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Interference between 3D → 3D electron-exchange transitions and interband excitations in MnO(100)
~ )
Solid State Communications, Vol. 82, No. 11, PP. 879-882, 1992. Printed in Great Britain.
0038-1098/9255.00 + . O0 Pergamon Press Ltd
INTERFERENCE BETWEEN 3D ~ 3D ELECTRON-EXCHANGE TRANSITIONS AND INTERBAND EXCITATIONS IN MnO (100) Shin-Puu Jeng* and Victor E. Henrich Department of Applied Physics, Yale University, New Haven, CT 06520, USA
(Received March 20, 1992 by R.H. Silsbee)
We report the observation of strong optically forbidden 3d ~ 3d transitions in the electron-energy-loss spectra of vacuum-cleaved MnO (100). A striking sensitivity of the relative amplitudes of the various 3d ~ 3d muhiplet transitions to the interference from other excitation channels is observed for the first time in the transition-metal oxides. A model reflecting the electron exchange nature of the 3d ~ 3d transition in MnO, similar to that used to describe the 4f ~ 4f transitions in rare-earth metals, is proposed to explain the large amplitudes of the transitions. However, unlike the case of the rare-earth metals, the energy dependence of the amplitude of the transitions cannot be explained simply in terms of a resonant electron emission process.
MnO are, therefore, at least three orders of magnitude weaker than those of optically allowed transitions. 7 In addition, the Mn 4s,4p band of MnO is well separated in energy from the 3d band, so there is no interference from optically allowed 3d -~ 4p transitions in the energy range of the 3d -~ 3d transitions. 9 Thus MnO is an ideal material in which to study the spin-forbidden 3d -~ 3d transitions (AE < 5 eV) by electron impact.
The selection rules for the filled-to-empty state electronic transitions in electron-energy-loss spectroscopy (ELS) are different from the dipole selection rules that govern optical transitions, due both to the finite amount of momentum possessed by the incident electrons and to their fennion nature. 1 Thus optically forbidden transitions can often be seen in ELS spectra, as has been reported for 4f -~ 4f transitions in the lanthanide rare-earth metals.2, 3 In the first-row transition metals and their compounds, the cation 3p ~ 3d core-valence transitions have been studied in detail by several groups using ELS. 4-6 In contrast to these inner-shell transitions, however, little attention has been paid to the electron-excited intra-atomic 3d ~ 3d transitions. Recently, Kemp et al. 7 have compared the 3d ~ 3d transitions excited by electrons and photons for several of the first-row transition-metal oxides. They found for all of the oxides studied that the transition energies of the electronexcited 3d ~ 3d spectra are consistent with those excited by photons, but that the intensities of the electron-excited transitions are at least an order of magnitude greater than expected based upon the intensities of the optical transitions. In this Communication we report the results of measurements of the incident-electron-energy dependence of ELS spectra for vacuum-cleaved single-crystal MnO (100). Based upon those results, we attribute the enhanced ELS intensity of MnO to a two-particle electron exchange process. Furthermore, the relative amplitudes of the various 3d ~ 3d multiplet transitions in the ELS spectra exhibit a striking sensitivity to interference from other intra- and interatomic transitions. MnO is conceptually one of the simplest transition-metal oxides since the large exchange splitting of the Mn2÷ 3d 5 ion separates the highly localized 3d level into occupied spin-up and unoccupied spin-down components. The Mn 2+ ions in the rocksalt structure are octahedrally coordinated with 02- anions, and the octahedral field further splits the degeneracy of the 3d orbitals into eg and t2g levels,8 yielding a 6Alg high-spin ground state. All of the intra-atomic 3d ~ 3d transitions in MnO are optically forbidden because excitation leads to lower spin states (AS = -2); their intensities in the optical absorption spectra of *
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Fig. 1 (a) ELS spectrum for MnO (100) showing various inter- arid intraionic transitions ~Ei = 150.4 eV); (b) Mn 3d ~ 3d ELS spectrum fox MnO (100) at Ei = 39.9 eV, showing final state electron configurations.
