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Auger energy shifts of free cesium halide molecules
Journal of Electron Spectroscopy
and Related Phenomena, 35 (1985)
Elsevier Science Publishers B.V., Amsterdam
AUGER
ENERGY
S. AKSELA,
l-6 - Printed in The Netherlands
SHIFTS OF FREE CESIUM HALIDE
H. AKSELA
MOLECULES
and S. LEINONEN
Department of Physics, University of Oulu, 90540 Oulu 54 (Finland) (Received
12 June 1984)
ABSTRACT The M4.5 N4 5 N+ 5 Auger spectra of Cs and the strongest Auger lines of its halides have been measured hr the vapour phase from CsF, CsCl, CsBr, and CsI molecules. The shifts of the molecular Auger lines of Cs have been analyzed both in terms of the Auger parameter, combining the Auger shifts with the recently published 3d binding energy shifts, and in terms of the calculated Auger energies for free ions, applying a simple point-charge model for the cesium halide molecules studied. The latter method is also applied to the kinetic energies of the main halide lines. INTRODUCTION
Alkali halide molecules and their electron spectra have been studied extensively from solid samples [l-3] and also from the vapour phase using UV excitation (e.g., refs. 4-6), but very few X-ray photoelectron [7,8] and Auger electron [9] measurements have been carried out on free molecules. The essential advantage of studying free molecules instead of solid samples is that crystal field effects on initial energy levels and extra molecular relaxation effects affecting the observed electron energies are absent, making the interpretation of the experimental results much more straightforward. Measurements from the vapour phase are usually more complicated due to the necessary high-temperature oven system. The temperatures needed to obtain the adequate vapour pressure of - 10m3 Torr (S 0.1 Pa) are rather low, in the range 400-800°C for most alkali halides, which can be reached conveniently by the usual resistance heating methods. The purpose of this paper is to present new experimental Auger electron results for cesium halides, and thus continue our efforts to study the electron spectra from free alkali halide molecules. The Auger spectra of cesium halides makes the publication of 3d photoelectron results by Mathews et al. [7, 81 especially interesting and timely. Furthermore, it is well known that the fraction of dimer present is lowest for the cesium hahdes. The Me, 5Nq, 5N4, 5 transitions of Cs occur well within the core levels
0368-2048/85/S
03.30
0 1986 Elsevier Science Publishers B.V.
2
and are thus very suitable for shift investigations. The first results of studies spectra of the CsI molecule have been published on the &,s%,sN~,s recently [9] and the corresponding studies for Auger spectra of sodium and potassium halides are also in progress.
EXPERIMENTAL
The Auger spectra have been measured by means of a cylindrical mirrortype electron spectrometer [lo]. A resistance-heated vapour oven was used to produce the necessary vapour pressure of S 0.1 Pa inside the oven, where primary ionization was brought about by an electron beam. The primary beam usually had current and voltage values of 1 mA and 3 kV. The measurement was controlled and the data collected by a microprocessor-based system. The energies of the Auger lines are determined by calibration with the ArL3M2,3MZ,3(1D2) and Ne KL 2,3L2,3(1D2) lines. The energies of free atoms have been used as reference values for the energy shifts. The experimental energy shifts for the most intense M4N4, 5N4, 5( iG4) line are given in Table 1, together with the binding energy shifts published recently by Banna’s group [ 81. DISCUSSION
Auger spectra of cesium Inspection of the results of Table 1 shows that the Cs Auger energies from all halides are slightly higher than those from free Cs atoms. Further, it can be seen that the Auger energies increase with increasing size of the halide, but that the changes in the Auger energies are smaller than those in the binding energies.
