alkaline earth core levels of high temperature superconductors are suggested to be a consequence of large Madelung energies at the cation sites, and may be a measure of the hole distribution on the Cu-0 planes. Positive core level binding energy shifts are observed when Ca and Sr metals are oxidized, while the bulk oxides exhibit negative core level shifts. This discrepancy may be due to differences in work functions or Fermi level pinning. The energy loss regions from the core levels of the Sr compounds have also been measured. A peak which has previously been assigned as a configuration interaction satellite of the Sr4s peak has been shown to be primarily a plasmon loss from the Sr4p peak. High intensity satellites reported for the outermost

238

filled cation s core level in compounds of the other heavier alkali and alkaline earth elements are likely to have a similar origin. ACKNOWLEDGMENTS

The research described in this paper was performed by the Center for Space Microelectronics Technology, Jet Propulsion Laboratory, California Institute of Technology. It was sponsored by the National Aeronautics and Space Administration (NASA), Office of Aeronautics and Space Technology, and by the Defense Advanced Research Projects Agency and the Strategic Defense Initiative Organization, Innovative Science and Technology Office, through Agreements with NASA.

REFERENCES 1

2 3

10 11 12 13

14 15 16

17 18 19 20 21 22

J.A.Th. Verhoeven and H. van Dover-n, Appl. Surf. Sci., 5 (1980) 361. H. van Dovern and J.A.Th. Verhoeven, J. Electron Spectrosc. Relat. Phenom., 21 (1980) 265. W.V. Lampert, K.D. Rachocki, B.C. Lamartine and T.W. Haas, J. Electron Spectrosc. Relat. Phenom., 26 (1982) 133. K. Jacobi, C. Astaidi, B. Frick and P. Geng, Phys. Rev. B, 36 (1987) 3079. G.K. Wertheim, J. Electron Spectrosc. Relat. Phenom., 34 (1984) 309. D.M. Hill, H.M. Meyer III and J.H. Weaver, Surf. Sci., 225 (1990) 63. C.S. Fadley, S.B.M. Hagstrom, M.P. Klein and D.A. Shirley, J. Chem. Phys., 48 ( 1968) 3779. P-H. Citrin and T.D. Thomas, J. Chem. Phys., 57 (1972) 4446. J.Q. Broughton and P.S. Bagus, J. Electron Spectrosc. Relat. Phenom., 20 (1980) 261; 21 (1980) 283. S.W. Gaarenstroom and N. Winograd, J. Chem. Phys., 67 (1977) 3500. R.P. Vasquez and R.M. Housiey, J. Appl. Phys., 67 (1990) 7141. S. Kohiki, T. Wada, S. Kawashima, H. Takagi, S. Uchida and S. Tanaka, Phys. Rev. B, 38 (1988) 8868. F.U. Hillebrecht, J. Fraxedas, L. Ley, H. J. TrodabI, J. Zaanen, W. Braun, M. Mast, H. Peterson, M. Schaible, L.C. Bourne, P. Pinsukanjana and A. Zettle, Phys. Rev. B, 39 (1989) 236. H.M. Meyer III, D.M. HiII, J.H. Weaver, D.L. Nelson and C.F. G&o, Phys. Rev. B, 38 (1988) 7144. P. Steiner, S. Hufner, A. Jungmann, S. Junk, V. Kinsinger, I. Sander, W.R. ThieIe, N. Backes and C. Politis, Physica C, 156 (1988) 213. P. Kulkami, S. Mahamuni, N. Chandrachood, IS. Mulla, A.P.B. Sinha, AS. Nigavekar and S.K. Kulkarni, J. Appl. Phys., 67 (1990) 3438. R-P. Vasquez, MC. Foote and B.D. Hunt, J. Appl. Phys,, 66 (1989) 4866. H.M. Meyer III, T.J. Wagener, J.H. Weaver and D.S. Ginley, Phys. Rev. B, 39 (1989) 7343. J.C. FuggIe and N. M&tensson, J. Electron Spectrosc. Relat. Phenom., 21 (1980) 275. H.F. Franzen, J. Merrick, M. Umana, A.S. Khan, D.T. Peterson, J.R. McCreary and R.J. Thorn, J. Electron Spectrosc. Relat. Phenom., 11 (1977) 439. V. Young and T. Otagawa, Appl. Surf. Sci., 20 (1985) 228. A.B. Christie, J. Lee, I. Sutherland and J.M. Walls, Appl. Surf. Sci., 15 (1983) 224.

