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X-ray excited KLL and KLM Auger spectra of manganese
JOURNAL OF ELECTRON SPECTROSCOPY and RelatedPhenomena
ELSEVIER
Journal of Electron Spectroscopyand Related Phenomena 70 (1995) 183-192
X-ray excited K L L and K L M Auger spectra of manganese A . N 6 m e t h y a, L. K 6 v 6 r a'*, I. C s e r n y a, D . V a r g a ~, P . B . B a r n a b aInstitute of Nuclear Research of the Hungarian Academy of Sciences, P.O. Box 51, H-4001 Debrecen, Hungary bResearch Institutefor Technical Physics of the Hungarian Academy of Sciences, P.O. Box 132, H-1518 Budapest, Hungary
First received20 May 1994; in final form 28 June 1994
Abstract
High resolution (AE < 1.5 eV) manganese KLL and K L M Auger spectra, obtained by photoexcitation, are presented, and the transition energies and intensities are compared with the experimental and theoretical data published earlier. Significant disagreement can be observed between the respective experimental intensity values in the cases of the most intense lines, and large differences are found between the experimental and the available theoretical transition energy values of the main peaks. The disagreements between the experimental data can be explained rather by the different chemical states of the manganese samples investigated in the separate experiments than by the different mechanisms of the Auger processes. Keywords: Auger; XAES; XPS
1. Introduction
There are various electron spectroscopic methods for studying Auger transitions, and a great many papers have already been published dealing with low (below 3 keV) energy transitions; however, few accurate experimental data are available for the higher energy transitions owing to the difficulties regarding the effective ionization of the deep inner shells and the precise energy analysis of high energy Auger electrons. Experimental study of the K Auger transitions is essential from the point of view of the adequate description of Auger processes, in particular in the atomic number region Z = 15-30, where intermediate coupling is usually applied in theoretical calculations. Accurate experimental data are * Corresponding author.
needed for checking the validities of the different theoretical models. There are only a few publications available on experimental manganese K Auger spectra [1-3]. In those studies, the K Auger spectra of manganese from 55Fe decay were examined. During radioactive decay a l s electron is captured into the nucleus, whereby the nuclear charge is decreased and a K-shell vacancy is produced. At the same time an outer electron finds itself excited to an unoccupied state. At the end of the nuclear decay process the atom remains neutral, because both the nuclear charge and the number of electrons in the atom have decreased by the same quantity. In consequence of the above-mentioned processes, rearrangements - - e.g. shake-up or other excitations - - are taking place in the electron shells. In the case of photoionization, however, an electron leaves the atom due to photoexcitation, while
0368-2048/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0368-2048(94)02236-4
A. N~methy et al./Journal of Electron Spectroscopy and Related Phenomena 70 (1995) 183-192
184
a core hole is produced and a positive ion is created. These processes can be accompanied by electronic rearrangements similar to those in the case of electron capture. For Auger processes following electron capture or photoionization the initial state electron configurations are different. For manganese in the two cases there are different numbers of electrons in the 3d subshell. During electron capture, owing to the type of excitation, there are no disturbing photoelectron peaks and the continuous background is low as well. In the case of photoexcitation using bremsstrahlung, the peak to background ratio is smaller because the induced photoelectrons increase
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the continuous background. Combining XAES (X-ray excited Auger electron spectroscopy) with XPS (X-ray excited photoelectron spectroscopy), however, gives the possibility of checking the quality and composition of the sample. Moreover, a necessary condition for the excitation of transitions following electron capture is to have an existing radioactive isotope - - with a half-life suitable from the point of view of the experiments that creates the necessary initial state (this isotope is 55Fe in the case of manganese [1-3]), and for this reason the field of the application of this method is very limited. An alternative method for exciting Auger spectra is to use an electron beam. This method, however,
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Fig. 1. The measured Mn KLL Auger spectrum excited by Mo bremsstrahlung with a resolution of 1.3 eV, compared with the spectrum obtained earlier [3] from 55Fe decay (inset) with a resolution of 7 eV.
A. NOmethy et al./Journal of Electron Spectroscopy and Related Phenomena 70 (1995) 183 192
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Fig. 2. Photoexcited Mn KL23Lz3 Auger spectrum from the present work with a resolution of 1.3 eV, compared with the spectrum [3] obtained from Auger transitions following electron capture in 5SFe (inset) with a resolution of 2.2 eV.
leads to a large background and a very small peak to background ratio in the high energy range and requires an electron beam energy about four times the binding energy of the inner shell to be ionized. In this paper we present the results of high resolution X-ray excited Auger measurements of manganese compared with the best earlier Auger spectra from 55Fe decay [3] and with our theoretical estimations and those of others [4-8].