TABLE 1 EXPERIMENTALLY OBSERVED SHIFTS FOR THE Cs M4N4,s N,,s (lG4) AUGER LINE AND 3d5,2 BINDING ENERGY WITH REFERENCE TO CORRESPONDING FREE ATOM VALUES (IN eV)
CSF CkCl CsBr CkI
-0.5 0.1 0.1 0.6
0.8 0.8 1.0 1.2
0.3 0.9 1.1 1.8
3
The Auger energies of a given atom in different chemical environments depend, analogously with core ionization energies, on two factors; namely, on the change of the potential in the original molecule at the atom to be core-ionized, and on the electronic relaxation process of the surrounding charge distribution associated to core ionization. From a single photoelectron or an Auger electron measurement it is not usually possible to separate these two contributions to the observed shift. Very recently Bomben et al. [ll] have presented an interesting method to determine extra-atomic relaxation energies by applying the Manne-Aberg sum rule to the full photoelectron spectrum. Using the similar dependence of Auger energies on the initial state chemical shifts, but an essentially stronger dependence on relaxation effects, the Auger parameter cx turns out to be a very useful quantity. The Auger parameter is defined as a sum of the Auger and the core binding energies of the atom of interest. Several authors [12-171 have shown that Aol Z 2AR, where AR is the extra-atomic relaxation in a single core ionization. This approximation is justified for the shifts between chemically similar compounds, but the higher-order correction terms may contribute significantly [14] to the shifts between compounds of the element studied that are chemically very different. Keeping these limitations in mind for the present cesium halide molecules we will, as a first approximation, apply it to the values given in Table 1. The values of AR thus range from 0.15eV for CsF to 0.9eV for CsI. The magnitude and the trend of increasing extra-atomic relaxation energies on going to heavier halides are correct. The very small value for CsF is in agreement with the low polarizability of F. As a second estimate we will calculate the semiempirical Auger energies for completely ionic molecules, applying the point charge model and assuming that the differences between these estimates and measured Auger energies arise solely from the extra-atomic relaxation energies. We followed the same procedure outlined in our recent paper [9] for CsI. The Auger energy of the Cs+ ion was obtained from relativistically-calculated ASCF Auger energy, corrected by the difference between the similarly calculated TABLE 2 SEMIEMPIRICALLY CALCULATED AUGER ENERGIES (IN eV)
Cs (CsF) cs (C&l) Cs (CsBr) Cs WI)
2.34534 2.9064 3.07224 3.31519
6.12 4.95 4.68 4.34
557.22 556.03 555.76 556.42
1.42 2.56 3.01 3.68
0.5 0.9 1.0 1.2
IIMd'4.&.5
('WWsBr)
('QdPBr) 515.46
1388.9
1.44
3.32
4.34
4.68
6.12
Br-, BrWf4,~&,5
1.51
2.32
4.95
181.98
659.10
DiracrFock
clC1L3M1,3M1,3(1D~)(CsC1)
TABLE3 AUGERENERGIESOFHALIDES
509.68
1380.92
175.52
650.66
509.60
1381.67
174.57
650.30
-0.03
+0.25
-0.32
-0.12
5
and the experimental Auger values for the isoelectronic Xe atom. Assuming that the bonding is completely ionic, the Auger energies from cesium halide molecules deviate from the energies of the free Cs+ ion by the static potential energy shift caused by the counter ion at the interatomic distance and by the extra-atomic relaxation energy contribution. Using the interatomic distances given in Table 2, the potential energy shifts and thus the corrected Cs Mq, 5N4,5N4,5 ( ‘G4) energies of CsX molecules were obtained. These values do not include the extra-atomic relaxation energies and their deviations from the experimental values give the second estimates for the extraatomic relaxation energies. In order to get the extra-atomic relaxation energy AR per single core hole (photoemission), the Auger shifts should be divided by a factor of three. It can be seen that the values obtained are in rather good agreement with the Auger parameter values, and again show the same increasing trend on going from lighter to heavier halides. Spectra of halides A similar consideration of the spectra of halides in CsX molecules was performed. The spectra investigated were F (KLL), Cl (,L2,3MM), Br (L,, 3M4,sMq, 5), and I (M4, 5N4, 5N4, 5). The Auger energies of free halide ions were again obtained from ASCF Dirac-Fock calculations and corrected by the deviation between the calculated and the experimental values of neighbouring isoelectronic rare gas element. They are further corrected by the potential contribution of the Cs ion at the inter-atomic distance. These values are shown in Table 3, with the measured values. Again, the difference between the experimental and “calculated” value should describe the extraatomic relaxation contribution to the Auger energy. From the last column of Table 3 it can be seen that these values are about zero within the accuracies of the experimental results and correction terms. F (KLL) and Cl (L,, ,MM) Auger transitions lead to double hole final states in the valance orbitals, and therefore are not as reliable for this kind of study. The negative relaxation values may be indications of this. On the other hand, the systematically diminishing nominal relaxation contributions support the reasoning of Mathews et al. [ 71 that the assumption of completely ionic molecules leads to a high potential shift and errors in atomic relaxation contributions. These errors have a cancelling character for Cs, but are additive for halides.
ACKNOWLEDGEMENT
This work has been supported
by the Finnish Academy
of Science.