239 23 24

25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61

H.F. Franzen, M.X. Umana, J.R. McCreary and R.J. Thorn, J. Solid State Chem., 18 (1976) 363. C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Mouider and GE. Muiienberg, Handbook of Xray Photoelectron Spectroscopy, Perk&Elmer Corp., Eden Prairie, MN, 1979 (and references cited therein). M. Murata, K. Wakino and S. Ikeda, J. Electron Spectrosc. Relat. Phenom., 6 (1975) 459. D.M. Hill, H.M. Meyer III and J.H. Weaver, J. Appl. Phys., 65 (1989) 4943. R.P. Vasquez, B.D. Hunt and M.C. Foote, J. Electrochem. Sot., 137 (1990) 2344. W.E. Morgan, J.R. Van Wazer and W.J. Stec, J. Am. Chem. Sot., 95 (1973) 751. S.P. Kowalczyk, F.R. McFeely, L. Ley, R.A. Pollak and D.A. Shirley, Phys. Rev. B, 9 (1974) 3573. A. Barrie and F.J. Street, J. Electron Spectrosc. Relat. Phenom., 7 (1975 ) 1. L.-G. Petersson and S.-E. Karlsson, Phys. Ser., 16 (1977) 425. C.Y. Su, I. Lindau, P.W. Chye, S.-J. Oh and W.E. Spicer, J. Electron Spectrosc. Relat. Phenom., 31 (1983) 221. I. Ikemoto, K. Ishii, S. Kinoshita and H. Kuroda, J. Electron Spectrosc. Relat. Phenom., 11 (1977) 251. M. Scrocco, Phys. Rev. B, 32 (1985) 1301. N.A.W. Holzwarth, D.P. DiVincenzo, R.C. Tatar and S. Rabii, Int. J. Quantum Chem., 23 (1983) 1223. M.E. Preil, J.E. Fischer, S.B. DiCenzo and G.K. Wertheim, Phys. Rev. B, 30 (1934) 3536. J.P. Albert, C. Jouanin and C. Gout, Phys. Rev. B, 16 (1977) 4619. R.A. Heaton and C.C. Lin, Phys. Rev. B, 22 (1980) 3629. N.C. Amaral, B. Maffeo and D. Guenzburger, Phys. Status Solidi B, 117 (1983) 141. A. Hasegawa and Y, Yanase, J. Phys. C, 13 (1980) 1995. O.E. Taurian, M. Springborg and N.E. Christensen, Solid State Commun., 55 (1985) 351. I.-S. Yang, A.G. Schrott and CC. Tsuei, Phys. Rev. B, 41 (1990) 8921. B. Szpunar and V.H. Smith, Jr., Phys. Rev. B, 37 (1988) 7525. T. J. Wagener, Y. Gao, J.H. Weaver, A. J. Arko, B. Flandermeyer and D.W. Capone II, Phys. Rev. B, 36 (1987) 3899. J.F. Herbst, Phys. Rev. B, 24 (1981) 608. J. Vayrynen, T.T. Rantala, E. Minni and E. Suoninen, J. Electron Spectrosc. Relat. Phenom., 31 (1983) 293. A. Fujimori, J.H. Weaver and A. Franciosi, Phys. Rev. B, 31 (1985) 3549. J.A. Leiro and E.E. Minni, Phys. Rev. B, 31 (1985) 8248. N.G. Krishnan, W.N. Delgass and W.D. Robertson, J. Phys. F, 7 (1977) 2623. C.D. Wagner, Faraday Discuss. Chem. Sot., 60 (1975) 291. J.Q. Broughton and P.S. Bagus, J. Electron Spectrosc. Relat. Phenom., 20 (1980) 127. J.R. Tessman, A.H. Kahn and W. Shockley, Phys. Rev., 92 (1953) 890. J. Shanker and SC. Agarwal, Indian J. Pure Appl. Phys., 14 ( 1976) 79. R.C. Weast and M.J. Astle (&is.), CRC Handbook of Chemistry and Physics, 61st edn. CRC Press, Boca Raton, FL, 1980. J.S.H.Q. Perera, D.C. Frost, C.A. McDowell, C.S. Ewig, R.J. Key and MS. Banna, J. Chem. Phys., 77 (1982) 3308. B.F. Levine, J. Chem. Phys., 59 (1973) 1463. D.R. Jennkon and A.B. Kunz, Phys. Rev. B, 13 (1976) 5597. R.D. Mathews, A.R. Slaughter, R.J. Key and M.S. Banna, J. Electron Spectrosc. Relat. Phenom., 26 ( 1982) 271. N.F. Mott and M.J. Littleton, Trans. Faraday Sot., 34 (1938) 485. H-B. Michaelson, J. Appl. Phys., 48 (1977) 4729. A-H. Nethercot, Jr., Phys. Rev. I&t., 33 (1974) 1088.

240 62

N.F. Wright, G.S. Painter, W.R. Busing and W.H. Butler, in M.B. Brodsky, R.C. Dynes, K. Kitazawa and H.L. ‘I’uller (Eds. ), High Temperature Superconductors, MRS Symposium Proceedings Vol. 99, Materials Research Society, Pittsburgh, PA, 1988, p. 539. 63 J. Kondo, Y. Asai and S. Nagai, J. Phys. Sot. Jpn., 57 (1988) 4334. 64 J.B. Torrance and R.M. Metzger, Phys. Rev. I&t., 63 (1989 ) 1515. 65 T.L. Barr, C.R. Brundle, A. Klumb, Y .L. Liu, L.M. Chen and M.P. Yin, in G. Margaritondo, R. Joynt and M. One&on (l&Is.), High Tc Superconducting Thin Films, Devices, and Applications.AIP Conference Proceedings No. 182, American Institute of Physics, New York, 1989, p. 216. 66 G.W. Rubloff, Phys. Rev. B, 5 (1972) 662. 67 C. Sugiura, Phys. Rev. B, 9 (1974) 2679. 6i3 G. Ebbinghaus, W. Braun and A. Simon, Z. Naturforach., Teil B, 31 (1976) 1219. 69 C.R. Helms and W.E. Spicer, Phys. Rev. Lett., 28 (1972) 565. 70 K.A. Kress and G.J. Lapeyre, Phys. Rev. Lett., 28 ( 1972) 1639. 71 G.K. Wertheim, in C.R. BrundIe and A.D. Baker (Et%.), Electron Spectroscopy: Theory, Techniques, and Applications, Vol. 2, Academic Press, New York, 1978, p. 259. 72 G.K. Wertheim and A. Rosencwaig, Phys. Rev. L&t., 26 (1971) 1179. 73 J. Reader, Phys. Rev. A, 7 (1973) 1431. 74 D.P. Spears, H.J. Fischbeck and T.A. Carlson, Phys. Rev. A, 9 (1974) 1603.