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Soft x-ray appearance potential spectra of the N4,5 transitions in lanthanum
Volume 49A,
number 5
I October 1974
PHYSICS LETTERS
SOFT X-RAY APPEARANCE POTENTIAL SPECTRA OF THE N,,, TRANSITIONS IN LANTHANUM P.S. SZCZEPANEK and W.E. HARTE Laboratory for physical sciences, College park, Maryland 20740,
USA
Received 7 June 1974 Structure is observed in the soft X-ray appearance potential spectrum of lanthanum near the N4 5 binding energy. This structure is due to exchange splitting caused by the overlap of the 4d and 4f wavefun&ons as modified by the incident election.
Soft X-ray appearance potential spectroscopy (SXAPS) measures changes in the total X-ray fluorescence yield as a function of incident electron energy. Using the standard technique of potential modulation, the derivative of the X-ray yield in the vicinity of the core level excitation energies is obtained. The shapes of these derivative spectra have been assumed to be related to the unoccupied density of states of the material; the threshold energy being a measure of the binding energy of the core level. Recent measurements have isolated features in SXAPS spectra which are inconsistent with such a relationship and indicate that an alternate interpretation of the spectra is necessary
dl
i-r
100
ENERGY lcv)
150
Fig. 1. SXAPS of La near the N4,5 transitions of lanthanum.
VI* We have measured the SXAPS spectra of lanthanum in the region around the N4,5 threshold. The spectra obtained is displayed in fig. 1. Prominent in the spectra are two small peaks at 103 eV and 107 eV, a large and broader peak at 116 eV and at 123 eV another small peak. This structure in the vicinity of the N4,, threshold bears no resemblance to the anticipated density of unoccupied states of lanthanum. The SXAPS spectra differs significantly for the N4,5 levels from that due to the M,,, although they are of the same d symmetry. Zimkina et al. [2] have measured the absorption spectra of a number of rare earth elements and these measurements have been updated by Haensel et al. [3]. They both record a series of sharp peaks both below and above the binding energy of the N4,5 electrons followed by a broader peak. This structure has been interpreted as having its origin in an exchange interaction involving an atomic 4d hole and 4f electron resulting in the splitting of the final state energy
for transitions of the type 4d1u4f” +4dg 4f”+‘. The general features of the photoabsorption spectra of the rare earth elements including lanthanum are in good agreement with the theoretical analysis of Sugar [4]. For lanthanum the calculated photoabsorption energies near the 4d edge occur at 97.2, 101.3 and, 117.0 eV. We interpret the SXAPS spectra of lanthanum near the N4,5 binding energy as due to similar core configuration transitions. Because of the presence of the incident electron in appearance potential spectroscopy the excitation spectra of the core levels of lanthanum are modified from that recorded in photoabsorption measurements. The incident electron may be inelastically scattered either into a conduction band states or into a 4f state. In the former case the occupation of conduction states has little effect on the core levels and transitions of the type considered by Sugar for lanthanum, 4d” + 4dg 4f1, are excited. In the latter case in 377
Volume 49A, number 5
PHYSICS LETTERS
which the scattered incident electron occupies a 4f state, transition of the type 4d1’4f+ 4dg4f2 determine the excitation spectra. At present only a rough estimate of this absorption spectra is possible by associating it with the isoelectronic elements Ce+++. A more detailed calculation of the level structure is necessary for comparison. We would like to acknowledge interesting discussions with Dr. Jack Sugar and Dr. A.J. Leyendecker.