Present address: Texas Instruments, P.O. Box 655012, Dallas, TX 75265 879
ELECTRON-EXCHANGE TRANSITIONS
880
A single crystal of MnO was oriented and cut into rods of 4x4 mm 2 cross section having the (100) cleavage face normal to the rod axis. The sides of the cleavage rod were coated with -0.5 ~tm of A1 to reduce charging in ELS spectra. The sample was cleaved in situ in the spectrometer vacuum chamber at a base pressure of less than 1.5 x 10 -10 Tort. In spite of the insulating nature of bulk MnO, no charging effects were observed in room temperature measurements. The spectra were excited by unpolarized electrons with energies of 4 to 250 eV incident normal to the (100) surface and were measured with a double-pass cylindrical-mirror analyzer (CMA) whose axis was also normal to the (100) surface; the ELS spectra are thus semiangle integrated. The current density of the incident electron beam was kept very small in order to prevent electron-beaminduced reduction of the oxide surface. In order to maintain constant system resolution, the FWHM of the incident beam was set to the same value (about 0.6 eV) for all incident electron energies. Figure 1 (a) presents a survey of the ELS spectrum of vacuum-cleaved MnO (100), showing the regions where various intra- and inter-ionic transitions occur. The Mn 3d 3d region of the spectrum is shown on an expanded energy scale in Fig. 1 (b) for an incident electron energy, Ei, of 39.9 eV. Four 3d ~ 3d loss features are resolved at transition energies of 2.2, 2.8, 3.4 and 4.6 eV. According to the optical absorption data for MnO, these features are identified as transitions from the 6Alg ground state to TIg(G), 4T2g(G)+4AIg(G)+ 4Eg(G), 4T2g(D)+aEg(D), and TIg(P) excited states, respectively. 10 The loss energies in ELS are consistent with the optical absorption spectra within the experimental error o f + 0.2 eV; they also agree qualitatively with ELS spectra for MnO powder samples. 7 Evidence for the relaxation of optical selection rules for low incident-electron energies can be seen in Fig. 2, which presents the 1 - 15 eV region of the loss spectrum for incident-electron energies of 50, 150 and 250 eV. The 3d ~ 3d transitions are not visible for Ei = 250 eV, but increase in relative amplitude as Ei is reduced, until they are even larger than those of the optically allowed chargetransfer transitions O 2p ~ Mn 3d for Ei = 50 eV. In addition to their large overall amplitude, the relative amplitudes of the various components of the 3d ~ 3d loss vary dramatically with incident electron energy. The measured ELS spectra of the 3d ~ 3d excitations for incident energies between 4 eV and 100 eV are shown in Fig. 3 (a) and (b). The two most prominent features, at 2.8 eV and 3.4 eV, correspond to 4G- and 4D-derived excited states,
Vol. 82, No. 11
respectively. As Ei increases from 4 to 9 eV, the intensity of the 4D feature increases relative to that of the 4G; since the 4D loss energy is higher than that of the 4G, the domination of the 4G loss at lower Ei and the increase of the 4D loss at higher Ei are probably just threshold effects. However, as Ei increases above 10 eV, the amplitude of the 4G loss starts to increase; it dominates the spectrum up to Ei = 45 eV. Between Ei -- 25 and 45 eV, the 3d ~ 3d loss spectra exhibit only small changes. When the incident electron
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Fig. 2 Comparisonof the ELS spectrafor MnO at various incident electron
energies, showing the relaxation of optical selection rules at low incident electron energy.
Fig. 3 Mn 3d ~ 3d ELS spectra for MnO (100) at incident electron energies from (a) 4.1 to 30.2 eV, and (b) 30.4 to 100.l eV.
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E L E C T R O N - E X C H A N G E TRANSITIONS
energy exceeds the Mn 3p --> 3d optical excitation threshold at -- 50 eV, 11 the intensity of the 4D feature increases relative to that of the 4G, with the 4D loss transition dominating the spectrum for Ei > 65 eV. The increase in the relative amplitude of the optically forbidden 3d -o 3d loss transitions for low incident-electron energies results from the breakdown of optical dipole selection rules. The same effect has been observed in ELS spectra of 4f -> 4f transitions in the lanthanide rare earths.2, 3 In the case of MnO, where the 3d 5 6Alg ground state configuration has all spin-up levels occtipied and all spindown levels empty, the 3d --->3d transitions are not only dipole forbidden but also spin forbidden, since any direct 3d -o 3d excitation would involve a spin flip. We thus attribute this anomalously large intensity to a two-particle exchange process,2,3,12,13 in which a spin-down incident electron can fall into an unoccupied 3d state, emitting a spin-up electron from a filled 3d state (a spin-up incident electron could not participate in such a loss process). In this exchange process, no electron actually undergoes a reversal of spin, and so "spin-forbidden" transitions can occur. The 3d ~ 3d loss intensities diminish sharply with increasing E i due to the decreasing cross-section for electron exchange transitions. 13,14 The changes in the relative amplitude of the 4D- and 4G-derived loss features can be seen more clearly by plotting their ratio as a function of incident electron energy; this has been done in Fig. 4 (a). [The effects of diffraction on relative amplitudes and on the normalized total intensity in Fig. 4(b) should be minimal due to the semi-angleintergrated nature of CMA measurements and the small fractional energy losses for the 4G and 4D transitions.] In order to interpret the changes in relative intensity shown in Fig. 4 (a), we must consider the possible effects of other electron-excited transitions in MnO. The transitions that can occur in the incident-electron-energy range of interest are indicated by heavy bars on the ELS spectrum in Fig. 1 (a)
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Fig. 4 (a) The ratio of the intensitiesof the 4D and 40 features in the ELS spectra of MnO (I00) as a function of incident-electronenergy; (b) the incident-electron-energydependence of the total 3d ~ 3d loss intensity, normalized to that of the elastic peak (the dashed curve results from two three-point smoothings of the data).
