Energies and chemical shifts of the sulphur 1s level and the KL2L3(1D2) Auger line in H2S, SO2 and SF6

Journal ofEieceron Spectroscopy and Related Phenomena, 9 (1976) 371-380 @ Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

ENERGIES AND CHEMICAL SHIFTS OF THE SULPHUR THE KL,L,(‘D,) AUGER LINE IN HZS, SO2 AND SF6*

Is LEVEL

AND

0. KESKI-RAHKONEN Laboratory

of Physics,

Helsinki

University

of Technofogy,

02150 Otaniemi

(Finland)

M. 0. KRAUSE Transuranium ( W.S. A.)

Research

(Received 26 January

Laboratory,

Oak Ridge

National

Laboratory,

Oak

Ridge,

Tennessee

37830

1976; in final form 19 April 1976)

ABSTRACT

and KL2L3(lD,) Auger energies have been measured in gaseous H,S, SO, and SF,. The experimental data, including the chemical shifts, are compared with various theoretical ab initio results. Theoretical and experimental values agree within l-2 eV for the chemical shift and the binding energy of the 1s level, provided in the latter case relaxation, relativistic and correlation corrections are applied. Likewise, Shirley’s method2’, which uses empirical energies, predicts the Auger energies satisfactorily. The measured S 1s binding energies are 2478.5(l) eV, 2483.7(l) eV and 2490.1(l) eV, and KL,L,(‘D2) Auger energies are 2098.7(l) eV, 2095.5(2) eV, 2092.6(l) eV for H2S, SO2 and SF,, respectively. The chemical shift for the 1s electron is found to be greater than for the 2s or 2p electron and in better accord with the prediction of the potential model. Data suggest the molecular relaxation energy to be small compared with the atomic relaxation energy. The sulphur

1s binding

energies

INTRODUCTION

The core-level binding energies for almost all elements, and for hundreds of compounds of some elements, have been measured by photoelectron spectrometry using either Al Ko! or Mg Kor X-ray sources. However, there are only a few measurements of binding energies in free atoms or molecules for levels other than those that can be studied with the Al Km radiation of 1487 eV. Similarly, few measurements exist on chemical shifts in solid compounds for levels beyond the reach of the Al Ka * Research sponsored in part by the U.S. Energy Research and Development AdminIstration

under contract with the Union Carbide Corporation.

372 X-rays. Study of deeper core levels has several motivations. For example: (i) data on “shallower” levels are complemented and extended, especially data regarding the satellite structure, (ii) theoretical predictions of level energies can be checked more extensively, and (iii) chemical shifts are expected to closely correspond to the predictions of the potential model in ESCA in its simplest formulation. Data on Auger electron energies for transitions between core levels offer additional information on the atomic and molecular structure and dynamics. In particular, the molecular relaxation energy, which is due to the flow of negative charge toward the inner-shell ionized atom of a compound, can be probed under the more severe condition of the presence of two core vacancies. In this work the sulphur 1 s binding energies and the KL,L,(‘D,) Auger energies in gaseous H2S, S02, and SF, have been measured, and the chemical shifts have been determined for both the photo- and Auger lines. Silver and rhodium X-ray tubes served as excitation sources providing AgL and RhL lines between 2.7 keV and 3.2 keV. EXPERIMENTAL

The binding energy measurements photoelectric process hv = E,

+ Ekin

are based on the energy equation

of the

(1)

where hv is the energy of the absorbed photon, EB the binding energy of the orbital electron (referred to the vacuum level for free atoms or molecules) and E~in the kinetic energy of the photoelectron. If a photoelectron is accelerated (or retarded) before energy analysis, eqn. (1) becomes ELin = hv -

E,

+ eU

(14

where e is the electron charge and U the potential difference the electron traversed. Similarly, if an Auger electron of energy EA is accelerated through a potential difference U’, its final kinetic energy will be E&

= EA + eU’