2. Experimental Manganese films of 10 nm thickness were evaporated onto Si wafer substrates in vacuum with a deposition rate of 2 n m s -1, and were cleaned and
thinned in situ before measurements by argon ion sputtering with an AGS-2 ion gun using a beam energy of 4.5 keV and a current of 10-30 #A. The sputtered area was 1 cm 2. The manganese oxide (MnO) samples were analytically pure grade polycrystalline powders. X-ray induced manganese Auger spectra were measured with a home built high energy, high resolution electron spectrometer [11,12] consisting of a 180 ° hemispherical electrostatic analyser with a 250mm working radius, floatable up to 10kV and completed with a multi-element zoom electronoptical lens. The analyzer works in the pass energy range 20-500eV with a resolution of 5 x 10 -3. In the present measurements the fixed retardation ratio (FRR) working mode was used with a
A. N~methy et al./Journal of Electron Spectroscopy and Related Phenomena 70 (1995) 183-192
186
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Relative Energy (eV) Fig. 3. The Mn KLM Auger spectrum excited by Mo bremsstrahlung with a resolution of 1.5 eV, compared with that measured
previously[31from 55Fedecay(inset) with a resolutionof ,~ 7eV. retardation factor of 20, ensuring an absolute instrumental resolution of 1.25-1.5eV in the energy range of manganese K L L and K L M Auger spectra. A maximum 30 kV/30mA dual-anode (AI,Mo) X-ray source (with an A1 filter foil of 0.9 #m thickness) was used in the measurements. The Mn K Auger transitions were excited by using Mo bremsstrahlung (20-25 kV/20-25 mA) and photoelectron lines were measured by using AI K a excitation (15 kV/20mA) for checking the quality of the samples and Cu K a excitation for obtaining the manganese ls photoelectron spectra. During measurements the vacuum was better than 10-7 Pa. The energy calibration of the spectrometer for high energies was performed by using Cu Kal,2
excited photoelectron lines (Ag2s, Ag2pv2, Ag2p3/2, Au3p3/2, Au3d3/2 and Au3ds/2 in the kinetic energy range 4220-5840eV) of metallic silver and gold samples [13-15] cleaned by in situ Ar ion sputtering prior to measurements. Cu K a characteristic X-rays can be obtained from below the thin molybdenum anode layer at 25 kV per 25 mA. Energy calibration in the XPS region was made with the aid of A1Ka excited photopeaks (Ag3ds/2, Cu2p3/2, Cu3p and Au4f7/2) and Auger lines (Ag M 4 N N and Cu L a M M ) of the same polycrystalline silver, gold and copper samples [16]. Corrections for charging effects in the case of manganese oxide samples were made by using the C ls line from the surface hydrocarbon with
4. N~methy et al./Journal of Electron Spectroscopy and Related Phenomena 70 (1995) 183-192
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A. N~methy et al./Journal of Electron Spectroscopy and Related Phenomena 70 (1995) 183-192
Table 2 M n K L M Auger transition energies and intensities related to the most intensive Mn KL2M3(ID2) peak Peak
Energies (eV)
Intensities (%)
Experiment
KLIM l
Theory
Experiment
Theory
Present work
Ref. [3]
Ref. [7]
Ref. [8]
Present work
Ref. [3]
-156.6 (0.8)
-153.0 (1.0)
-155.8 [Is0] -151.0 [3S1]
-135.30
23.7 (2.0)
20.1 (1.0)
-118.3 (1.3)
-119.5 (1.0)
33.3 (2.0)
35.3 (1.5)
KL1M2 KL1M3 KLaN
-67.2 (1.4)
-70.0 (2.0)
KL~M l
-33.3 (1.1)
-38.0 (2.0)
KL3M l
-21.7 (0.8)
-24.0 (2.0)
-117.2 -115.8 -115.8 -115.8 -
KL3M2 9.57 (0.7)
11.0 (1.0)
KL3M3
[Ip~] [3p0] [3p1] [3p2]
Ref. [5]
Ref. [8]
50.0
50.4
26.5
26.6
48.4
48.8
-104.81 -103.50 -49.97
5.3 (1.1)
3.1 (0.8)
6.1
4.6
-36.1 [3p0] [~P~] -35.3
-31.80
4.7 (1.7)
5.9 (3.1)
24.0
23.9
-25.4 [3p2] [3p~] -24.2
-21.05
25.8 (1.6)
17.0 (3.8)
43.8
44.4
12.0 [3S1] 10.0 [3p2]
9.44
100.1
100.0
7.8 [3p0] 12.3 [3D2] 14.1 [3D3]
10.75
113.6
113.3
1.7 1.2 7.9 1.3 4.0
3.2 4.0 3.9 4.0
31.3 (2.4)
60.1 (5.1)
14.4 [1Pl]
KL2M4 KL2M5 KL3M4 KL3M 5 KL3 N
48.8 (0.9)
54.0 (2.0)
-
60.5 (0.9)
63.0 (1.0)
-
49.06 59.68 59.81 64.28
284.9 eV obtained from our XPS measurements performed on metallic silver samples. The measured spectra were evaluated by the computer program EWA [17]. Peak shapes were described by pseudo-Voigtians with inelastic tails Ebind =
4.9 (0.9)
2.3 (1.0)
5.9 (0.9)
9.7 (1.5)
[17] applying a Tougaard type inelastic background [18]. The energy, height and FWHM parameter values of each line in the spectra (except those of plasmon peaks) were fitted independently, while the remaining five parameters (such
Table 3 Mn KL2L 3 and KL2M 3 Auger energies in eV Peak
Theory
Experiment Present work
Present work
Ref. [3]
Metallic M n
MnO
Extra peak in M n O
"Metallic" M n
Extra peak
5208.2 (0.8)
5203.3 (1.2)
5195.4 (1.0)
5202.5 (2.5) a
5194.3 (2.5) a 5195.5 (2.5) a
KL2M3(1D2) 5837.6 (0.9)
5832.8 (1.7)
KLEL3(ID2)
a Auger transition following electron capture.
-
5830.8 (2.0) a
-
5192.41 5212.1 a 5823.9 5841.3 a
Ref. [6]
Ref. [7] Ref. [8]
5202.8 a
5191.7
-
5195.71
5787.15 5826.9
A. N~methy et al./Journal of Electron Spectroscopy and Related Phenomena 70 (1995) 183-192
as Lorentzian-Gaussian ratio, asymmetry parameter, B and C parameter of the Tougaard type inelastic tail and exponential slope) were assumed to be the same for all Auger lines. A common, constant component of the background was also assumed for the KLL spectra and a polynomial one for the KLM spectrum.
