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Influence of nondipolar parameters on the XPS intensities in solids
Journal of Electron Spectroscopy and Related Phenomena 125 (2002) 153–156 www.elsevier.com / locate / elspec
Influence of nondipolar parameters on the XPS intensities in solids a, a b b b V.I. Nefedov *, V.G. Yarzhemsky , R. Hesse , P. Streubel , R. Szargan a
Institute of General and Inorganic Chemistry RAS, Leninski Pr. 31, Moscow, Russia ¨ Physikalische und Theoretische Chemie der Universitat ¨ Leipzig, Linnestr ´ . 2, D-04103 Leipzig, Germany Wilhelm-Ostwald-Institut f ur
b
Received 23 October 2001; received in revised form 26 April 2002; accepted 15 May 2002
Abstract The influence of nondipolar parameters on the XPS intensities in solids was investigated experimentally for the first time. Two spectrometers were used with different angles (75 and 1258) between the directions of the X-ray flux and the photoelectrons. The intensities of XPS lines have been normalized using Auger transitions with approximately the same kinetic energy as the corresponding XPS line under investigation. The experimental relative intensities of N 1s, O 1s and F 1s lines for two different angles are in reasonable agreement with the theoretical values, if nondipolar parameters are taken into account. 2002 Elsevier Science B.V. All rights reserved. Keywords: Nondipolar parameters; Angular dependence of XPS line intensities
1. Introduction The angular distribution of photoelectrons excited from free atoms by unpolarized photons is given by [1] dsnl 4p F 5 ]] ? ] 5 1 2 0.25b (3 cos 2 u 2 1) dV snl 2
1 (0.5g sin u 1 d )cos u
(1)
where snl is the total photoionization cross-section of a subshell nl, b is the dipole parameter, u is the angle between the directions of the photon flux and the photoelectrons, g and d are the nondipolar parameters. The experimental parameters g and d derived from XPS angular distributions [2–4] and X-ray standing waves [5] were found to agree well *Corresponding author. Fax: 17-95-954-1279. E-mail address: [email protected] (V.I. Nefedov).
with the theory for free atoms indicating substantial nondipolar contributions to the photoemission intensity. In the case of photoemission from solids the angular distribution is influenced by electron scattering which can be taken into account by D1 5 (1 2 v )20.5 H (H5Chandrasekhar function) giving [6,7] 2
Fs 5 a[D1 2 0.25b (3 cos u 2 1) 1 (0.5g sin 2 u 1 d )cos u ]
(2)
where a512 v with v as the single scattering albedo (coefficient of scattering reflection). The XPS intensity I (Eq. 3) from semiinfinite solid samples is determined by Fs from Eq. (2), the X-ray intensity J, the detector efficiency E, the analyser efficiency D, the effective sample area S, the solid angle of the analyser acceptance V, the inelastic mean free path l of the photoelectrons in the solid
0368-2048 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0368-2048( 02 )00135-4
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V.I. Nefedov et al. / Journal of Electron Spectroscopy and Related Phenomena 125 (2002) 153 – 156
and the sample concentration n, giving for normal photoelectron emission I 5 (0.25snl /p )Fs JEDSVln
both spectrometers. As charging correction the reference EB (C 1s)5285.0 eV was used.
(3)
All the instrumental parameters given above may vary when comparing angle-dependent intensities obtained by using different spectrometers. In order to compensate these instrumental effects the intensities of Auger lines with kinetic energies close to the photoelectron energies under study can be used for normalization of the photoelectron intensities. This is possible because the process of Auger electron intensity of free atoms is independent from the angle between the direction of the X-ray flux and the electron emission. Consequently the angle dependence of Auger electrons measured at different spectrometers is nearly the same and depends only on the electron scattering in the solid valid just as for photoelectrons. The present paper reports on variations of normalized XPS intensities measured at different angles u between the directions of the photon flux and the photoelectron emission demonstrating for the first time the significance of nondipolar contributions to the line intensity of solids.