Using eqns. (la) and (2), the binding energy E, can be obtained from known photon and Auger electron energies and a measurement of the voltage difference AU = U - U’ and the energy interval AEki, = ELin - Eli,,. We have E,

= hv -

EA + eAU -

AEki,

(3)

In principle, a photo line and an Auger line can be shifted into the same energy position by the proper choice of U and U’ so that AE,i” = 0, but, in practice, the line positions can be matched only within the range lAEki,[ 5 0.5 eV. This is sufficient

373 for a precision measurement based on eqn. (3), since with A,!&, being small, it is unnecessary to know accurately the analyzer calibration factorl, and problems arising from analyzer work function, relativistic effects, etc. are circumvented. Care must be exercised, however, to avoid durchgriff of the acceleration field into the source region and disturbing lens effects in the acceleration slit structure. Durchgriff can be virtualIy eliminated by careful design of the slit system’ and lens effects by accelerating both the photoelectrons and the Auger electrons, requiring that both LI and U’ are greater than /Au/. The overall stability of the power supplies for the analyzer plate voltages and the acceleration slit system was better than 1 - 10P4, and the voltages were measured with a differential voltmeter calibrated to an accuracy of better than 2 - lo- ‘. Under these conditions, the ultimate error was governed2 by the error in AE,i, and hence essentially by the accuracy of the determination of the peak positions. Both Lorentzian and Gaussian shapes were fitted to the measured peaks after substraction of a linear background. The difference between the results of the two fits was usually smaller by one order of magnitude than the absolute fitting error. The experiment was performed with a spherical-plate electrostatic energy analyzer’ incorporating an acceleration slit structure’ between the source voIume and the dispersive element. A gaseous sulfur compound was mixed with neon gas

A.rZp REFERENCE

I

,\

250

240

c ENERGIES

(&‘I

SlS

/

OF LEVELS

OR TRANSITIONS

INDICATED

Figure 1. The 1s and 2p photoelectron lines and KLzL#Dz) Auger lines of sulphur in HzS, so2 and SF6 and the respective calibration lines. The S 2p and Ar 2p spectra are adapted from ref. 6.

374 using two needle valves in separate, parallel inlets. X-rays were used for excitation and continuous pumping of the gas cell was employed, so that signals coming from fragment ions could be neglected. The Ne KL2L3(lD2) Auger line (804.54 (2) eV) served as the standard. Sulphur Is photoelectrons were produced by AgLa, (2984.34(2) eV) in the case of H2S and SF,, and by AgL/?, (3 150.97(3) eV) in the case of SOz. The S KL2L,(1D2) Auger lines were measured in reference to AgLa,, RhLa, (2696.77(5) eV), and RhLj?, (2834&I(3) eV) X-rays converted by the Ne 1s (870.32(2) eV) level. The were converted to electron volts using the factor wavelengths given by Bearden ,I* = 1 .OOO0256(18) A/A* after Deslattes and Henins4, and the wavelength-energy product5 of l-2398520(32) nmeV. Auger lines, sulphur 1s photo lines, and their respective calibration standards that were measured in this study are shown in Fig. 1 with the S 2p photo lines and the Ar 2p reference line that were reported previously6. The entire S KLL Auger spectrum for each of the three sulphur compounds was also measured and will be discussed elsewhere’. RESULTS

AND DISCUSSION

The absolute values of the Auger electron energy of the strongest peak, KL2L3(1D2), in a KLL spectrum and of the binding energies of the sulphur core electrons for the gaseous molecules H,S, SO, and SF, are given in Table 1. The combined statistical and systematic errors, which amount to 0.1 eV in our work, are indicated in parentheses. This is the first measurement of the sulphur K-Auger line energy, and the first determination of the S 1s binding energy, in these molecules by the photoelectron method. LaVilla ‘9 ’ has recently derived the S 1s binding energy TABLE