3. Results and discussion
The spectra in Figs. 1, 2 and 3 demonstrate our better instrumental resolution compared with the spectra reported in ref. [3]. Especially, in several cases (see e.g. K L 3 L 3 (3p2), K L 3 M I , KL2,3M4,5,
and other lines), more resolved details can be
189
observed in our spectra. Moreover, our spectra show that the inelastic tails of the peaks can be considerably reduced using thin films as samples in the case of photoexcitation, and the peak to background ratios are comparable with (although still lower than) those obtainable by using very thin radioactive sources. The relative energies of the manganese Auger transitions are compared with the results of the previous experiment [3] and of different calculations for free atoms [6-8] in Tables 1 and 2. Our experimental values are similar to the data obtained from the earlier measurement [3] in most cases with the exception of the KL1L1(tSo) and KLIL2(1p1) lines. The disagreements can be explained by the different line shapes applied and
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Relative Energy (eV) Fig. 4. Photoexcited Mn KL2,3L2, 3 Auger spectrum of MnO powder with a resolution of 1.3 eV, compared with the Mn KL2,3L2, 3 spectrum [3] of "metallic" Mn obtained from Auger transitions following electron capture in 55Fe (inset) with a resolution of 3 eV.
190
A. NOmethyet al./Journal of Electron Spectroscopy and Related Phenomena 70 (1995) 183-192
the inelastic background being taken into consideration in different ways in the two studies. A better agreement can be observed between the present experimental data and the theoretical data obtained from earlier calculations [6,7] except in Ref. [8], which is a less accurate estimation because it assumes, e.g., that the binding energies of subshells are the same for neutral ground state atoms as for ionized atoms. Absolute energy values of KL2L3(1D2) and KL2M3(lD2) transitions are presented in Table 3. Our theoretical manganese KL2L3(1D2) and KL2M3(1D2) transition energies were calculated from the difference in the total energies of the initial and the final states for the cases of Auger transitions following photoionization and electron capture, respectively, using a relativistic D i r a c Fock approximation. For estimating the respective total shake probabilities, relativistic D i r a c Fock-Slater wavefunctions and sudden approximation were used [19]. The present experimental values are higher than the corresponding theoretical data, which can be explained by the fact that the calculations are related to free atoms and referred to the vacuum level whereas the experimental data are obtained in the case of solids and are related to the Fermi level. It is interesting to note that the present experimental Auger energy values are higher than those obtained following electron capture [3] in spite of the fact that the present and other theoretical estimations [6-8] predict the contrary, as can be seen in Table 3. Comparing the shape of the photoexcited manganese KLL spectrum of MnO with the manganese spectra obtained earlier [3] from 55Fe decay, however, they show a definite similarity (Fig. 4). In both spectra an extra peak can be found at the low energy side of the KL2L3(1D2) line at the same energy distance (Table 3). Moreover, the measured energy values of the photoexcited KLL and K L M transitions of the MnO sample agree with the respective data of the previous experiment (Table 3), similar to the case of the air-oxidized sample. An overall agreement can be seen between the relative Auger intensities from the respective experiments (Tables 1 and 2) with a few exceptions: the KLIL2(1pI), KLzL2(ISo) and KL3M2, 3
lines, which are, however, intense lines alongside the main peaks. The deviations can arise partly from the different chemical environments (in the present work pure metallic manganese samples were investigated, the quality of which was checked by XPS, whereas in the 55Fe sample [3] there were also undecayed iron atoms present as well as manganese atoms and the samples could become oxidized during preparation [3]). The determined intensity ratio values can be strongly dependent on the type of the inelastic background correction as well as on the peak shape applied. The disagreements between the respective experimental and theoretical intensity values (Tables 1 and 2) can be explained by the less exact theoretical estimation methods applied in the case of ref. [8], as mentioned above. The calculation referred to in [4], which used intermediate coupling with configuration interaction, gives a better agreement for the manganese KLL intensities (although there are still significant differences between those and the present experimental values) than the calculation in ref. [5], which used j - j coupling, or the estimation referred to in [8]. In the manganese KL2,3L2,3 spectrum measured by Kovalik et al. [3], an extra peak can be seen at the low energy side of the KL2L3(1D2) line (see the inset in Fig. 2). In the photoexcited KL2,3L2,3 spectrum, however, no extra line can be identified at that position (Figs. 1 and 2) in spite of the better instrumental resolution used. This difference in the results of the two experiments cannot be attributed to the different atomic excitation probabilities on the basis of the calculated intensity values [9,10] presented in Table 4, because in the case of the photoionization the estimated total shake probability is ~50 times higher than in the case of electron capture (at least for free atoms). On the basis of the absolute KLL
Table 4 Total shake probabilities (%) for 1s photoionizationof Mn and ls electroncapture of 55Fe Process
Present Ref.[9] Ref.[10] work
Photoionization (ls 1... 3d5) Electron capture (ls I ... 3d6)
26.64 0.51
33.10 0.68
0.38
A. Nrmethy et al./Journal of Electron Spectroscopy and Related Phenomena 70 (1995) 183-192
191
Table 5 Manganese K Auger parameters from experimental data for metallic Mn: a ~ = EKin(Auger line)+Ebina(photoelectron line) Lines
Is
KL1L l KL~L2 KLxL2, 3 KL1L 3 KL2L 2 KL2L 3
11509.5 11616.6 11645.1 11654.3 11728.3 11746.5 11757.0 11765.9 12219.3 12257.6 12308.7 12342.6 12354.2 12375.9 12385.5 12424.7 12436.4
KL3L3 KL1M 1 KL1M2, 3 KLaN KL2M ~ KLaM ~ KL2M 3 KL3M2,3 KL2M4,5 KL3M4, 5
(1.6) (0.5) (0.8) (0.6) (0.6) (0.6) (0.6) (0.6) (0.6) (1.1) (1.1) (1.0) (0.7) (0.7) (0.7) (0.8) (1.0)
2s
2pl/2
5740.8 (1.8) 5847.9 (0.7) 5876.4 (1.0) 5885.6 (0.8) 5959.6 (0.8) 5977.8 (0.8) 5988.3 (0.8) 5997.2 (0.8) 6450.6 (0.9) 6488.9 (1.3) 6540.0 (1.3) 6573.9 (1.3) 6585.5 (1.0) 6607.2 (1.0) 6616.8 (1.0) 6656.0 (1.1) 6667.7 (1.2)
5621.8 5728.9 5757.4 5766.6 5840.6 5858.8 5869.3 5878.2 6331.6 6369.9 6421.0 6454.9 6466.5 6488.2 6497.8 6537.0 6548.7
2p3/2 (1.3) (0.5) (0.6) (0.4) (0.4) (0.4) (0.5) (0.5) (0.6) (1.1) (1.1) (1.0) (0.6) (0.5) (0.6) (0.8) (0.9)
5610.9 5718.0 5746.5 5755.5 5829.7 5847.9 5858.4 5867.3 6320.7 6359.0 6410.1 6444.0 6455.6 6477.3 6486.9 6526.1 6537.8
(1.2) (0.4) (0.5) (0.4) (0.4) (0.3) (0.5) (0.5) (0.5) (1.0) (1.0) (0.9) (0.5) (0.4) (0.5) (0.7) (0.8)
Values attached to a main peak are given in bold type and data attached to a photoelectron line which does not take part in the Auger transition are shown in small type.