6 REFERENCES 1 P.H. Citrin and T.D. Thomas, J. Chem. Phys., 67 (1972) 4446. 2 S.P. Kowalczyk, F.R. McFeely, L. Ley, R.A. Pollak and D.A. Shirley, Phys. Rev. B,9 (1974) 3573. 3 R.T. Poole, J-G. Jenkin, J. Liesegang and R.C.G. Leckey, Phys. Rev. B, 11 (1975) 6179. 4 J. Berkowitz, in P. Davidovits and D.L. McFadden (Eds.), Alkali Halide Vapors, Structure, Spectra and Reaction Dynamics, Academic Press, New York, 1979, Chap. 6. 5 J. Berkowitz, in C.R. Brundle and A.D. Baker (Eds.), Electron Spectroscopy, Theory, Techniques and Applications, Vol. 1, Academic Press, New York, 1977, pp. 355433. 6 J. Berkowitz, C.H. Batson and G.L. Goodman, J. Chem. Phys., 71 (1979) 2624. 7 R.D. Mathews, R.J. Key, A. Sur, C.S. Ewig and M.S. Bonna, J. Chem. Phys., 74 (1981) 6407. 8 R.D. Mathews, AR. Slaughter, R.J. Key and MS. Banna, J. Electron Spectrosc. Relat. Phenom., 26 (1982) 271. 9 S. Aksela and H. Aksela, Chem. Phys. Lett., 94 (1983) 592. 10 J. Viiyrynen and S. Aksela, J. Electron Spectrosc. Relat. Phenom., 16 (1979) 423. 11 K.D. Bomben, J.K. Gimzewski and T.D. Thomas, J. Chem. Phys., 78 (1983) 5437. 12 CD. Wagner, Anal. Chem., 44 (1972) 973. 13 CD. Wagner and P. Biloen, Surf. Sci., 35 (1973) 82. 14 T.D. Thomas, J. Electron Spectrosc. Relat. Phenom., 20 (1980) 117. 15 SW. Gaarenstroom and N. Winograd, J. Chem. Phys., 67 (1977) 3500. 16 A.R. Williams and N.D. Lang, Phys. Rev. Let&, 40 (1978) 964. 17 E. J. Aitken, M.K. Bahl, K,D, Bomben, J.K. Gimzewski, G.S. Nalen and T.D. Thomas, J. Am. Chem. Sot., 102 (1980) 4873.
and Related Phenomena, 35 (1985)
Elsevier Science Publishers B.V., Amsterdam
AUGER
ENERGY
S. AKSELA,
l-6 - Printed in The Netherlands
SHIFTS OF FREE CESIUM HALIDE
H. AKSELA
MOLECULES
and S. LEINONEN
Department of Physics, University of Oulu, 90540 Oulu 54 (Finland) (Received
12 June 1984)
ABSTRACT The M4.5 N4 5 N+ 5 Auger spectra of Cs and the strongest Auger lines of its halides have been measured hr the vapour phase from CsF, CsCl, CsBr, and CsI molecules. The shifts of the molecular Auger lines of Cs have been analyzed both in terms of the Auger parameter, combining the Auger shifts with the recently published 3d binding energy shifts, and in terms of the calculated Auger energies for free ions, applying a simple point-charge model for the cesium halide molecules studied. The latter method is also applied to the kinetic energies of the main halide lines. INTRODUCTION
Alkali halide molecules and their electron spectra have been studied extensively from solid samples [l-3] and also from the vapour phase using UV excitation (e.g., refs. 4-6), but very few X-ray photoelectron [7,8] and Auger electron [9] measurements have been carried out on free molecules. The essential advantage of studying free molecules instead of solid samples is that crystal field effects on initial energy levels and extra molecular relaxation effects affecting the observed electron energies are absent, making the interpretation of the experimental results much more straightforward. Measurements from the vapour phase are usually more complicated due to the necessary high-temperature oven system. The temperatures needed to obtain the adequate vapour pressure of - 10m3 Torr (S 0.1 Pa) are rather low, in the range 400-800°C for most alkali halides, which can be reached conveniently by the usual resistance heating methods. The purpose of this paper is to present new experimental Auger electron results for cesium halides, and thus continue our efforts to study the electron spectra from free alkali halide molecules. The Auger spectra of cesium halides makes the publication of 3d photoelectron results by Mathews et al. [7, 81 especially interesting and timely. Furthermore, it is well known that the fraction of dimer present is lowest for the cesium hahdes. The Me, 5Nq, 5N4, 5 transitions of Cs occur well within the core levels
0368-2048/85/S
03.30
0 1986 Elsevier Science Publishers B.V.