378
7 October 1974
References [l] W.E. Harte, P.S. Szczepanek and A.J. Leyendecker (to be published). [ 21 T.M. Zimkina, V.A. Fomichev, S.A. Gribovskii and 1.1. Zhukova, Sov. Phys. Sol. State 9, (1967) 1128. [3] R. Haensel, P. Rabe and B. Sonntag, Sol. State Commun. 8, (1970) 1845. [4] J. Sugar, Phys. Rev. B, 5, (1972) 1785.
number 5
I October 1974
PHYSICS LETTERS
SOFT X-RAY APPEARANCE POTENTIAL SPECTRA OF THE N,,, TRANSITIONS IN LANTHANUM P.S. SZCZEPANEK and W.E. HARTE Laboratory for physical sciences, College park, Maryland 20740,
USA
Received 7 June 1974 Structure is observed in the soft X-ray appearance potential spectrum of lanthanum near the N4 5 binding energy. This structure is due to exchange splitting caused by the overlap of the 4d and 4f wavefun&ons as modified by the incident election.
Soft X-ray appearance potential spectroscopy (SXAPS) measures changes in the total X-ray fluorescence yield as a function of incident electron energy. Using the standard technique of potential modulation, the derivative of the X-ray yield in the vicinity of the core level excitation energies is obtained. The shapes of these derivative spectra have been assumed to be related to the unoccupied density of states of the material; the threshold energy being a measure of the binding energy of the core level. Recent measurements have isolated features in SXAPS spectra which are inconsistent with such a relationship and indicate that an alternate interpretation of the spectra is necessary
dl
i-r
100
ENERGY lcv)
150
Fig. 1. SXAPS of La near the N4,5 transitions of lanthanum.
VI* We have measured the SXAPS spectra of lanthanum in the region around the N4,5 threshold. The spectra obtained is displayed in fig. 1. Prominent in the spectra are two small peaks at 103 eV and 107 eV, a large and broader peak at 116 eV and at 123 eV another small peak. This structure in the vicinity of the N4,, threshold bears no resemblance to the anticipated density of unoccupied states of lanthanum. The SXAPS spectra differs significantly for the N4,5 levels from that due to the M,,, although they are of the same d symmetry. Zimkina et al. [2] have measured the absorption spectra of a number of rare earth elements and these measurements have been updated by Haensel et al. [3]. They both record a series of sharp peaks both below and above the binding energy of the N4,5 electrons followed by a broader peak. This structure has been interpreted as having its origin in an exchange interaction involving an atomic 4d hole and 4f electron resulting in the splitting of the final state energy
for transitions of the type 4d1u4f” +4dg 4f”+‘. The general features of the photoabsorption spectra of the rare earth elements including lanthanum are in good agreement with the theoretical analysis of Sugar [4]. For lanthanum the calculated photoabsorption energies near the 4d edge occur at 97.2, 101.3 and, 117.0 eV. We interpret the SXAPS spectra of lanthanum near the N4,5 binding energy as due to similar core configuration transitions. Because of the presence of the incident electron in appearance potential spectroscopy the excitation spectra of the core levels of lanthanum are modified from that recorded in photoabsorption measurements. The incident electron may be inelastically scattered either into a conduction band states or into a 4f state. In the former case the occupation of conduction states has little effect on the core levels and transitions of the type considered by Sugar for lanthanum, 4d” + 4dg 4f1, are excited. In the latter case in 377
Volume 49A, number 5
PHYSICS LETTERS
which the scattered incident electron occupies a 4f state, transition of the type 4d1’4f+ 4dg4f2 determine the excitation spectra. At present only a rough estimate of this absorption spectra is possible by associating it with the isoelectronic elements Ce+++. A more detailed calculation of the level structure is necessary for comparison. We would like to acknowledge interesting discussions with Dr. Jack Sugar and Dr. A.J. Leyendecker.
378
7 October 1974
References [l] W.E. Harte, P.S. Szczepanek and A.J. Leyendecker (to be published). [ 21 T.M. Zimkina, V.A. Fomichev, S.A. Gribovskii and 1.1. Zhukova, Sov. Phys. Sol. State 9, (1967) 1128. [3] R. Haensel, P. Rabe and B. Sonntag, Sol. State Commun. 8, (1970) 1845. [4] J. Sugar, Phys. Rev. B, 5, (1972) 1785.