Auger energies of metallic Mn and MnO (see Table 3) obtained experimentally, however, it can be assumed that the sample investigated by Kovalik et al. was not in a metallic but in a different, probably an oxidized, chemical state. This assumption is supported by the shape of the measured MnO Auger spectrum in which there is an extra peak at the low energy side of the main line (Fig. 4) at an energy distance of 7.9 eV. The extra peak may originate from a shake process or a multiplet splitting [20] in a manganese compound (e.g. in MnO the Mn 2p3/2 line shows a satellite structure at an energy distance of ~ 6 - 7 eV (21,22]) and in small part from inelastic energy losses in the MnO sample itself (based on our
electron energy loss spectroscopic measurements), although in the earlier work [3] the authors were reluctant to consider the extra line to be a plasmon peak. Manganese Auger parameters are presented in Table 5 for metallic manganese and in Table 6 for manganese oxide. The values obtained from experimental data are calculated with the a ' = EKin (Auger line)+EBind (Photoelectron line) formula. This kind of Auger parameter is related to the chemical environment of the ionized atom and is independent of reference level and charging effects. Therefore it is an important source of chemical information. Auger parameters reflect extra-atomic
Table 6 Manganese K Auger parameters from experimental data for MnO Lines
1s
KL2L 2 KL2L 3
11724.0 11743.5 11753.3 11764.8
KL3L3
(1.9) (1.9) (1.9) (1.9)
2s
2pl/2
5955.5 (2.2) 5975.0 (2.1) 5984.8 (2.2) 5996.3 (2.2)
5837.4 5856.9 5866.7 5878.2
2p3/2 (1.3) (1.2) (1.3) (1.3)
5825.9 5845.4 5855.2 5866.7
(1.3) (1.2) (1.3) (1.3)
Values attached to a main peak are given in bold type and data attached to a photoelectron line which does not take part in the Auger transition are shown in small type.
192
A. N~methy et al./Journal of Electron Spectroscopy and Related Phenomena 70 (1995) 183 192
relaxation and screening effects. An advantage of the photoexcitation method (as opposed to electron capture) is that photopeaks can be measured using characteristic X-ray radiation, and the calculation of Auger parameters is possible.
4. Summary In this work, precise experimental data are presented for the manganese KLL and K L M Auger transitions excited by X-rays. Indispensable conditions for obtaining adequate results were a high energy, high resolution spectrometer, good quality thin film samples and spectrum analysis based on physically reliable models. Our results show that precise data can be obtained for high energy Auger transitions using bremsstrahlung X-ray excitation. Comparing the experimental data to the theoretical ones, in spite of the significant disagreements between the respective measured and estimated absolute energy and relative intensity values, calculation [4] using intermediate coupling with configuration interaction seems to give the best agreement with the experimental results for the manganese KLL Auger transitions. Comparing the photoexcited experimental data with the measured values obtained from 55Fe radioactive decay, the disagreements and the extra peak close to the KLzL3(1D2) line can be explained by the different chemical state of the samples rather than by the different mechanism of Auger processes.
Acknowledgement This work was partially supported by the research projects COST/D5/12014 (CEC) and OTKA/T007274/1993 (Hungarian).