2. Experimental The N 1s, O 1s and F 1s lines from solid samples of Na 2 B 4 O 7 , Na 3 Co(NO 2 ) 6 , and NaF emitting Na KLL and F KLL Auger peaks close to the photoelectron lines (see Fig. 1) were measured at the angles u 5758 and u 51258 between the directions of the photon flux and the photoelectron emission. The very fine powder of each sample was prepared on the sample holder with a double-sided adhesive tape. The spectra were recorded with VG ESCA3 (u 5758) and VG ESCALAB 220iXL (u 51258) using nonmonochromatized Al Ka radiation and a take-off angle of 608 (related to the surface normal). The same electron take-off angle for both spectrometers was chosen to minimize the influence of possible different adventitious overlayers on the results. The peak intensities were calculated applying the fitting program UNIFIT [8] using a Shirley background and equal energy widths at the line basis for
3. Results and discussion In Fig. 1 different kinetic energy regions of the survey spectra of Na 2 B 4 O 7 , Na 3 Co(NO 2 ) 6 and NaF recorded with different spectrometers are shown. The O 1s, N 1s and F 1s intensities in comparison with the relative intensity of the Na KLL Auger line in the case of u 5758 (ESCA3) are considerably larger than in the case of u 51258 (ESCALAB). This behavior can be explained already by the dipole approximation for free atoms of Eq. (1): with b 52 for 1s photoelectron transitions one obtains F(u 5758) / F(u 51258)¯1.40. In order to study the effect of nondipolar transitions more in detail, the intensity ratios I75 /I125 of the O 1s, N 1s and F 1s lines normalized using the Na KLL or F KLL Auger spectra were calculated (see Table 1). Unfortunately, also the small difference between the kinetic energies of the photoelectrons and the Auger electrons influences the ratio of the normalized intensities. The smallest difference of the kinetic energies DEkin is 35 eV (O 1s–Na KLL), the largest is 188 eV (F 1s–Na KLL) (see Fig. 1).The error of the measured intensity ratios I75 /I125 depending on DEkin is first of all influenced by different electron analyser efficiency functions D of ESCA3 and ESCALAB, but also by nonidentical adventitious overlayers on the samples in the two spectrometers. The absolute value of the error was estimated by comparison of calculated different model examples with different transition functions E 2n kin and overlayer models exp[22d i / l(E kin )] as well as by statistics of the measured results as #10%. In Table 1 the experimental ratios are compared with theoretical values derived from Eq. (2). The D1 values were obtained using Chandrasekhar functions H calculated according Tilinin’s paper [9]. Using the dipolar parameter b 52 and the nondipolar parameters g 50.674 (N 1s); 0.606 (O 1s); 0.524 (F 1s) and d 50 from relativistic calculations [10] together with a weighted sum of atomic a values from elements [11] considering the stoichiometry of the compounds, theoretical ratios I75 /I125 were calculated.
V.I. Nefedov et al. / Journal of Electron Spectroscopy and Related Phenomena 125 (2002) 153 – 156
155
Table 1 Experimental ratios I75 /I125 of 1s line intensities normalized using Na KL 23 L 23 Auger intensities (estimated error: u10u% or smaller) compared with theoretical ratios from Eq. (2) in dipolar (I) and nondipolar approximation (II) I75 a /I125 b
Experiment Theory (I) Theory (II)
Na 2 B 4 O 7
Na 3 Co(NO 2 ) 6
NaF
O 1s
O 1s
N 1s
F 1s
1.73 1.34 1.54
1.52 1.33 1.54
1.51 1.33 1.58
1.66 1.33 1.45
1.63 c
a
u 5758. u 51258. c Normalized using F KLL Auger lines. b
ty ratios are in better agreement with the results of the theory when nondipolar transitions are included (difference between experiment and theory (II) #15%, between experiment and theory (I) #30%).
4. Conclusions The nondipolar transitions considerably influence the angular dependence of XPS lines of solids. In most cases the differences between the experimental results and results from a theory which takes into account nondipolar transitions (theory (II)) does not exceed 15%. The deviation may be caused mainly by the normalization procedure which disregards the differing instrumental effects at different kinetic energies of photoelectrons and related Auger electrons. Some inaccuracy in D1 and a values may also contribute to the deviations. The differences between the experimental results and results from the theory with dipolar approximation (theory (I)) are in all cases essentially larger (up to 30%) than with theory (II).
Fig. 1. Survey spectra of Na 2 B 4 O 7 (a), Na 3 Co(NO 2 ) 6 (b) and NaF (c) recorded with VG ESCA3 (u 5758) and with VG ESCALAB (u 51258).
