Auger and photoelectron spectra of K+ in solids at resonant 2p6 to 2p53d excitation

Journal of Electron Spectroscopy and Related Phenomena 72 (1995) 127-131

Auger and photoelectron spectra of K + in solids at resonant 2p 6 to 2p53d excitation M. Elango a, A. Ausmees b*, A. Kikasb*, A. Maisteb, E. N6mmisteb+, A. Saarb a Department of Experimental Physics and Technology, Tartu University, Ulikooli 18, EE2400Tartu, Estonia b Institute of Physics, Estonian Academy of Sciences, Riia 142, EE2400 Tartu, Estonia We present here the 2p-3d-resonant photoelectron and Auger spectra of K + in KMnF 3 and compare them with analogous spectra of KCI. The spectra are very similar for both compounds. The most conspicious structure which arises during the LMM Auger spectator decay of the 2p53d configuration is due to the 3p'23d final configuration. Its remarkable property, the linear dispersion in the whole resonance region, demonstrates that i) the Auger transition is so fast that at resonant excitation it is not possible separate it from the photon absorption process, ii) the L2L3M45 Coster-Kronig transition is an important link in the nonradiative decay of the resonantly-created L2 hole, and iii) the crystal-field splitting of the intermediate 2p53d configuration does not determine the general mechanism of nonradiative decay of this configuration.

The resonantly excited Auger and photoelectron spectra of K + ions as constituents of potassium halides have turned out to be sensitive probes to distinguish from each other the intraatomic and solid-state contributions to the cation-related electron structure of ionic solids 11-3]. At incident photon energies, h v, coinciding with the 2p6 -> 2p53d resonances in the L23 absorption spectra at 295 - 305 eV, the spectra of electrons emitted by K + ions contain a clearly distinguishable spectator structure at the high-energy side of the L23A423M23 normal Auger structure. With increasing photon energy this structure shifts to higher kinetic energy and thus resembles the ordinary photoelectron lines. Both empirical considerations and Hartree-FockPauli calculations have shown that the spectator structure originates from the 3p-23d final configuration. It was concluded on the ground of these findings that the spectator structure arises from the Auger-resonant inelastic scattering process 2p6 + h v -> 3p-23d + e. In addition, the 3s and 3p photoemission peaks of K + exhibit a resonant enhancement due to the 2p53d-> 3p-1 (3s-l) + e participator transitions. The absence of such an enhancement for the valence band of potassium halides indicates that the 3d electron of K + is

strongly localized at the parent ion and does not interact with the valence electrons, i. e. once again manifests the high ionicity of potassium halides. It is natural, therefore, that most of the pecularities of the autoionization phenomena involving the 2p53d configuration of K + in halides can be understood in terms of an atomic treatment. Among problems still open after these studies is how much the crystal-field splitting of the 3d orbital affects the decay of the 2p53d excited configuration of K +. To study this point in detail we compare here with each other these spectra for K + situated in KCI where the crystal field causes a splitting of about leV between t2g and eg components of the 3d orbital, and in a perovskite-structure KMnF3 where the crystal-field splitting of the 3d orbital of K + happens to be close to zero [41. The experiments were carried out using synchrotron radiation from beamline 22 at the MAX-laboratory, Lund University, Sweden [5]. The monochromator was a modified SX-700 plane grating monochromator with energy resolution better than 0.3 eV in the photon energy region of interest. The electron spectra were recorded by a hemispherical analyzer Scienta SES-200 with energy resolution set to 0.3 eV. The samples were

*Present address: Department of Physics, University of Uppsala, Box 530, S-75121 Uppsala +Present address: Filmish Synchrotron Radiation Facility at Max-lab, Box 118, S-22100 Lund, Sweden

0368-2048/95 $09.50 © 1995 Elsevier Science B.V. All rights reserved

128 films of KCI and KMnF 3 evaporated in situ from a molybdenum boat onto a stainless steel substrate in vacuum below 5 x 10-7 Torr. The film thickness (about 100 A) was controlled by a crystal quarz monitor. The vacuum in experimental chamber was typically about 2 x 10-10 Torr during the measurements. In Fig. 1 the electron spectra of KMnF 3 induced by photons with different energies in the region of the K L23 absorption edge are shown. The absorption spectrum measured in the total electron yield mode is shown in the inset. It is very close to the corresponding spectrum of Ref. 4. Its two maxima at 297.3 and 299.9 eV are due to

KMnF 3

296 298 300 302 PHOTON ENERGY (eV)

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250 270 290 KINETIC ENERGY (eV)

Figure 1. Photoemission spectra of KMnF 3 normalized to the photon flux. The inset shows the K L23 absorption spectrum. The number at each spectrum corresponds to the vertical bar with the same number in the inset and shows the photon energy used. The energy levels and the corresponding Auger transitions are indicated for most of the spectral structures.