References [1] Y.Y. Lui and R.G. Albridge, Nucl. Phys., A92 (1967) 139. [2] V. Brabec, O. Dragoun, M. Ry~avT~,M. Fi~er, A. Kovalik, I. Cserny and I. K/tdfir, Z. Phys. A, 311 (1983) 37. [3] A. Kovalik, V. Brabec, J. Nov/tk, O. Dragoun, V.M. Gorozhankin, A.F. Novgorodov and Ts. Vylov, J. Electron Spectrosc. Relat. Phenom., 50 (1990) 89. [4] M.H. Chen, B. Crasemann and H. Mark, Phys. Rev. A, 21 (1980) 442. [5] M.H. Chen, B. Crasemann and H. Mark, At. Data Nucl. Data Tables, 24 (1979) 13. [6] L. Bandurina, O. Zatsarinny, A. Lengyel, E.Yu Remeta, X'93 Conference, Debrecen, 1993, Book of Abstracts. [7] F.P. Larkins, At. Data Nucl. Data Tables, 20 (1977) 311. [8] S.T. Perkins, D.E. Cullen, M.H. Chen, J.H. Hubbel, J. Rathkopf, J. Scofield, Tables and Graphs of Atomic Subshells and Relaxation Data Derived from the LLNL Evaluated Atomic Data Library (EADL), Z = 1 100, Lawrence Livermore National Laboratory, Livermore, CA, UCRL-50400, Vol. 30 (1991). [9] E. Arndt, G. Brunner and E. Hartmann, J. Phys. B, 15 (1982) L887. [10] B. Crasemann, M.H. Chen, J.P. Briand, P. Chevallier, A. Chetioui and M. Tavernier, Phys. Rev. C, 19 (1979) 1042. [11] D. Varga, L. K6v~r, I. Cserny, J. T6th and K. T6k~si, Surf. Interface Anal., 19 (1992) 9. [12] L. K6v~r, D. Varga, I. Cserny, J. T6th and K. T6k~si, to be published. [13] C. Nordling, Ark. Fys., 15 (1959) 397. [14] A. Fahlman and S. Hagstr6m, Ark. Fys., 27 (1964) 69. [15] P. Bergvall, O. H6rnfeldt and C. Nordling, Ark. Fys., 17 (1960) 113. [16] M.P. Scab, G.C. Smith and M.T. Anthony, Surf. Interface Anal., 15 (1990) 293. [17] J. V~gh, PhD. Thesis, Institute of Nuclear Research of the Hungarian Academy of Sciences, Debrecen, 1990. [18] S. Tougaard, Surf. Interface Anal., 11 (1988)453. [19] T. Mukoyama and K. Taniguchi, Bull. Inst. Chem. Res. Kyoto Univ., 70 (1992) 1. [20] T.A. Carlson, Photoelectron and Auger Spectroscopy, Plenum Press, New York, 1975. [21] J. van Elp, R.H. Potze, H. Eskes, R. Berger and G.A. Sawatzky, Phys. Rev. B, 44 (1991) 4. [22] R.J. Lad and V.E. Henrich, Phys. Rev. B, 38 (1988) 15.
ELSEVIER
Journal of Electron Spectroscopyand Related Phenomena 70 (1995) 183-192
X-ray excited K L L and K L M Auger spectra of manganese A . N 6 m e t h y a, L. K 6 v 6 r a'*, I. C s e r n y a, D . V a r g a ~, P . B . B a r n a b aInstitute of Nuclear Research of the Hungarian Academy of Sciences, P.O. Box 51, H-4001 Debrecen, Hungary bResearch Institutefor Technical Physics of the Hungarian Academy of Sciences, P.O. Box 132, H-1518 Budapest, Hungary
First received20 May 1994; in final form 28 June 1994
Abstract
High resolution (AE < 1.5 eV) manganese KLL and K L M Auger spectra, obtained by photoexcitation, are presented, and the transition energies and intensities are compared with the experimental and theoretical data published earlier. Significant disagreement can be observed between the respective experimental intensity values in the cases of the most intense lines, and large differences are found between the experimental and the available theoretical transition energy values of the main peaks. The disagreements between the experimental data can be explained rather by the different chemical states of the manganese samples investigated in the separate experiments than by the different mechanisms of the Auger processes. Keywords: Auger; XAES; XPS
1. Introduction
There are various electron spectroscopic methods for studying Auger transitions, and a great many papers have already been published dealing with low (below 3 keV) energy transitions; however, few accurate experimental data are available for the higher energy transitions owing to the difficulties regarding the effective ionization of the deep inner shells and the precise energy analysis of high energy Auger electrons. Experimental study of the K Auger transitions is essential from the point of view of the adequate description of Auger processes, in particular in the atomic number region Z = 15-30, where intermediate coupling is usually applied in theoretical calculations. Accurate experimental data are * Corresponding author.
needed for checking the validities of the different theoretical models. There are only a few publications available on experimental manganese K Auger spectra [1-3]. In those studies, the K Auger spectra of manganese from 55Fe decay were examined. During radioactive decay a l s electron is captured into the nucleus, whereby the nuclear charge is decreased and a K-shell vacancy is produced. At the same time an outer electron finds itself excited to an unoccupied state. At the end of the nuclear decay process the atom remains neutral, because both the nuclear charge and the number of electrons in the atom have decreased by the same quantity. In consequence of the above-mentioned processes, rearrangements - - e.g. shake-up or other excitations - - are taking place in the electron shells. In the case of photoionization, however, an electron leaves the atom due to photoexcitation, while
0368-2048/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0368-2048(94)02236-4
A. N~methy et al./Journal of Electron Spectroscopy and Related Phenomena 70 (1995) 183-192
184
a core hole is produced and a positive ion is created. These processes can be accompanied by electronic rearrangements similar to those in the case of electron capture. For Auger processes following electron capture or photoionization the initial state electron configurations are different. For manganese in the two cases there are different numbers of electrons in the 3d subshell. During electron capture, owing to the type of excitation, there are no disturbing photoelectron peaks and the continuous background is low as well. In the case of photoexcitation using bremsstrahlung, the peak to background ratio is smaller because the induced photoelectrons increase
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the continuous background. Combining XAES (X-ray excited Auger electron spectroscopy) with XPS (X-ray excited photoelectron spectroscopy), however, gives the possibility of checking the quality and composition of the sample. Moreover, a necessary condition for the excitation of transitions following electron capture is to have an existing radioactive isotope - - with a half-life suitable from the point of view of the experiments that creates the necessary initial state (this isotope is 55Fe in the case of manganese [1-3]), and for this reason the field of the application of this method is very limited. An alternative method for exciting Auger spectra is to use an electron beam. This method, however,
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Fig. 1. The measured Mn KLL Auger spectrum excited by Mo bremsstrahlung with a resolution of 1.3 eV, compared with the spectrum obtained earlier [3] from 55Fe decay (inset) with a resolution of 7 eV.
A. NOmethy et al./Journal of Electron Spectroscopy and Related Phenomena 70 (1995) 183 192
185
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Fig. 2. Photoexcited Mn KL23Lz3 Auger spectrum from the present work with a resolution of 1.3 eV, compared with the spectrum [3] obtained from Auger transitions following electron capture in 5SFe (inset) with a resolution of 2.2 eV.
leads to a large background and a very small peak to background ratio in the high energy range and requires an electron beam energy about four times the binding energy of the inner shell to be ionized. In this paper we present the results of high resolution X-ray excited Auger measurements of manganese compared with the best earlier Auger spectra from 55Fe decay [3] and with our theoretical estimations and those of others [4-8].