1

EXPERIMENTAL

BINDING

AND AUGER

ENERGIES

OF’SULPHUR

COMPOUNDS

(eV)a

Compound

l&S

so2

Sfi

SKLZL~(~DZ)~ S 1s this workc S 1s LaVillad s 2se S 2p1/ze

2098.7( 1) 2478.5(l) 2478.3(3) 234.5

2095.5(2) 2483.7(l) -

2092.6(l) 2490.1(l) 2490.0(3) 244.7

s 2p3/2e

17i.5 170.2

-

181.7

180.4

174.8

In this work, X-ray calibration wavelengths taken from Bearden are converted to electron volts using the conversion factors of Deslattes and Henins and Cohen and Taylor5. This work, data calibrated against Ne(Ag Lal), Ne(Rh L~I) and Ne(Rh L/%) photoelectron lines. Excited by Ag La1 and Ag L/?I lines. Calibrated against Ne KLzLa(lD2) Auger line. Derived from absorption spectra, refs, 8 and 9. Ref. 6.

375 from X-ray absorption spectra, and obtained values that are in excellent agreement with our photoelectron spectrometric values (cf. Table 1). This accord demonstrates that a similar precision was achieved in both types of determinations; more important, it shows that the inherently complex absorption spectra were correctly interpreted. The S 2s and S 2p binding energies are taken from Siegbahn et al.6 Using the entries in Table 1, the energy of the S Kal, 2 X-ray line in SF6 is calculated to be 2309.3 eV, which is exactly the same value that was measured directly in emission’. Up to now, theoretical ab initio calculations of energies of core electrons and Auger electrons in molecules have not proceeded to the high level of accuracy that binding energy has been reached in the case of atoms’ ‘. Instead of the Hartree-Fock EEF, which is equal to the difference of the total energies of the initial and final states of the atom (or molecule), usually only the eigenvalue E is available for a core level of a molecule. This eigenvalue, if given in the Hartree-Fock limit, sHF, differs from E,HF by the relaxation energy AE,.,. EiF

= -aHF

+ AERLX

has a quasi-atomic

where AE,,,

(4) and a molecular

component,

namely

The quasi-atomic relaxation energy can be visualized to arise from the contraction of the local charge density around the atom with a core vacancy, and the molecular or extra-atomic relaxation energy from a negative-charge flow toward this atom from other regions of the molecule”* i2. The binding energy of an electron can be expressed by EB = EiF + AE or E,

=

-cHF

i- AE,,,

i- AE

(6bl

where the additional term AE = AECoRR + AEREL + AEED corresponds to the correlation, relativistic and quantum electrodynamic energies13. For the molecules studied in this work, eigenvalues that are presumably close to the Hartree-Fock . . limit, E z &HF,have been reported for the 1s level of sulphur14* 15, but no calculations have been made for the other terms in eqn. (6b). A rigorous comparison between csperimental and theoretical binding energies EB is therefore not possible, On the other hand, assuming that the calculated eigenvalues E are “exact” and that AE remains unchanged, one can use atomic values for AE and AERLX, and then regard the difference between experimental and theoretical values of E, as being due mainly to molecular or extra-atomic relaxation. In Table 2, the “experimental” value of the relaxation energy, AERLX, (using eqn. (6b)) is compared with different theoretical predictionsi I7 of the relaxation energy for the S atom (act. to eqn. (4)) and with semiempirical values” ’ 1 *. If we adopt a value of 30 eV for the atomic relaxation energy, we might then attribute - 2 eV to the

376 TABLE 2 RELAXATION

ENERGY

EB

Corn-

OF Is LEVEL OF SULPHUR

AE~

- EHF

pound

2478.5 2483.7 2490.1

H2S so2

SFe

2502.81 b 2507.79b 2515.57c

7.6 I

COMPOUNDS

(eV)

AERLX

AERLx~*~~

Expt.