transitions into states related to the 2p5(3s23p6)3d configuration of K + and reflect the spin-orbit splitting of about 2.6-eV of the 2p shell. Differently from the spectra of KCl (see, e. g. I2-4]) the t2g - eg crystal-field splitting of the 3d orbital is absent here as explained above. The numbered vertical bars in the inset show the photon energies used to induce the electron spectra. The photoelectron lines at the high-kineticenergy part of Fig. 1 correspond to photoionization of 3s and 3p shells of K +, 3s, 3p, and 3d shells of Mn2+ , and- 2s and 2p shells of F'. If the photon enrgy matches the 2p 6 -> 2p53d resonances the 3s and 3p electron lines of K + exhibit a resonant enhancement. This effect is due to the participation of the 3d electron of K + in the Auger decay channels L23MIM45 and L23M23M45 of the configuration 2p53s23p63d of K +. The absence of a similar effect for the electron shells of Mn 2+ and F" in the K + resonance region manifests the high ionicity of the chemical bond in KMnF 3. The low-kinetic-energy part of Fig. 1 is dominated by strong L23M23M23 Auger-electron lines of K + at 240 - 250 eV. They are very similar to analogous lines in the spectra of KC1 [2,3]. The prominent effect here is that small changes of the energies of the exciting photons lead to drastic changes in the L23A423M23 Auger spectra. It is obvious the appearence of a new structure on the high-energy side of the normal spectrum. This extra structure originates from the spectator transitions to the 3p-23d final configuration of K +, as shown conclusively in the case of KCI [2]. The appearence of the normal Auger structure in the resonanceexcitation region, starting from spectrum 4, shows that besides the participator and spectator transitions the excited 3d electron has a certain probablility to be delocalized to the conduction band of the crystal. Such a delocalization is purely a solid-state effect. At higher photon energies, starting from spectrum 8 in Fig. 1, exactly the same process takes place for the 2pl/2-13d (instead of 2p3/2-13d) configuration of K +. An essential point is that besides the L2M23M23 Auger structure the L3M23M23 structure appears also, in spite of the fact that the L 3 subshell is not excited at these photon energies. The appearence of the L3M23M23 structure here is a consequence of the fast L2L3M45 Coster-Kronig process which removes the 3d

129 electron and replaces the L 2 hole by the L 3 hole before the Auger transition is completed. Remarkable is the dispersion of the spectator structure: as seen in Fig. 2, it is very close to the linear throughout the whole resonance region, similarly to the dispersion of photolines. This is definitely different from the dispersionless behaviour of the normal Auger structure. In a somewhat less elegant form the same property of the spectator structure has been revealed for KCI [2]. This property indicates that the final states arising from the L3 and L 2 resonances are always the same, have definite energies and long lifetimes and do not depend on how much and which kind of the 2p53d states are possibly involved, and they are the final states of the Auger-resonant inelastic scattering process [6]. It may be concluded, therefore, that the spectator structure arises as a product of the Auger-resonant inelastic scattering of incident photons

2p63s23p6 + h v -> (2p53s23p63d) -> 2p 63s23p43d + e, in which the 2p-hole states serve as the intermediate states. In Fig. 3 the excitation spectra for the high-

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Figure 2. The energy positions of the K 3s photoelectron line (closed triangles), the highenergy line of the spectator structure (open triangles) and the 3p peak of the L23M23M23 Auger structure as functions of the photon energy.

energy band of the spectator structure, the 3p band of the L3M23M23 normal Auger structure as well as for the 3p and 3s photoemission lines are shown. The behavior of these spectra reflects the complicated nature of the decay of the 2p'13d excitations. The efficiency to excite the 3p "l and 3s "l participator bands seems to follow the absorption spectrum. The efficences of the spectator and normal structures are somewhat suppressed in the region of the second absorption maximum. Moreover, the second (but not the first) maximum of these spectra for the 3s "l participator band and the spectator band is shifted to lower kinetic (i. e. higher binding) energy by comparison with the corresponding absorption maximum. The strong configuration interaction between the 3s "1 and 3p'23d configurations and/or a Fanoqype resonance may be the reasons for this effect. For complete understanding of the complicated mixture of several processes (two participator transitions, the spectator transition, the Coster-Kronig transition, the escape of the excated 3d electron) which seem to have comparable rates a very complex multiconfiguration calculation with including the solid-state effects is needed. Another interesting topic is the possible influence of the crystal-field splitting of the 3d electron states on the resonant scattering process. As pointed out above, the crystal field at the K + site has remarkable strenght in KCI, but is very weak in K]k4nF3. Thus, the linear dispersion of the spectator structure, characteristic for both KCI and KMnF 3, has little if at all to do with the crystal-field splitting of the 3d electron states. Also small is the effect of splitting on the excitation spectra of different decay channels of the 2p'13d configuration: it reduces almost entirely to the appearence of maxima in the excitation spectra in accordance with maxima in the absorption spectrum. However, the 2p-13d -> 3p'23d spectator transitions seem to be influenced by this splitting. In Fig. 4 a detailed view of the high-kinetic-energy band of the spectator structure is shown. For KMnF 3 only the spectrum excited at the first absorption maximum is shown. Within accuracy limits of our experiments the shape of this band does not depend on the excitation energy. For KCI the spectra excited at four prominent absorption maxima are shown. It is evident that the excitation at the first and third absorption maxima, which produces the 2p'13d(t2g) states, leads to a