2. Experimental Manganese films of 10 nm thickness were evaporated onto Si wafer substrates in vacuum with a deposition rate of 2 n m s -1, and were cleaned and
thinned in situ before measurements by argon ion sputtering with an AGS-2 ion gun using a beam energy of 4.5 keV and a current of 10-30 #A. The sputtered area was 1 cm 2. The manganese oxide (MnO) samples were analytically pure grade polycrystalline powders. X-ray induced manganese Auger spectra were measured with a home built high energy, high resolution electron spectrometer [11,12] consisting of a 180 ° hemispherical electrostatic analyser with a 250mm working radius, floatable up to 10kV and completed with a multi-element zoom electronoptical lens. The analyzer works in the pass energy range 20-500eV with a resolution of 5 x 10 -3. In the present measurements the fixed retardation ratio (FRR) working mode was used with a
A. N~methy et al./Journal of Electron Spectroscopy and Related Phenomena 70 (1995) 183-192
186
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~ / / J
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Relative Energy (eV) Fig. 3. The Mn KLM Auger spectrum excited by Mo bremsstrahlung with a resolution of 1.5 eV, compared with that measured
previously[31from 55Fedecay(inset) with a resolutionof ,~ 7eV. retardation factor of 20, ensuring an absolute instrumental resolution of 1.25-1.5eV in the energy range of manganese K L L and K L M Auger spectra. A maximum 30 kV/30mA dual-anode (AI,Mo) X-ray source (with an A1 filter foil of 0.9 #m thickness) was used in the measurements. The Mn K Auger transitions were excited by using Mo bremsstrahlung (20-25 kV/20-25 mA) and photoelectron lines were measured by using AI K a excitation (15 kV/20mA) for checking the quality of the samples and Cu K a excitation for obtaining the manganese ls photoelectron spectra. During measurements the vacuum was better than 10-7 Pa. The energy calibration of the spectrometer for high energies was performed by using Cu Kal,2
excited photoelectron lines (Ag2s, Ag2pv2, Ag2p3/2, Au3p3/2, Au3d3/2 and Au3ds/2 in the kinetic energy range 4220-5840eV) of metallic silver and gold samples [13-15] cleaned by in situ Ar ion sputtering prior to measurements. Cu K a characteristic X-rays can be obtained from below the thin molybdenum anode layer at 25 kV per 25 mA. Energy calibration in the XPS region was made with the aid of A1Ka excited photopeaks (Ag3ds/2, Cu2p3/2, Cu3p and Au4f7/2) and Auger lines (Ag M 4 N N and Cu L a M M ) of the same polycrystalline silver, gold and copper samples [16]. Corrections for charging effects in the case of manganese oxide samples were made by using the C ls line from the surface hydrocarbon with
4. N~methy et al./Journal of Electron Spectroscopy and Related Phenomena 70 (1995) 183-192
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188
A. N~methy et al./Journal of Electron Spectroscopy and Related Phenomena 70 (1995) 183-192
Table 2 M n K L M Auger transition energies and intensities related to the most intensive Mn KL2M3(ID2) peak Peak
Energies (eV)
Intensities (%)
Experiment
KLIM l
Theory
Experiment
Theory
Present work
Ref. [3]
Ref. [7]
Ref. [8]
Present work
Ref. [3]
-156.6 (0.8)
-153.0 (1.0)
-155.8 [Is0] -151.0 [3S1]
-135.30
23.7 (2.0)
20.1 (1.0)
-118.3 (1.3)
-119.5 (1.0)
33.3 (2.0)
35.3 (1.5)
KL1M2 KL1M3 KLaN
-67.2 (1.4)
-70.0 (2.0)
KL~M l
-33.3 (1.1)
-38.0 (2.0)
KL3M l
-21.7 (0.8)
-24.0 (2.0)
-117.2 -115.8 -115.8 -115.8 -
KL3M2 9.57 (0.7)
11.0 (1.0)
KL3M3
[Ip~] [3p0] [3p1] [3p2]
Ref. [5]
Ref. [8]
50.0
50.4
26.5
26.6
48.4
48.8
-104.81 -103.50 -49.97
5.3 (1.1)
3.1 (0.8)
6.1
4.6
-36.1 [3p0] [~P~] -35.3
-31.80
4.7 (1.7)
5.9 (3.1)
24.0
23.9
-25.4 [3p2] [3p~] -24.2
-21.05
25.8 (1.6)
17.0 (3.8)
43.8
44.4
12.0 [3S1] 10.0 [3p2]
9.44
100.1
100.0
7.8 [3p0] 12.3 [3D2] 14.1 [3D3]
10.75
113.6
113.3
1.7 1.2 7.9 1.3 4.0
3.2 4.0 3.9 4.0
31.3 (2.4)
60.1 (5.1)
14.4 [1Pl]
KL2M4 KL2M5 KL3M4 KL3M 5 KL3 N
48.8 (0.9)
54.0 (2.0)
-
60.5 (0.9)
63.0 (1.0)
-
49.06 59.68 59.81 64.28
284.9 eV obtained from our XPS measurements performed on metallic silver samples. The measured spectra were evaluated by the computer program EWA [17]. Peak shapes were described by pseudo-Voigtians with inelastic tails Ebind =
4.9 (0.9)
2.3 (1.0)
5.9 (0.9)
9.7 (1.5)
[17] applying a Tougaard type inelastic background [18]. The energy, height and FWHM parameter values of each line in the spectra (except those of plasmon peaks) were fitted independently, while the remaining five parameters (such
Table 3 Mn KL2L 3 and KL2M 3 Auger energies in eV Peak
Theory
Experiment Present work
Present work
Ref. [3]
Metallic M n
MnO
Extra peak in M n O
"Metallic" M n
Extra peak
5208.2 (0.8)
5203.3 (1.2)
5195.4 (1.0)
5202.5 (2.5) a
5194.3 (2.5) a 5195.5 (2.5) a
KL2M3(1D2) 5837.6 (0.9)
5832.8 (1.7)
KLEL3(ID2)
a Auger transition following electron capture.