By taking into account nondipolar transitions, substantially higher intensity ratios (theory (II)) were obtained with respect to the values from the dipolar approximation (theory (I)). The experimental intensi-
Acknowledgements V.I. Nefedov is grateful to the Alexander von Humboldt Foundation for the possibility to perform this work in cooperation with German scientists. The authors V.I.N. and V.G.Y. thank the Russian Fund for Fundamental research for financial support. Helpful comments of the referees are grateful acknowledged.
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V.I. Nefedov et al. / Journal of Electron Spectroscopy and Related Phenomena 125 (2002) 153 – 156
References [1] J.W. Cooper, Phys. Rev. A 47 (1993) 1841. [2] E.W.B. Dias, H.S. Chakraborty, P.C. Deshmukh, S.T. Manson, O. Hemmers, P. Glans, D.L. Hansen, H. Wang, S.B. Whitfield, D.W. Lindle, R. Wehlitz, J.C. Levin, I.A. Selin, R.C.C. Perera, Phys. Rev. Lett. 78 (1997) 4553. [3] M. Jung, B. Kraessig, D.S. Gemmell, E.P. Kanter, T. Le Brun, S.H. Southworth, L. Young, Phys. Rev. A 54 (1996) 2127. [4] A. Derevjanko, W.R. Johnson, K.T. Cheng, Atomic Data Nucl. Data Tables 73 (1999) 153. [5] C.J. Fisher, R. Ithin, R.G. Jones, G.J. Jackson, D.P. Woodruff, B.C.C. Cowie, J. Phys. Condens. Matter. 10 (1998) L623.
[6] V.I. Nefedov, I.S. Nefedova, J. Electron Spectrosc. Relat. Phenom. 107 (2000) 131. [7] V.I. Nefedov, I.S. Nefedova, J. Electron Spectrosc. Relat. Phenom. 113 (2000) 3. ´ R. Szargan, Fresenius J. Anal. Chem. [8] R. Hesse, T. Chasse, 365 (1999) 48. [9] I.S. Tilinin, W.S.M. Werner, Surf. Sci. 290 (1993) 119. [10] V.I. Nefedov, V.G. Yarzhemsky, I.S. Nefedova, M.B. Trzhaskovskaya, I.M. Band, J. Electron Spectrosc. Relat. Phenom. 107 (2000) 123. [11] V.I. Nefedov, I.S. Nefedova, J. Electron Spectrosc. Relat. Phenom. 95 (1998) 281.
Influence of nondipolar parameters on the XPS intensities in solids a, a b b b V.I. Nefedov *, V.G. Yarzhemsky , R. Hesse , P. Streubel , R. Szargan a
Institute of General and Inorganic Chemistry RAS, Leninski Pr. 31, Moscow, Russia ¨ Physikalische und Theoretische Chemie der Universitat ¨ Leipzig, Linnestr ´ . 2, D-04103 Leipzig, Germany Wilhelm-Ostwald-Institut f ur
b
Received 23 October 2001; received in revised form 26 April 2002; accepted 15 May 2002
Abstract The influence of nondipolar parameters on the XPS intensities in solids was investigated experimentally for the first time. Two spectrometers were used with different angles (75 and 1258) between the directions of the X-ray flux and the photoelectrons. The intensities of XPS lines have been normalized using Auger transitions with approximately the same kinetic energy as the corresponding XPS line under investigation. The experimental relative intensities of N 1s, O 1s and F 1s lines for two different angles are in reasonable agreement with the theoretical values, if nondipolar parameters are taken into account. 2002 Elsevier Science B.V. All rights reserved. Keywords: Nondipolar parameters; Angular dependence of XPS line intensities
1. Introduction The angular distribution of photoelectrons excited from free atoms by unpolarized photons is given by [1] dsnl 4p F 5 ]] ? ] 5 1 2 0.25b (3 cos 2 u 2 1) dV snl 2
1 (0.5g sin u 1 d )cos u
(1)
where snl is the total photoionization cross-section of a subshell nl, b is the dipole parameter, u is the angle between the directions of the photon flux and the photoelectrons, g and d are the nondipolar parameters. The experimental parameters g and d derived from XPS angular distributions [2–4] and X-ray standing waves [5] were found to agree well *Corresponding author. Fax: 17-95-954-1279. E-mail address: [email protected] (V.I. Nefedov).