2
and are thus very suitable for shift investigations. The first results of studies spectra of the CsI molecule have been published on the &,s%,sN~,s recently [9] and the corresponding studies for Auger spectra of sodium and potassium halides are also in progress.
EXPERIMENTAL
The Auger spectra have been measured by means of a cylindrical mirrortype electron spectrometer [lo]. A resistance-heated vapour oven was used to produce the necessary vapour pressure of S 0.1 Pa inside the oven, where primary ionization was brought about by an electron beam. The primary beam usually had current and voltage values of 1 mA and 3 kV. The measurement was controlled and the data collected by a microprocessor-based system. The energies of the Auger lines are determined by calibration with the ArL3M2,3MZ,3(1D2) and Ne KL 2,3L2,3(1D2) lines. The energies of free atoms have been used as reference values for the energy shifts. The experimental energy shifts for the most intense M4N4, 5N4, 5( iG4) line are given in Table 1, together with the binding energy shifts published recently by Banna’s group [ 81. DISCUSSION
Auger spectra of cesium Inspection of the results of Table 1 shows that the Cs Auger energies from all halides are slightly higher than those from free Cs atoms. Further, it can be seen that the Auger energies increase with increasing size of the halide, but that the changes in the Auger energies are smaller than those in the binding energies.
TABLE 1 EXPERIMENTALLY OBSERVED SHIFTS FOR THE Cs M4N4,s N,,s (lG4) AUGER LINE AND 3d5,2 BINDING ENERGY WITH REFERENCE TO CORRESPONDING FREE ATOM VALUES (IN eV)
CSF CkCl CsBr CkI
-0.5 0.1 0.1 0.6
0.8 0.8 1.0 1.2
0.3 0.9 1.1 1.8
3
The Auger energies of a given atom in different chemical environments depend, analogously with core ionization energies, on two factors; namely, on the change of the potential in the original molecule at the atom to be core-ionized, and on the electronic relaxation process of the surrounding charge distribution associated to core ionization. From a single photoelectron or an Auger electron measurement it is not usually possible to separate these two contributions to the observed shift. Very recently Bomben et al. [ll] have presented an interesting method to determine extra-atomic relaxation energies by applying the Manne-Aberg sum rule to the full photoelectron spectrum. Using the similar dependence of Auger energies on the initial state chemical shifts, but an essentially stronger dependence on relaxation effects, the Auger parameter cx turns out to be a very useful quantity. The Auger parameter is defined as a sum of the Auger and the core binding energies of the atom of interest. Several authors [12-171 have shown that Aol Z 2AR, where AR is the extra-atomic relaxation in a single core ionization. This approximation is justified for the shifts between chemically similar compounds, but the higher-order correction terms may contribute significantly [14] to the shifts between compounds of the element studied that are chemically very different. Keeping these limitations in mind for the present cesium halide molecules we will, as a first approximation, apply it to the values given in Table 1. The values of AR thus range from 0.15eV for CsF to 0.9eV for CsI. The magnitude and the trend of increasing extra-atomic relaxation energies on going to heavier halides are correct. The very small value for CsF is in agreement with the low polarizability of F. As a second estimate we will calculate the semiempirical Auger energies for completely ionic molecules, applying the point charge model and assuming that the differences between these estimates and measured Auger energies arise solely from the extra-atomic relaxation energies. We followed the same procedure outlined in our recent paper [9] for CsI. The Auger energy of the Cs+ ion was obtained from relativistically-calculated ASCF Auger energy, corrected by the difference between the similarly calculated TABLE 2 SEMIEMPIRICALLY CALCULATED AUGER ENERGIES (IN eV)
Cs (CsF) cs (C&l) Cs (CsBr) Cs WI)
2.34534 2.9064 3.07224 3.31519
6.12 4.95 4.68 4.34
557.22 556.03 555.76 556.42
1.42 2.56 3.01 3.68
0.5 0.9 1.0 1.2
IIMd'4.&.5
('WWsBr)
('QdPBr) 515.46
1388.9
1.44
3.32
4.34
4.68
6.12
Br-, BrWf4,~&,5