-
5830.8 (2.0) a
-
5192.41 5212.1 a 5823.9 5841.3 a
Ref. [6]
Ref. [7] Ref. [8]
5202.8 a
5191.7
-
5195.71
5787.15 5826.9
A. N~methy et al./Journal of Electron Spectroscopy and Related Phenomena 70 (1995) 183-192
as Lorentzian-Gaussian ratio, asymmetry parameter, B and C parameter of the Tougaard type inelastic tail and exponential slope) were assumed to be the same for all Auger lines. A common, constant component of the background was also assumed for the KLL spectra and a polynomial one for the KLM spectrum.
3. Results and discussion
The spectra in Figs. 1, 2 and 3 demonstrate our better instrumental resolution compared with the spectra reported in ref. [3]. Especially, in several cases (see e.g. K L 3 L 3 (3p2), K L 3 M I , KL2,3M4,5,
and other lines), more resolved details can be
189
observed in our spectra. Moreover, our spectra show that the inelastic tails of the peaks can be considerably reduced using thin films as samples in the case of photoexcitation, and the peak to background ratios are comparable with (although still lower than) those obtainable by using very thin radioactive sources. The relative energies of the manganese Auger transitions are compared with the results of the previous experiment [3] and of different calculations for free atoms [6-8] in Tables 1 and 2. Our experimental values are similar to the data obtained from the earlier measurement [3] in most cases with the exception of the KL1L1(tSo) and KLIL2(1p1) lines. The disagreements can be explained by the different line shapes applied and
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Relative Energy (eV) Fig. 4. Photoexcited Mn KL2,3L2, 3 Auger spectrum of MnO powder with a resolution of 1.3 eV, compared with the Mn KL2,3L2, 3 spectrum [3] of "metallic" Mn obtained from Auger transitions following electron capture in 55Fe (inset) with a resolution of 3 eV.
190
A. NOmethyet al./Journal of Electron Spectroscopy and Related Phenomena 70 (1995) 183-192
the inelastic background being taken into consideration in different ways in the two studies. A better agreement can be observed between the present experimental data and the theoretical data obtained from earlier calculations [6,7] except in Ref. [8], which is a less accurate estimation because it assumes, e.g., that the binding energies of subshells are the same for neutral ground state atoms as for ionized atoms. Absolute energy values of KL2L3(1D2) and KL2M3(lD2) transitions are presented in Table 3. Our theoretical manganese KL2L3(1D2) and KL2M3(1D2) transition energies were calculated from the difference in the total energies of the initial and the final states for the cases of Auger transitions following photoionization and electron capture, respectively, using a relativistic D i r a c Fock approximation. For estimating the respective total shake probabilities, relativistic D i r a c Fock-Slater wavefunctions and sudden approximation were used [19]. The present experimental values are higher than the corresponding theoretical data, which can be explained by the fact that the calculations are related to free atoms and referred to the vacuum level whereas the experimental data are obtained in the case of solids and are related to the Fermi level. It is interesting to note that the present experimental Auger energy values are higher than those obtained following electron capture [3] in spite of the fact that the present and other theoretical estimations [6-8] predict the contrary, as can be seen in Table 3. Comparing the shape of the photoexcited manganese KLL spectrum of MnO with the manganese spectra obtained earlier [3] from 55Fe decay, however, they show a definite similarity (Fig. 4). In both spectra an extra peak can be found at the low energy side of the KL2L3(1D2) line at the same energy distance (Table 3). Moreover, the measured energy values of the photoexcited KLL and K L M transitions of the MnO sample agree with the respective data of the previous experiment (Table 3), similar to the case of the air-oxidized sample. An overall agreement can be seen between the relative Auger intensities from the respective experiments (Tables 1 and 2) with a few exceptions: the KLIL2(1pI), KLzL2(ISo) and KL3M2, 3
lines, which are, however, intense lines alongside the main peaks. The deviations can arise partly from the different chemical environments (in the present work pure metallic manganese samples were investigated, the quality of which was checked by XPS, whereas in the 55Fe sample [3] there were also undecayed iron atoms present as well as manganese atoms and the samples could become oxidized during preparation [3]). The determined intensity ratio values can be strongly dependent on the type of the inelastic background correction as well as on the peak shape applied. The disagreements between the respective experimental and theoretical intensity values (Tables 1 and 2) can be explained by the less exact theoretical estimation methods applied in the case of ref. [8], as mentioned above. The calculation referred to in [4], which used intermediate coupling with configuration interaction, gives a better agreement for the manganese KLL intensities (although there are still significant differences between those and the present experimental values) than the calculation in ref. [5], which used j - j coupling, or the estimation referred to in [8]. In the manganese KL2,3L2,3 spectrum measured by Kovalik et al. [3], an extra peak can be seen at the low energy side of the KL2L3(1D2) line (see the inset in Fig. 2). In the photoexcited KL2,3L2,3 spectrum, however, no extra line can be identified at that position (Figs. 1 and 2) in spite of the better instrumental resolution used. This difference in the results of the two experiments cannot be attributed to the different atomic excitation probabilities on the basis of the calculated intensity values [9,10] presented in Table 4, because in the case of the photoionization the estimated total shake probability is ~50 times higher than in the case of electron capture (at least for free atoms). On the basis of the absolute KLL