881
and in Fig. 4. Except for the Mn 3d ~ 3d transition, the assignments of transitions are based on the calculated imaginary part of the complex dielectric function. 9 The O 2p --->Mn 3d transition occurs at nearly the same loss energy as does the Mn 3d -o Mn 4s,4p; however, according to Ref. 15, the latter transition cannot be seen in reflection ELS due to its large exciton depletion in the near-surface region.15 The changes in the relative amplitudes of the 4D and 4G features are seen to correspond to the onset of other transitions, suggesting the possibility of interference between the 3d ~ 3d transitions and other inter- or intraionic transitions. Particularly striking are the large drop in the 4D/4G ratio at the onset of the O 2p ~ Mn 3d transitions, and the sharp increase in the ratio at the energy of the Mn 3p -o 3d transition. For incident electron energies above the Mn 3p ~ 3d excitation threshold, the Mn 3d --> 3d transition can couple to the Mn 3p --->3d transition as follows: (1) energy released from trapping of the incident electron in an empty 3d level can excite a spin-down electron in the 3p level into an empty 3d level of the same spin; this process can be represented as 3p63d 5 + ei --> (3p53d7) *, where * denotes an excited state; (2) the detected electron, eloss, results from an M2,3M4,sM4,5 super-Coster-Kronig decay of the excited state: (3p53dT) * ~ (3p63dS) * + eloss. The (3p63d5) * state is the same final state as that resulting from a direct 3d -o 3d transitions: ei + 3p63d 5 ~ (3p63dS) * + eloss. Interference between the two channels is analogous to the situation in resonant photoemission, 11 and the changes in amplitude of the 4f --, 4f excitations in the ELS spectra for the lanthanide metals as a function of incident-electron energy were, in fact, interpreted simply in terms of such a resonance and associated Fano-type lineshapes. 2,3 However, the situation for MnO is significantly different, since the overall intensity of the 3d -o 3d excitations does not exhibit a Fano-like lineshape, as shown in Fig. 4 (b). The interference between the Mn 3d -o 3d and O 2p --->Mn 3d transitions can occur in a similar manner via interionic transitions: 2p6(O)3dS(Mn) + ei -o [2pS(O)3dT(Mn)] *, followed by [2p5(O)3dT(Mn)] * --, [2p6(O)3d5(Mn)] * + eloss. However, we cannot totally ignore possible interference from the O 2p --->Mn 4s,4p. The preferential excitation of either the 4G or the 4D losses at different incident electron energies (Fig. 3) and the absence of Fano-like changes in overall Mn 3d --->3d intensity [Fig. 4 (b)] should be explainable in terms of the interactions between the final-state multiplet structures and the intermediate excited states. However, due to the complexity of the multiplet structure of the Mn 2+ ion, and the fact that the 4(; and 4D features in the ELS spectra each contain several multiplet configurations, this cannot be done in any simple way. It would be necessary to calculate how each of the multiplets coupled to the Mn 3p ~ 3d or O 2p ~ Mn 3d transitions in order to compare even semiquantitatively with the experimental results presented here. To date such calculations have not been performed. It is interesting to note that the energies of the Mn 3d --->3d losses do not vary with Ei over the range investigated. In contrast, the energies of the 3p -o 3d transitions in transition metals measured by ELS shift to higher values with increasing Ei above threshold.5, 6 This shift has been interpreted as arising from a post-collision interaction in which some of the energy given up by the excited electron as it relaxes to screen the 3p core hole is transferred to the scattered electron. This process becomes less effective for higher scattered electron energies, thereby giving larger measured energy losses. 6 The lack of such a post-collision interaction in the 3d ~ 3d transition in MnO is due to the absence of a tightly bound positive core hole after excitation.