Th. I, 26

Th. 3e

Emp. I’

Emp. 28

-29.5

29.7

-31.0

-30.2

-31.9 -31.7 -33.0

I

a AE = AECORR + AEREL + AEED for sulphur atom, (ref. 13) with AECoRR = 0.6 eV, AEREL = 6.5 eV, and AEED = 0.5 eV. D Ref. 14; c ref. 15; d sulphur atom, ref. 16: t: sulphur atom, ref. 17 from HF average energies; PSuIphur atom, ref. 11; g Sulphur atom, ref. 18.

TABLE 3 SEMIEMPIRICAL CALCULATION OF SULPHUR KLzL3(‘Dz) AUGER ENERGIES SOME COMPOUNDS (eV), AND ESTIMATE OF THE RELAXATION ENERGY ER Compound

HzS soa SF6

FOR

Expt. energy

energy&

Expf.

Th.b

2098.7 2095.5 2092.6

2098 2094 2089

19 20 22

18

* See eqn. (7); b for sulphur atom, ref. 20.

molecular part, a result which agrees with an expected small, but not negligible molecular relaxation energy. The observed change of this energy from H,S to SF, by about 1 eV is probably real. We note that a recent calculation of the relaxation energy by Chong et a1.l’ is at variance with the above results in the case of the 1s level, but in good agreement in the case of the 2s and 2p levels. In terms of the single-electron binding energies E(K), E(L,), E(L,) and the two-electron integral Ei”(2p2p), the energy of the Auger electron KL2L,(‘D,) is given in intermediate coupling by” E(KL,L#D,))

= E(K) - E(L,)

- E(L,)

+

ER -

F0(2p2p) + smaller terms

(7)

whereby ER is the relaxation energy referred to the final state L,L,. As for S Is, ER contains an atomic and a molecular or extra-atomic part’l; for the atomic part, one can expect E;;‘“” w 2AERLx (L,, 3) to hold where the relaxation energy for the L,, 3 shell can be obtained from eqn. (3). It is about 9.6 eV1’. In Table 3, the experimental data for the principal KLL Auger line are compared with the values calculated with

377 TABLE 4 CHEMICAL SHIFTS OF KLzL3(‘Dz) COMPOUNDS (eV)

AUGER

LINES AND

A(w,S - Sod

S KLzL~(~Dz)

s 1s S 2s s 2P3/2 SR(L2L3p

CORE LEVELS OF SULPHUR

AC&S - SFs)

Expt.

Th.

Expt.

Th.

3.2(2) -5.2(2) -5.1” -4.6” -0.8

-

&l(2) - 11.6(2)b -10.2e - 10.2e - 2.7

-

-5.0a -5.1* -5.5’ -

-12.8& - 12.2e -

a S 1s of HzS and SOZ from ref. 14 and S 1s of SF6 from ref. 15. b 11.7 eV according to ref. 8; c refs. 6 and 24; d ref. 14; - 5.5 eV according to refs. 24 and 25; e ref. 6; f ref. 26; g defined as 6R = GE(KLL) - &T(K) + 2&T(L) according to ref. 21.