Table 4 Total shake probabilities (%) for 1s photoionizationof Mn and ls electroncapture of 55Fe Process
Present Ref.[9] Ref.[10] work
Photoionization (ls 1... 3d5) Electron capture (ls I ... 3d6)
26.64 0.51
33.10 0.68
0.38
A. Nrmethy et al./Journal of Electron Spectroscopy and Related Phenomena 70 (1995) 183-192
191
Table 5 Manganese K Auger parameters from experimental data for metallic Mn: a ~ = EKin(Auger line)+Ebina(photoelectron line) Lines
Is
KL1L l KL~L2 KLxL2, 3 KL1L 3 KL2L 2 KL2L 3
11509.5 11616.6 11645.1 11654.3 11728.3 11746.5 11757.0 11765.9 12219.3 12257.6 12308.7 12342.6 12354.2 12375.9 12385.5 12424.7 12436.4
KL3L3 KL1M 1 KL1M2, 3 KLaN KL2M ~ KLaM ~ KL2M 3 KL3M2,3 KL2M4,5 KL3M4, 5
(1.6) (0.5) (0.8) (0.6) (0.6) (0.6) (0.6) (0.6) (0.6) (1.1) (1.1) (1.0) (0.7) (0.7) (0.7) (0.8) (1.0)
2s
2pl/2
5740.8 (1.8) 5847.9 (0.7) 5876.4 (1.0) 5885.6 (0.8) 5959.6 (0.8) 5977.8 (0.8) 5988.3 (0.8) 5997.2 (0.8) 6450.6 (0.9) 6488.9 (1.3) 6540.0 (1.3) 6573.9 (1.3) 6585.5 (1.0) 6607.2 (1.0) 6616.8 (1.0) 6656.0 (1.1) 6667.7 (1.2)
5621.8 5728.9 5757.4 5766.6 5840.6 5858.8 5869.3 5878.2 6331.6 6369.9 6421.0 6454.9 6466.5 6488.2 6497.8 6537.0 6548.7
2p3/2 (1.3) (0.5) (0.6) (0.4) (0.4) (0.4) (0.5) (0.5) (0.6) (1.1) (1.1) (1.0) (0.6) (0.5) (0.6) (0.8) (0.9)
5610.9 5718.0 5746.5 5755.5 5829.7 5847.9 5858.4 5867.3 6320.7 6359.0 6410.1 6444.0 6455.6 6477.3 6486.9 6526.1 6537.8
(1.2) (0.4) (0.5) (0.4) (0.4) (0.3) (0.5) (0.5) (0.5) (1.0) (1.0) (0.9) (0.5) (0.4) (0.5) (0.7) (0.8)
Values attached to a main peak are given in bold type and data attached to a photoelectron line which does not take part in the Auger transition are shown in small type.
Auger energies of metallic Mn and MnO (see Table 3) obtained experimentally, however, it can be assumed that the sample investigated by Kovalik et al. was not in a metallic but in a different, probably an oxidized, chemical state. This assumption is supported by the shape of the measured MnO Auger spectrum in which there is an extra peak at the low energy side of the main line (Fig. 4) at an energy distance of 7.9 eV. The extra peak may originate from a shake process or a multiplet splitting [20] in a manganese compound (e.g. in MnO the Mn 2p3/2 line shows a satellite structure at an energy distance of ~ 6 - 7 eV (21,22]) and in small part from inelastic energy losses in the MnO sample itself (based on our
electron energy loss spectroscopic measurements), although in the earlier work [3] the authors were reluctant to consider the extra line to be a plasmon peak. Manganese Auger parameters are presented in Table 5 for metallic manganese and in Table 6 for manganese oxide. The values obtained from experimental data are calculated with the a ' = EKin (Auger line)+EBind (Photoelectron line) formula. This kind of Auger parameter is related to the chemical environment of the ionized atom and is independent of reference level and charging effects. Therefore it is an important source of chemical information. Auger parameters reflect extra-atomic
Table 6 Manganese K Auger parameters from experimental data for MnO Lines
1s
KL2L 2 KL2L 3
11724.0 11743.5 11753.3 11764.8
KL3L3
(1.9) (1.9) (1.9) (1.9)
2s
2pl/2
5955.5 (2.2) 5975.0 (2.1) 5984.8 (2.2) 5996.3 (2.2)
5837.4 5856.9 5866.7 5878.2
2p3/2 (1.3) (1.2) (1.3) (1.3)
5825.9 5845.4 5855.2 5866.7
(1.3) (1.2) (1.3) (1.3)
Values attached to a main peak are given in bold type and data attached to a photoelectron line which does not take part in the Auger transition are shown in small type.
192
A. N~methy et al./Journal of Electron Spectroscopy and Related Phenomena 70 (1995) 183 192
relaxation and screening effects. An advantage of the photoexcitation method (as opposed to electron capture) is that photopeaks can be measured using characteristic X-ray radiation, and the calculation of Auger parameters is possible.
4. Summary In this work, precise experimental data are presented for the manganese KLL and K L M Auger transitions excited by X-rays. Indispensable conditions for obtaining adequate results were a high energy, high resolution spectrometer, good quality thin film samples and spectrum analysis based on physically reliable models. Our results show that precise data can be obtained for high energy Auger transitions using bremsstrahlung X-ray excitation. Comparing the experimental data to the theoretical ones, in spite of the significant disagreements between the respective measured and estimated absolute energy and relative intensity values, calculation [4] using intermediate coupling with configuration interaction seems to give the best agreement with the experimental results for the manganese KLL Auger transitions. Comparing the photoexcited experimental data with the measured values obtained from 55Fe radioactive decay, the disagreements and the extra peak close to the KLzL3(1D2) line can be explained by the different chemical state of the samples rather than by the different mechanism of Auger processes.
Acknowledgement This work was partially supported by the research projects COST/D5/12014 (CEC) and OTKA/T007274/1993 (Hungarian).
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