with the theory for free atoms indicating substantial nondipolar contributions to the photoemission intensity. In the case of photoemission from solids the angular distribution is influenced by electron scattering which can be taken into account by D1 5 (1 2 v )20.5 H (H5Chandrasekhar function) giving [6,7] 2
Fs 5 a[D1 2 0.25b (3 cos u 2 1) 1 (0.5g sin 2 u 1 d )cos u ]
(2)
where a512 v with v as the single scattering albedo (coefficient of scattering reflection). The XPS intensity I (Eq. 3) from semiinfinite solid samples is determined by Fs from Eq. (2), the X-ray intensity J, the detector efficiency E, the analyser efficiency D, the effective sample area S, the solid angle of the analyser acceptance V, the inelastic mean free path l of the photoelectrons in the solid
0368-2048 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0368-2048( 02 )00135-4
154
V.I. Nefedov et al. / Journal of Electron Spectroscopy and Related Phenomena 125 (2002) 153 – 156
and the sample concentration n, giving for normal photoelectron emission I 5 (0.25snl /p )Fs JEDSVln
both spectrometers. As charging correction the reference EB (C 1s)5285.0 eV was used.
(3)
All the instrumental parameters given above may vary when comparing angle-dependent intensities obtained by using different spectrometers. In order to compensate these instrumental effects the intensities of Auger lines with kinetic energies close to the photoelectron energies under study can be used for normalization of the photoelectron intensities. This is possible because the process of Auger electron intensity of free atoms is independent from the angle between the direction of the X-ray flux and the electron emission. Consequently the angle dependence of Auger electrons measured at different spectrometers is nearly the same and depends only on the electron scattering in the solid valid just as for photoelectrons. The present paper reports on variations of normalized XPS intensities measured at different angles u between the directions of the photon flux and the photoelectron emission demonstrating for the first time the significance of nondipolar contributions to the line intensity of solids.
2. Experimental The N 1s, O 1s and F 1s lines from solid samples of Na 2 B 4 O 7 , Na 3 Co(NO 2 ) 6 , and NaF emitting Na KLL and F KLL Auger peaks close to the photoelectron lines (see Fig. 1) were measured at the angles u 5758 and u 51258 between the directions of the photon flux and the photoelectron emission. The very fine powder of each sample was prepared on the sample holder with a double-sided adhesive tape. The spectra were recorded with VG ESCA3 (u 5758) and VG ESCALAB 220iXL (u 51258) using nonmonochromatized Al Ka radiation and a take-off angle of 608 (related to the surface normal). The same electron take-off angle for both spectrometers was chosen to minimize the influence of possible different adventitious overlayers on the results. The peak intensities were calculated applying the fitting program UNIFIT [8] using a Shirley background and equal energy widths at the line basis for
3. Results and discussion In Fig. 1 different kinetic energy regions of the survey spectra of Na 2 B 4 O 7 , Na 3 Co(NO 2 ) 6 and NaF recorded with different spectrometers are shown. The O 1s, N 1s and F 1s intensities in comparison with the relative intensity of the Na KLL Auger line in the case of u 5758 (ESCA3) are considerably larger than in the case of u 51258 (ESCALAB). This behavior can be explained already by the dipole approximation for free atoms of Eq. (1): with b 52 for 1s photoelectron transitions one obtains F(u 5758) / F(u 51258)¯1.40. In order to study the effect of nondipolar transitions more in detail, the intensity ratios I75 /I125 of the O 1s, N 1s and F 1s lines normalized using the Na KLL or F KLL Auger spectra were calculated (see Table 1). Unfortunately, also the small difference between the kinetic energies of the photoelectrons and the Auger electrons influences the ratio of the normalized intensities. The smallest difference of the kinetic energies DEkin is 35 eV (O 1s–Na KLL), the largest is 188 eV (F 1s–Na KLL) (see Fig. 1).The error of the measured intensity ratios I75 /I125 depending on DEkin is first of all influenced by different electron analyser efficiency functions D of ESCA3 and ESCALAB, but also by nonidentical adventitious overlayers on the samples in the two spectrometers. The absolute value of the error was estimated by comparison of calculated different model examples with different transition functions E 2n kin and overlayer models exp[22d i / l(E kin )] as well as by statistics of the measured results as #10%. In Table 1 the experimental ratios are compared with theoretical values derived from Eq. (2). The D1 values were obtained using Chandrasekhar functions H calculated according Tilinin’s paper [9]. Using the dipolar parameter b 52 and the nondipolar parameters g 50.674 (N 1s); 0.606 (O 1s); 0.524 (F 1s) and d 50 from relativistic calculations [10] together with a weighted sum of atomic a values from elements [11] considering the stoichiometry of the compounds, theoretical ratios I75 /I125 were calculated.