the aid of eqn. 7 and the use of the pertinent binding energies of Table 1, and the two-electron integrals by Mann”. As seen from Table 3, the agreement between the directly-measured value and the value derived from eqn. (7) is very good, indicating that the relaxation energy is determined by the atomic component, which was the only part used in eqn. (7) for arriving at E(KL,L,(lD)). Going from H,S to SF,, the change in relaxation energy is almost 3 eV. This is more than double the variation for the 1s level and reasonable in view of the fact that Auger process produces 2 holes in a core level of sulphur. In Table 4, the chemical shifts for the is, 2s, 2p,,, levels and the Auger line KL,L3(lD,) are givenz3-26. Al so listed is the change in relaxation energy, GR(L,L,), which is defined as follows2’ GR(L,L,) = GE(KL,L&D,)) - 6,??(K) + 6E(L,) + 6E(L,) (8) and thereby expresses the fact that extra-atomic relaxation is an important link in relating Auger energy shifts to binding energy shiftsz7. As a major result, we find the chemical shift larger for the Is level than for the 2p level. This is particularly noticeable for SF6 relative to H2S or SO1. The observed shift S( 1s) is in good agreement with calculations that are based on the simple potential model in ESCA6’ 12. This is not surprising since the approximations inherent in the model should apply well for a deep core level such as the 1s level of S. On the other hand the 2p shell is in sufficient proximity to the valence electrons that the limitations of the potential model become apparent. More detailed considerations of the potential distribution are then needed to account for the shifts of each of the “shallower” core levels. The relaxation energy shift of 2.7 eV from H,S to SF, is less important than the net binding-energy shift of 8.8 eV in determining the Auger shift of - 6.1 eV. This is in contrast to the situation in a metal-metal oxide system21 where 6(R) is predominant. However, the charge flow occurs freely in a metal and is unfavored in a molecule like SF,.

378 CONCLUSIONS

The S 1s binding energies and S KLZL3(‘DZ) Auger energies have been measured with high accuracy for gaseous H,S, SO, and SF,. The binding energy values in H2S and SF, are in excellent agreement with those from X-ray absorption measurements. Comparison of the data with theoretical or semiempirical predictions shows the molecular part of the relaxation energy to be small compared with the atomic part, Differences of the molecular relaxation for the different molecules are discernible, however, in both S Is and S KL,L,(lD,) and amount to about 1 and 3 eV, respectively, when going from H2S to SF+ The chemical shift for the 1s level is found to be greater than for the 2p level and in satisfactory agreement with the predictions of the potential model in its simplest formulation. This indicates that the Is level in second-row elements is especially suited for chemical-shift measurements and their ready interpretations. APPENDIX

Several properties of the X-ray lines used in this study and others that might be useful for probing the deeper core levels are summarized in Table 5. With these X-rays, especially RhL, AgL, and Ti Kol, the K shells of the second-row elements can be excited. However, only Ti Ka possesses the natural width and intensity desirable TABLE

5

PROPERTIES OF X-RAY FROM 2 TO 5 keV” Element

Line

LINES

FROM

Energy (e V

SUITABLE

Width (FWHM)”

ANODES

IN THE

ENERGY

RANGE

Target element for K-shell exc.C

Anode preparationd

Na to Si

Br, EV

(eV)

La1 LB1

2042.4 2124.4

1.6

MO

La1 LB1

2293.2 2394.8

1.9 2.2

NatoP

Br

Rh

La1 LB1

2838.6 2990.2

2.4 2.8

Na to Cl

ED

Ai?

La1 LB1

2984.3 3150.9

2.6

Na to Cl

Br, ED, EV

Ti

Km

4510.9

1.2

Na to Ca

EV

Zl-

1.9

3.1

n Further anodes can be fabricated from Y, Nb, Sb, and Sn. b Taken from 0. Keski-Rahkonen and M. 0. Krause, At. Data Nucl. Data Tables, 14 (1974) 139. c In most PES studies, first-row elements, Na and Mg are excited preferentially by Mg Ka and Al Ka. d Possible method of preparation: Br = brazed; ED = electrodeposit; EV = evaporation or sputtering.

379 in customary PES studies. The L lines of the elements Y to Sn are relatively broad and weak and would require monochromatization for investigations in which highresolution and a good line-to-background ratio are of prime importance. If needed, natural crystals would serve as suitable monochromators for these X-ray sources with energies ranging from 2-5 keV. The Zr L spectrum shown elsewhere” can be viewed as a representative example for the L spectra of Y to Sn. The La, and Lb, lines are predominant and have an intensity ratio I(Lj?,)/I(Lo(,) of approximately 0.5. Although a systematic study of the line intensities of RhL and AgL relative to Mg Kcc or Al Km was not undertaken, a practical order-of-magnitude result can be given. We found for the uncorrected intensity ratio of the Ne 1s(Mg Kcc) to Ne ls(Ag La,) photo-lines a value of - 44: 1 for the peak values and 13: 1 for the area values at maximum power dissipation of the respective anodes. Signal-to-noise ratios were about 100: 1 for Ne ls(Mg Kcz) and 20: 1 for Ne ls(Ag Lee,). However, in more spacious electron source chambers such as those used in ref. 1, these ratios can be improved by a factor of 24. It is hoped that this description, though fragmentary and approximate, might be useful to those who wish to employ in PES X-ray sources with energies greater than that of Al Kor. ACKNOWLEDGEMENTS