ELECTRON-EXCHANGE TRANSITIONS
882
In summary, an electron exchange process is proposed to account for the large amplitudes of the optically forbidden Mn 3d ~ 3d transitions observed in the ELS spectra of MnO. A striking sensitivity of the amplitude of the 3d ~ 3d transitions to interference from Mn 3p --, Mn 3d and O 2p --¢ Mn 3d excitation channels is observed for the first time in the transition-metal oxides. In such an exchange process the inelastically scattered electrons are strongly polarized because of the 3d 5 high-spin ground-state configuration of MnO, even for an unpolarized electron source. It is therefore possible to use these internally polarized electrons for electron exchange diffraction measurements analogous to those reported recently by Hermsmeier et al.16 using the multiplet-split 3s core photoelectrons in MnO, with the additional flexibility of easily changing the polarized electron energy. Acknowledgements - Partial support for this work by NSF Solid State Chemistry Grant No. DMR-87-11423, and useful discussions of MnO data with P.A. Cox, J.C. Fuggle and C.J. Powell are gratefully acknowledged.
4.
5. 6. 7. 8. 9. 10. 11. 12.
References 1.
2. 3.
See, for example: J.M. Auerhammer and P. Rez, Phys. Rev. B40, 2024 (1989); Y. Ohno, Phys. Rev. B36, 7500 (1987); J.A.D. Matthew, G. Strasser and F.P. Netzer, Phys. Rev. B27, 5839 (1983); L. Papagno, L.S. Caputi, M.D. CYescenzi and R. Rosei, Phys. Rev. B26, 2320 (1982); and R. Ludeke and A. Korea, Phys. Rev. Lett. 34, 817 (1975). S. Modesti, G. Paolucci and E. Tosatti, Phys. Rev Lett. 55, 2995 (1985). F. Della Valle and S. Modesti, Phys. Rev. B40, 933 (1989).
13. 14. 15. 16.
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See, for example: S.D. Bader, G. Zajac and J. Zak, Phys. Rev. Lett. 50, 1211 (1983); G. Zajac, S.D. Bader, A.J. Arko and J. Zak, Phys. Rev. B29, 5491 (1984); and J.M. McKay, M.H. Mohamed and V.E. Henrich, Phys. Rev. B35, 5491 (1987). C.J. Powell and N.E. Erickson, Phys. Rev. Lett. 51, 61 (1983). Y. Sakisaka, T. Miyano and M. Onchi, Phys. Rev. Lett. 54, 714 (1985). J.P. Kemp, S.T.P. Davies and P.A. Cox, J. Phys.: Condens. Matter 1, 5313 (1989). J.B. Goodenough, Prog. Solid State Chem. 5, 145 (1972), ed. H. Reiss (Pergamon, New York, 1972), p. 145. J. Hugel and C. Carabatos, Solid State Commun. 60, 369 (1986). G.W. Pratt and R. Coelho, Phys. Rev. 116, 281 (1959); and D.R. Huffman, R.L. Wild and M. Shinmei, J. Chem. Phys. 50, 4092 (1969). R.J. Lad and V.E. Henrich, Phys. Rev. B38, 10860 (1988). For a detailed description of electron exchange process in solids, see: J. Kirschner, Polarized Electrons at Surfaces (Springer-Verlag, Berlin, 1985); and Polarized Electrons in Surface Physics, ed. R. Feder (World Scientific, Singapore, 1985). J. Kessler, Polarized Electrons (Springer-Verlag, Berlin, 1985). H.S.W. Massey and E.H.S. Burhop, Electronic and Ionic Impact Phenomena (Oxford, London, 1969). K. Akimoto, Y. Sakisaka, M. Nishijima and M. Onchi, J. Phys. C: Solid State Phys. 11, 2535 (1978). B. Hermsmeier, J. Osterwalder, D.J. Friedman and C.S. Fadley, Phys. Rev. Lett. 62, 478 (1989).
Solid State Communications, Vol. 82, No. 11, PP. 879-882, 1992. Printed in Great Britain.