V.I. Nefedov et al. / Journal of Electron Spectroscopy and Related Phenomena 125 (2002) 153 – 156
155
Table 1 Experimental ratios I75 /I125 of 1s line intensities normalized using Na KL 23 L 23 Auger intensities (estimated error: u10u% or smaller) compared with theoretical ratios from Eq. (2) in dipolar (I) and nondipolar approximation (II) I75 a /I125 b
Experiment Theory (I) Theory (II)
Na 2 B 4 O 7
Na 3 Co(NO 2 ) 6
NaF
O 1s
O 1s
N 1s
F 1s
1.73 1.34 1.54
1.52 1.33 1.54
1.51 1.33 1.58
1.66 1.33 1.45
1.63 c
a
u 5758. u 51258. c Normalized using F KLL Auger lines. b
ty ratios are in better agreement with the results of the theory when nondipolar transitions are included (difference between experiment and theory (II) #15%, between experiment and theory (I) #30%).
4. Conclusions The nondipolar transitions considerably influence the angular dependence of XPS lines of solids. In most cases the differences between the experimental results and results from a theory which takes into account nondipolar transitions (theory (II)) does not exceed 15%. The deviation may be caused mainly by the normalization procedure which disregards the differing instrumental effects at different kinetic energies of photoelectrons and related Auger electrons. Some inaccuracy in D1 and a values may also contribute to the deviations. The differences between the experimental results and results from the theory with dipolar approximation (theory (I)) are in all cases essentially larger (up to 30%) than with theory (II).
Fig. 1. Survey spectra of Na 2 B 4 O 7 (a), Na 3 Co(NO 2 ) 6 (b) and NaF (c) recorded with VG ESCA3 (u 5758) and with VG ESCALAB (u 51258).
By taking into account nondipolar transitions, substantially higher intensity ratios (theory (II)) were obtained with respect to the values from the dipolar approximation (theory (I)). The experimental intensi-
Acknowledgements V.I. Nefedov is grateful to the Alexander von Humboldt Foundation for the possibility to perform this work in cooperation with German scientists. The authors V.I.N. and V.G.Y. thank the Russian Fund for Fundamental research for financial support. Helpful comments of the referees are grateful acknowledged.
156
V.I. Nefedov et al. / Journal of Electron Spectroscopy and Related Phenomena 125 (2002) 153 – 156
References [1] J.W. Cooper, Phys. Rev. A 47 (1993) 1841. [2] E.W.B. Dias, H.S. Chakraborty, P.C. Deshmukh, S.T. Manson, O. Hemmers, P. Glans, D.L. Hansen, H. Wang, S.B. Whitfield, D.W. Lindle, R. Wehlitz, J.C. Levin, I.A. Selin, R.C.C. Perera, Phys. Rev. Lett. 78 (1997) 4553. [3] M. Jung, B. Kraessig, D.S. Gemmell, E.P. Kanter, T. Le Brun, S.H. Southworth, L. Young, Phys. Rev. A 54 (1996) 2127. [4] A. Derevjanko, W.R. Johnson, K.T. Cheng, Atomic Data Nucl. Data Tables 73 (1999) 153. [5] C.J. Fisher, R. Ithin, R.G. Jones, G.J. Jackson, D.P. Woodruff, B.C.C. Cowie, J. Phys. Condens. Matter. 10 (1998) L623.
[6] V.I. Nefedov, I.S. Nefedova, J. Electron Spectrosc. Relat. Phenom. 107 (2000) 131. [7] V.I. Nefedov, I.S. Nefedova, J. Electron Spectrosc. Relat. Phenom. 113 (2000) 3. ´ R. Szargan, Fresenius J. Anal. Chem. [8] R. Hesse, T. Chasse, 365 (1999) 48. [9] I.S. Tilinin, W.S.M. Werner, Surf. Sci. 290 (1993) 119. [10] V.I. Nefedov, V.G. Yarzhemsky, I.S. Nefedova, M.B. Trzhaskovskaya, I.M. Band, J. Electron Spectrosc. Relat. Phenom. 107 (2000) 123. [11] V.I. Nefedov, I.S. Nefedova, J. Electron Spectrosc. Relat. Phenom. 95 (1998) 281.