One of us (0. K-R.) gratefully acknowledges financial support from the ASLA/Fulbright-Hays foundation and the University of Tennessee under contract AT-(40-1)- 4447 for his stay at the Oak Ridge National Laboratory in 1973.

REFERENCES

7 8 9 10 11 12 13 14 15 16

M. 0. Krause, A&. X-Ray Anal., 16 (1973) 74; F. Wuilleumier and M. 0. Krause, Phys. Rev. A, 10 (1974) 242. 0. Keski-Rahkonen and M. 0. Krause, Phys. Rev. A, in press. J. A. Bearden, Rev. Mod. Phys., 39 (1967) 78. R. D. Deslattes and A. Henins, Phys. Rev. Lett., 31 (1973) 972. E. R. Cohen and B. N. Taylor, J. Phys. Chem. Ref. Data, 2 (1973) 663. K. Siegbahn et al., ESCA Applied to Free Molecules, North-Holland, Amsterdam, London, 1969, p. 132. 0. Keski-Rahkonen, to be published. R. E. LaVilla, J. Chem. Phys., 62 (1975) 2209. R. E. LaVilla, J. Chem. Phys., 57 (1972) 899. H. P. Kelly, Phys. Rev. A, 11 (1975) 556. D. A. ShirIey, Chem. Phys. Len., 16 (1972) 220. U. Gelius, Phys. Scv., 9 (1974) 133. T. Aberg, Phys. Rev., 162 (1967) 5. B. Roos and P. Siegbahn, Theor. Chim. Acta, 21 (1971) 368. U. Gelius, private communication. HF calculations, U. Gelius et al., J. Electron Spectrosc. Relat. Phenom., 1 (1972173) 285; DiracFwk calculation gave identical results for S ls, and S 2p, M. Suvanen, personal communication.

380 17 18 19 20 21 22 23 24 25 26 27 28

B. Breuckmann, V. Schmidt, personal communication. L. C. Snyder, J. Chem. Whys., 55 (1971) 95. D. P. Chong, F. G. Herring and D. McWilliams, J. Chem. Whys., 61 (1974) 3567. D. A. Shirley, Whys. Rev. A, 7 (1973) 1520. S. P. Kowalczyk, L. Ley, F. R. MC Feely, R. A. Pollak and D. A. Shirley, Phys. Rev. B, 9 (1974) 381; C. D. Wagner and P. Biloen, Surf. Sci., 35 (1973) 82. J. B. Mann, Afomic Structure Calculations, LA-3690, Los Alamos, 1967, unpublished report. K. Siegbahn, J. Electron Spectrosc. Relar. Phenom., 5 (1974) 3. S. Rothenberg and H. F. Schaefer, III, J. Chem. Phys., 53 (1970) 3014. S. Rothenberg, R. H. Young and H. F. Schaefer, III, J. Am. Chem. Sot., 92 (1970) 3243. U. Gelius, B. Roos and P. Siegbahn, Chem. Phys. L&t., 4 (1970) 471. C. D, Wagner, Faraduy Discuss. Chem. Sac., 60 (1975) 291, introduces the so-called Auger parameter to describe this effect. M. 0. Krause, F. Wuilleumier and C. W. Nestor, Jr., Phys. Rev. A, 6 (1972) 871.