0038-1098/9255.00 + . O0 Pergamon Press Ltd
INTERFERENCE BETWEEN 3D ~ 3D ELECTRON-EXCHANGE TRANSITIONS AND INTERBAND EXCITATIONS IN MnO (100) Shin-Puu Jeng* and Victor E. Henrich Department of Applied Physics, Yale University, New Haven, CT 06520, USA
(Received March 20, 1992 by R.H. Silsbee)
We report the observation of strong optically forbidden 3d ~ 3d transitions in the electron-energy-loss spectra of vacuum-cleaved MnO (100). A striking sensitivity of the relative amplitudes of the various 3d ~ 3d muhiplet transitions to the interference from other excitation channels is observed for the first time in the transition-metal oxides. A model reflecting the electron exchange nature of the 3d ~ 3d transition in MnO, similar to that used to describe the 4f ~ 4f transitions in rare-earth metals, is proposed to explain the large amplitudes of the transitions. However, unlike the case of the rare-earth metals, the energy dependence of the amplitude of the transitions cannot be explained simply in terms of a resonant electron emission process.
MnO are, therefore, at least three orders of magnitude weaker than those of optically allowed transitions. 7 In addition, the Mn 4s,4p band of MnO is well separated in energy from the 3d band, so there is no interference from optically allowed 3d -~ 4p transitions in the energy range of the 3d -~ 3d transitions. 9 Thus MnO is an ideal material in which to study the spin-forbidden 3d -~ 3d transitions (AE < 5 eV) by electron impact.
The selection rules for the filled-to-empty state electronic transitions in electron-energy-loss spectroscopy (ELS) are different from the dipole selection rules that govern optical transitions, due both to the finite amount of momentum possessed by the incident electrons and to their fennion nature. 1 Thus optically forbidden transitions can often be seen in ELS spectra, as has been reported for 4f -~ 4f transitions in the lanthanide rare-earth metals.2, 3 In the first-row transition metals and their compounds, the cation 3p ~ 3d core-valence transitions have been studied in detail by several groups using ELS. 4-6 In contrast to these inner-shell transitions, however, little attention has been paid to the electron-excited intra-atomic 3d ~ 3d transitions. Recently, Kemp et al. 7 have compared the 3d ~ 3d transitions excited by electrons and photons for several of the first-row transition-metal oxides. They found for all of the oxides studied that the transition energies of the electronexcited 3d ~ 3d spectra are consistent with those excited by photons, but that the intensities of the electron-excited transitions are at least an order of magnitude greater than expected based upon the intensities of the optical transitions. In this Communication we report the results of measurements of the incident-electron-energy dependence of ELS spectra for vacuum-cleaved single-crystal MnO (100). Based upon those results, we attribute the enhanced ELS intensity of MnO to a two-particle electron exchange process. Furthermore, the relative amplitudes of the various 3d ~ 3d multiplet transitions in the ELS spectra exhibit a striking sensitivity to interference from other intra- and interatomic transitions. MnO is conceptually one of the simplest transition-metal oxides since the large exchange splitting of the Mn2÷ 3d 5 ion separates the highly localized 3d level into occupied spin-up and unoccupied spin-down components. The Mn 2+ ions in the rocksalt structure are octahedrally coordinated with 02- anions, and the octahedral field further splits the degeneracy of the 3d orbitals into eg and t2g levels,8 yielding a 6Alg high-spin ground state. All of the intra-atomic 3d ~ 3d transitions in MnO are optically forbidden because excitation leads to lower spin states (AS = -2); their intensities in the optical absorption spectra of *
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.
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....
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....
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....
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Fig. 1 (a) ELS spectrum for MnO (100) showing various inter- arid intraionic transitions ~Ei = 150.4 eV); (b) Mn 3d ~ 3d ELS spectrum fox MnO (100) at Ei = 39.9 eV, showing final state electron configurations.