130 low-binding-energy maximum in the spectator structure, which does not arise by excitation at the second and fourth maxima, where the 2p'13d(ee) states are produced. This comparison and the comparison with the KMnF 3 data lead to the conclusion that the extra maximum of the spectator structure is due to the crystal-field splitting of the initial as well as the final 3p'23d states of the spectator transition and probably originates from

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2p'13d(t2_ ) -> 3p'23d(t2g) transitions. It is also possible that the crystal-field splitting of the 3d orbital may change the probability of the shakeup process in the resonant Auger transition [7]. From the atomic viewpoint it is obvious that the 3d electron of the 2p'13d configuration of K + is collapsed which makes the probability of its shakeup during the L23M23M23 Auger process small (about 14% [2]). In the crystal field the 3d(t2g) orbital retains much of its atomic character, the 3d(e,) orbital, however, is somewhat hybridized and has larger spatial extent. This means its stronger contraction during the Auger process and a higher shakeup probability. Such effect seems to be the case at least for KF [7]. To conclude, our nmin result is that the photoelectron and Auger spectra of K + excited throughout the 2p6 2p53d resonance region the

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Figure 3. The normalized excitation spectra for the 3p (squares) and 3s (closed triangles) photoemission lines of KMnF 3, as well as for the high-energy spectator structure (open triangles) and the 3p L3M23M23 Auger line (circles), as compared with the K L23 edge (the bottom spectrum).

48

46 44 42 BINDING ENERGY (eV)

Figure 4. The low-binding-energy pan of the spectator structure in the resonant L23M23M23 Auger spectra of KMnF 3 and KCI. The energy of exciting photons is indicated for each spectrum.

131 (photon energies 290 - 310 eV) are very similar for KCI and KMnF3. The photoelectron spectra show that during the participator Auger deexcitation of the 2p53d comqguration the excited 3d electron interacts only with the 3s and 3p electrons of K + (but not with the electron shells which originate from manganese and fluorine) and thus manifest a high ionicity of K + in both compounds. The most conspicious structure which arises during the LMM Auger spectator decay of the 2p53d configuration is due to the 3p-23d final configutation and has again similar spectral and dispersion properties for both solids. Its remarkable properly, the linear shift to higher energies with increasing photon energy in the resonance region, demonstrates that i) the Auger transition is so fast that at resonant excitation it is not possible to separate it from the photon absorption process, ii) the L2L3M45 Coster-Kronig transition is an important link of the nonradiative decay of the resonantly-created L2 hole, and iii) the crystal-field splitting of the intermediate 2p53d configuration does not determine the general mechanism of the nonradiative decay of this configuration. However, the crystal-field splitting may modify the resulting spectra and probabilities of possible shake processes. These investigations are financially supported by the Estonian Science Foundation, the Swedish

Natural Sciences Research Council, the Crafoord Foundation, the Royal Swedish Academy of Sciences, and the International Science Foundation. The autors wish to thank Dr. J. N. Andersen, Dr. R. Nyholm, Prof. I. Martinson, Dr. A. Maaroos, R. Ruus, and A. Borissenko for their assistance. REFERENCES

1. A. Kikas, A. Ausmees, M. Elango, J. N. Andersen, R. Nyholm, and I. Martinson, Europhys. Lett., 15 (1991) 683. 2. M. Elango, A. Ausmees, A. Kikas, E. N6mmiste, R. Ruus, A. Saar, J. F. van Acker, J. N. Andersen, R. Nyholm, and I. Martinson, Phys. Rev. B, 47 (1993) 11736. 3. A. Kikas, A. Ausmees, M. Elango, E. N6mmiste, R. Ruus, and A. Saar, J. Electron Spectr. Relat. Phen., 68 (1994) 287. 4. F. Sette, B. Sinkovic, Y. J. Ma, and C. T. Chen, Phys. Rev. B, 39 (1989) 11125. 5. J. N. Andersen, O. Bj6rneholm, A. Sandell, R. Nyholm, J. Torsell, L. Th~nell, A. Nilsson, and N. M~rtenson, Synchrotron Radiat. News, 4 (1991) 15. 6. T. Aberg, Physica Scripta, T41 (1992) 71. 7. E. Kukk, S. Aksela, H. Aksela, E. N6mmiste, A. Kikas, A. Ausmees, and M. Elango, Phys. Rev. B (submitted).