1.51
2.32
4.95
181.98
659.10
DiracrFock
clC1L3M1,3M1,3(1D~)(CsC1)
TABLE3 AUGERENERGIESOFHALIDES
509.68
1380.92
175.52
650.66
509.60
1381.67
174.57
650.30
-0.03
+0.25
-0.32
-0.12
5
and the experimental Auger values for the isoelectronic Xe atom. Assuming that the bonding is completely ionic, the Auger energies from cesium halide molecules deviate from the energies of the free Cs+ ion by the static potential energy shift caused by the counter ion at the interatomic distance and by the extra-atomic relaxation energy contribution. Using the interatomic distances given in Table 2, the potential energy shifts and thus the corrected Cs Mq, 5N4,5N4,5 ( ‘G4) energies of CsX molecules were obtained. These values do not include the extra-atomic relaxation energies and their deviations from the experimental values give the second estimates for the extraatomic relaxation energies. In order to get the extra-atomic relaxation energy AR per single core hole (photoemission), the Auger shifts should be divided by a factor of three. It can be seen that the values obtained are in rather good agreement with the Auger parameter values, and again show the same increasing trend on going from lighter to heavier halides. Spectra of halides A similar consideration of the spectra of halides in CsX molecules was performed. The spectra investigated were F (KLL), Cl (,L2,3MM), Br (L,, 3M4,sMq, 5), and I (M4, 5N4, 5N4, 5). The Auger energies of free halide ions were again obtained from ASCF Dirac-Fock calculations and corrected by the deviation between the calculated and the experimental values of neighbouring isoelectronic rare gas element. They are further corrected by the potential contribution of the Cs ion at the inter-atomic distance. These values are shown in Table 3, with the measured values. Again, the difference between the experimental and “calculated” value should describe the extraatomic relaxation contribution to the Auger energy. From the last column of Table 3 it can be seen that these values are about zero within the accuracies of the experimental results and correction terms. F (KLL) and Cl (L,, ,MM) Auger transitions lead to double hole final states in the valance orbitals, and therefore are not as reliable for this kind of study. The negative relaxation values may be indications of this. On the other hand, the systematically diminishing nominal relaxation contributions support the reasoning of Mathews et al. [ 71 that the assumption of completely ionic molecules leads to a high potential shift and errors in atomic relaxation contributions. These errors have a cancelling character for Cs, but are additive for halides.
ACKNOWLEDGEMENT
This work has been supported
by the Finnish Academy
of Science.
6 REFERENCES 1 P.H. Citrin and T.D. Thomas, J. Chem. Phys., 67 (1972) 4446. 2 S.P. Kowalczyk, F.R. McFeely, L. Ley, R.A. Pollak and D.A. Shirley, Phys. Rev. B,9 (1974) 3573. 3 R.T. Poole, J-G. Jenkin, J. Liesegang and R.C.G. Leckey, Phys. Rev. B, 11 (1975) 6179. 4 J. Berkowitz, in P. Davidovits and D.L. McFadden (Eds.), Alkali Halide Vapors, Structure, Spectra and Reaction Dynamics, Academic Press, New York, 1979, Chap. 6. 5 J. Berkowitz, in C.R. Brundle and A.D. Baker (Eds.), Electron Spectroscopy, Theory, Techniques and Applications, Vol. 1, Academic Press, New York, 1977, pp. 355433. 6 J. Berkowitz, C.H. Batson and G.L. Goodman, J. Chem. Phys., 71 (1979) 2624. 7 R.D. Mathews, R.J. Key, A. Sur, C.S. Ewig and M.S. Bonna, J. Chem. Phys., 74 (1981) 6407. 8 R.D. Mathews, AR. Slaughter, R.J. Key and MS. Banna, J. Electron Spectrosc. Relat. Phenom., 26 (1982) 271. 9 S. Aksela and H. Aksela, Chem. Phys. Lett., 94 (1983) 592. 10 J. Viiyrynen and S. Aksela, J. Electron Spectrosc. Relat. Phenom., 16 (1979) 423. 11 K.D. Bomben, J.K. Gimzewski and T.D. Thomas, J. Chem. Phys., 78 (1983) 5437. 12 CD. Wagner, Anal. Chem., 44 (1972) 973. 13 CD. Wagner and P. Biloen, Surf. Sci., 35 (1973) 82. 14 T.D. Thomas, J. Electron Spectrosc. Relat. Phenom., 20 (1980) 117. 15 SW. Gaarenstroom and N. Winograd, J. Chem. Phys., 67 (1977) 3500. 16 A.R. Williams and N.D. Lang, Phys. Rev. Let&, 40 (1978) 964. 17 E. J. Aitken, M.K. Bahl, K,D, Bomben, J.K. Gimzewski, G.S. Nalen and T.D. Thomas, J. Am. Chem. Sot., 102 (1980) 4873.