Present address: Texas Instruments, P.O. Box 655012, Dallas, TX 75265 879
ELECTRON-EXCHANGE TRANSITIONS
880
A single crystal of MnO was oriented and cut into rods of 4x4 mm 2 cross section having the (100) cleavage face normal to the rod axis. The sides of the cleavage rod were coated with -0.5 ~tm of A1 to reduce charging in ELS spectra. The sample was cleaved in situ in the spectrometer vacuum chamber at a base pressure of less than 1.5 x 10 -10 Tort. In spite of the insulating nature of bulk MnO, no charging effects were observed in room temperature measurements. The spectra were excited by unpolarized electrons with energies of 4 to 250 eV incident normal to the (100) surface and were measured with a double-pass cylindrical-mirror analyzer (CMA) whose axis was also normal to the (100) surface; the ELS spectra are thus semiangle integrated. The current density of the incident electron beam was kept very small in order to prevent electron-beaminduced reduction of the oxide surface. In order to maintain constant system resolution, the FWHM of the incident beam was set to the same value (about 0.6 eV) for all incident electron energies. Figure 1 (a) presents a survey of the ELS spectrum of vacuum-cleaved MnO (100), showing the regions where various intra- and inter-ionic transitions occur. The Mn 3d 3d region of the spectrum is shown on an expanded energy scale in Fig. 1 (b) for an incident electron energy, Ei, of 39.9 eV. Four 3d ~ 3d loss features are resolved at transition energies of 2.2, 2.8, 3.4 and 4.6 eV. According to the optical absorption data for MnO, these features are identified as transitions from the 6Alg ground state to TIg(G), 4T2g(G)+4AIg(G)+ 4Eg(G), 4T2g(D)+aEg(D), and TIg(P) excited states, respectively. 10 The loss energies in ELS are consistent with the optical absorption spectra within the experimental error o f + 0.2 eV; they also agree qualitatively with ELS spectra for MnO powder samples. 7 Evidence for the relaxation of optical selection rules for low incident-electron energies can be seen in Fig. 2, which presents the 1 - 15 eV region of the loss spectrum for incident-electron energies of 50, 150 and 250 eV. The 3d ~ 3d transitions are not visible for Ei = 250 eV, but increase in relative amplitude as Ei is reduced, until they are even larger than those of the optically allowed chargetransfer transitions O 2p ~ Mn 3d for Ei = 50 eV. In addition to their large overall amplitude, the relative amplitudes of the various components of the 3d ~ 3d loss vary dramatically with incident electron energy. The measured ELS spectra of the 3d ~ 3d excitations for incident energies between 4 eV and 100 eV are shown in Fig. 3 (a) and (b). The two most prominent features, at 2.8 eV and 3.4 eV, correspond to 4G- and 4D-derived excited states,
Vol. 82, No. 11
respectively. As Ei increases from 4 to 9 eV, the intensity of the 4D feature increases relative to that of the 4G; since the 4D loss energy is higher than that of the 4G, the domination of the 4G loss at lower Ei and the increase of the 4D loss at higher Ei are probably just threshold effects. However, as Ei increases above 10 eV, the amplitude of the 4G loss starts to increase; it dominates the spectrum up to Ei = 45 eV. Between Ei -- 25 and 45 eV, the 3d ~ 3d loss spectra exhibit only small changes. When the incident electron
4G
4D
Z
~
Et
~
Primary ,
,
,
i
2
. . . .
i
. . . .
i
. . . .
3 4 Energy Loss (eV)
L
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~ Z
f
/ ~
~
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2
4
~
r
I
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I
I
12
14
3
4
Energy Loss (eV)
5
Fl~.r~ 3(hi
Fig. 2 Comparisonof the ELS spectrafor MnO at various incident electron
energies, showing the relaxation of optical selection rules at low incident electron energy.
Fig. 3 Mn 3d ~ 3d ELS spectra for MnO (100) at incident electron energies from (a) 4.1 to 30.2 eV, and (b) 30.4 to 100.l eV.
Vol. 82, No. 11
E L E C T R O N - E X C H A N G E TRANSITIONS
energy exceeds the Mn 3p --> 3d optical excitation threshold at -- 50 eV, 11 the intensity of the 4D feature increases relative to that of the 4G, with the 4D loss transition dominating the spectrum for Ei > 65 eV. The increase in the relative amplitude of the optically forbidden 3d -o 3d loss transitions for low incident-electron energies results from the breakdown of optical dipole selection rules. The same effect has been observed in ELS spectra of 4f -> 4f transitions in the lanthanide rare earths.2, 3 In the case of MnO, where the 3d 5 6Alg ground state configuration has all spin-up levels occtipied and all spindown levels empty, the 3d --->3d transitions are not only dipole forbidden but also spin forbidden, since any direct 3d -o 3d excitation would involve a spin flip. We thus attribute this anomalously large intensity to a two-particle exchange process,2,3,12,13 in which a spin-down incident electron can fall into an unoccupied 3d state, emitting a spin-up electron from a filled 3d state (a spin-up incident electron could not participate in such a loss process). In this exchange process, no electron actually undergoes a reversal of spin, and so "spin-forbidden" transitions can occur. The 3d ~ 3d loss intensities diminish sharply with increasing E i due to the decreasing cross-section for electron exchange transitions. 13,14 The changes in the relative amplitude of the 4D- and 4G-derived loss features can be seen more clearly by plotting their ratio as a function of incident electron energy; this has been done in Fig. 4 (a). [The effects of diffraction on relative amplitudes and on the normalized total intensity in Fig. 4(b) should be minimal due to the semi-angleintergrated nature of CMA measurements and the small fractional energy losses for the 4G and 4D transitions.] In order to interpret the changes in relative intensity shown in Fig. 4 (a), we must consider the possible effects of other electron-excited transitions in MnO. The transitions that can occur in the incident-electron-energy range of interest are indicated by heavy bars on the ELS spectrum in Fig. 1 (a)
0 O(2p) . - ~ M n ( 4 s , 4 p )
~+
ol 0
;
k
~,
I
>,
/
Y
I
+'%--,-*'"
O(2p) . - ~ M n ( 3 d )
2tO
Mn(3p) '-~ Mn(ad)
4'0
6'0
(a) 8JO
1 O0
O(2p) ' - ~ M n ( 3 d )
3
oI
(b)
o o'~, ~o
Mn(3p).-~ Mn(3d)
L*
I
d,o '~
t
I
o °-"
e - -°" - D - "°" - ~"°'d
O(2p) . . ~ M n ( 4 s . 4 p )
0
2tO
i K'
o "°
o o'o- - - -~_ o o 0- o--,i
i
40 60 80 Incident Electron Energy (eV)
%e-___.o i
100
Fig. 4 (a) The ratio of the intensitiesof the 4D and 40 features in the ELS spectra of MnO (I00) as a function of incident-electronenergy; (b) the incident-electron-energydependence of the total 3d ~ 3d loss intensity, normalized to that of the elastic peak (the dashed curve results from two three-point smoothings of the data).
881
and in Fig. 4. Except for the Mn 3d ~ 3d transition, the assignments of transitions are based on the calculated imaginary part of the complex dielectric function. 9 The O 2p --->Mn 3d transition occurs at nearly the same loss energy as does the Mn 3d -o Mn 4s,4p; however, according to Ref. 15, the latter transition cannot be seen in reflection ELS due to its large exciton depletion in the near-surface region.15 The changes in the relative amplitudes of the 4D and 4G features are seen to correspond to the onset of other transitions, suggesting the possibility of interference between the 3d ~ 3d transitions and other inter- or intraionic transitions. Particularly striking are the large drop in the 4D/4G ratio at the onset of the O 2p ~ Mn 3d transitions, and the sharp increase in the ratio at the energy of the Mn 3p -o 3d transition. For incident electron energies above the Mn 3p ~ 3d excitation threshold, the Mn 3d --> 3d transition can couple to the Mn 3p --->3d transition as follows: (1) energy released from trapping of the incident electron in an empty 3d level can excite a spin-down electron in the 3p level into an empty 3d level of the same spin; this process can be represented as 3p63d 5 + ei --> (3p53d7) *, where * denotes an excited state; (2) the detected electron, eloss, results from an M2,3M4,sM4,5 super-Coster-Kronig decay of the excited state: (3p53dT) * ~ (3p63dS) * + eloss. The (3p63d5) * state is the same final state as that resulting from a direct 3d -o 3d transitions: ei + 3p63d 5 ~ (3p63dS) * + eloss. Interference between the two channels is analogous to the situation in resonant photoemission, 11 and the changes in amplitude of the 4f --, 4f excitations in the ELS spectra for the lanthanide metals as a function of incident-electron energy were, in fact, interpreted simply in terms of such a resonance and associated Fano-type lineshapes. 2,3 However, the situation for MnO is significantly different, since the overall intensity of the 3d -o 3d excitations does not exhibit a Fano-like lineshape, as shown in Fig. 4 (b). The interference between the Mn 3d -o 3d and O 2p --->Mn 3d transitions can occur in a similar manner via interionic transitions: 2p6(O)3dS(Mn) + ei -o [2pS(O)3dT(Mn)] *, followed by [2p5(O)3dT(Mn)] * --, [2p6(O)3d5(Mn)] * + eloss. However, we cannot totally ignore possible interference from the O 2p --->Mn 4s,4p. The preferential excitation of either the 4G or the 4D losses at different incident electron energies (Fig. 3) and the absence of Fano-like changes in overall Mn 3d --->3d intensity [Fig. 4 (b)] should be explainable in terms of the interactions between the final-state multiplet structures and the intermediate excited states. However, due to the complexity of the multiplet structure of the Mn 2+ ion, and the fact that the 4(; and 4D features in the ELS spectra each contain several multiplet configurations, this cannot be done in any simple way. It would be necessary to calculate how each of the multiplets coupled to the Mn 3p ~ 3d or O 2p ~ Mn 3d transitions in order to compare even semiquantitatively with the experimental results presented here. To date such calculations have not been performed. It is interesting to note that the energies of the Mn 3d --->3d losses do not vary with Ei over the range investigated. In contrast, the energies of the 3p -o 3d transitions in transition metals measured by ELS shift to higher values with increasing Ei above threshold.5, 6 This shift has been interpreted as arising from a post-collision interaction in which some of the energy given up by the excited electron as it relaxes to screen the 3p core hole is transferred to the scattered electron. This process becomes less effective for higher scattered electron energies, thereby giving larger measured energy losses. 6 The lack of such a post-collision interaction in the 3d ~ 3d transition in MnO is due to the absence of a tightly bound positive core hole after excitation.
ELECTRON-EXCHANGE TRANSITIONS
882
In summary, an electron exchange process is proposed to account for the large amplitudes of the optically forbidden Mn 3d ~ 3d transitions observed in the ELS spectra of MnO. A striking sensitivity of the amplitude of the 3d ~ 3d transitions to interference from Mn 3p --, Mn 3d and O 2p --¢ Mn 3d excitation channels is observed for the first time in the transition-metal oxides. In such an exchange process the inelastically scattered electrons are strongly polarized because of the 3d 5 high-spin ground-state configuration of MnO, even for an unpolarized electron source. It is therefore possible to use these internally polarized electrons for electron exchange diffraction measurements analogous to those reported recently by Hermsmeier et al.16 using the multiplet-split 3s core photoelectrons in MnO, with the additional flexibility of easily changing the polarized electron energy. Acknowledgements - Partial support for this work by NSF Solid State Chemistry Grant No. DMR-87-11423, and useful discussions of MnO data with P.A. Cox, J.C. Fuggle and C.J. Powell are gratefully acknowledged.
4.
5. 6. 7. 8. 9. 10. 11. 12.
References 1.
2. 3.
See, for example: J.M. Auerhammer and P. Rez, Phys. Rev. B40, 2024 (1989); Y. Ohno, Phys. Rev. B36, 7500 (1987); J.A.D. Matthew, G. Strasser and F.P. Netzer, Phys. Rev. B27, 5839 (1983); L. Papagno, L.S. Caputi, M.D. CYescenzi and R. Rosei, Phys. Rev. B26, 2320 (1982); and R. Ludeke and A. Korea, Phys. Rev. Lett. 34, 817 (1975). S. Modesti, G. Paolucci and E. Tosatti, Phys. Rev Lett. 55, 2995 (1985). F. Della Valle and S. Modesti, Phys. Rev. B40, 933 (1989).
13. 14. 15. 16.
Vol. 82, No. 11
See, for example: S.D. Bader, G. Zajac and J. Zak, Phys. Rev. Lett. 50, 1211 (1983); G. Zajac, S.D. Bader, A.J. Arko and J. Zak, Phys. Rev. B29, 5491 (1984); and J.M. McKay, M.H. Mohamed and V.E. Henrich, Phys. Rev. B35, 5491 (1987). C.J. Powell and N.E. Erickson, Phys. Rev. Lett. 51, 61 (1983). Y. Sakisaka, T. Miyano and M. Onchi, Phys. Rev. Lett. 54, 714 (1985). J.P. Kemp, S.T.P. Davies and P.A. Cox, J. Phys.: Condens. Matter 1, 5313 (1989). J.B. Goodenough, Prog. Solid State Chem. 5, 145 (1972), ed. H. Reiss (Pergamon, New York, 1972), p. 145. J. Hugel and C. Carabatos, Solid State Commun. 60, 369 (1986). G.W. Pratt and R. Coelho, Phys. Rev. 116, 281 (1959); and D.R. Huffman, R.L. Wild and M. Shinmei, J. Chem. Phys. 50, 4092 (1969). R.J. Lad and V.E. Henrich, Phys. Rev. B38, 10860 (1988). For a detailed description of electron exchange process in solids, see: J. Kirschner, Polarized Electrons at Surfaces (Springer-Verlag, Berlin, 1985); and Polarized Electrons in Surface Physics, ed. R. Feder (World Scientific, Singapore, 1985). J. Kessler, Polarized Electrons (Springer-Verlag, Berlin, 1985). H.S.W. Massey and E.H.S. Burhop, Electronic and Ionic Impact Phenomena (Oxford, London, 1969). K. Akimoto, Y. Sakisaka, M. Nishijima and M. Onchi, J. Phys. C: Solid State Phys. 11, 2535 (1978). B. Hermsmeier, J. Osterwalder, D.J. Friedman and C.S. Fadley, Phys. Rev. Lett. 62, 478 (1989).