Strong multielectron absorption effects in the L-edge EXAFS spectra of elements from I to Ce

ELSEVIER

Physica B 208&209 (1995) 379-380

Strong multielectron absorption effects in the L-edge EXAFS spectra of elements from I to Ce A. Kodre a, I Ar~on a'*, M. Hribar a, M. Stuhec a, F. Villain b, W. Drube ¢, L. Tr6geff aJokef Stefan Institute and Department of Physics, University of LjubO'ana, 61111 LjubO'ana, SIovenia b LURE, Univ. Paris-Sud, 91404 Orsay. France CHASYL,4B, DESY, D-22603 Hamburg, Germany

Abstract Strong multielectron photoabsorption structure, comparable in size to EXAFS amplitudes, is found in L-edge EXAFS spectra of Xe-isoelectronic ions Cs ÷-La 3+. The spectra have been measured on amorphous samples (solutions, gels, and glasses) with a small single-wave EXAFS component. The structure is identified as a resonant transition to [2p4d]5d 2 state: it is missing in the spectrum of iodine, but continues into the lanthanide series.

Multielectron photoexcitations (MPEs), although a much studied subject for themselves [1], are seldom considered as a possible perturbation in EXAFS spectra [2]: the rather strong core + valence shell MPEs, within 50 eV above the edge, are hidden in the near edge structure. Much weaker excitations of the core + subvalence shells do occur at energies in the EXAFS region, but, for most transition elements, the rich multiplet splitting of their open-shell configurations smears out the MPE resonances and edges into a wide unresolved continuum [3]. Atoms and ions with near closed-shells, however, exhibit prominent sharp MPE with peak cross-section comparable to EXAFS amplitudes. Such a feature, a compound resonance [2p4d]5d 2, first recognized in xenon [4], is also found in the absorption spectra of the isoelectronic ions Cs +, Ba 2 +, and La a + in aqueous solutions, where EXAFS is limited to a single harmonic component [5]. Identical features are found on the ions in disordered solids such as xerogels and glasses, where EXAFS is similarly simple. For comparison, the two adjacent atomic systems were also studied: for I in ICH3

* Corresponding author.

gas, the MPE is below detection limit, while for Ce 2 + in a xerogel, the result is similar to that of La 3+. The [-2p4d] MPE of similar magnitude was found in spectra of several lanthanides [6]. The L3 EXAFS spectra of the ions in 0.3 M aqueous solutions of CsCI, BaCI2 and La(NOa)3 were measured

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A Kodre et al./ Physica B 208&209 (1995) 379-380

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AE (eV) Fig. 2. MPE structures in the L 3 EXAFS spectra of I, Xe, CS ÷, Ba 2÷, La 3--, and Ce 2÷, after subtraction of the single-wave ansatz (a 2nd-order polynomial in the case of Xe). MCDF energy ranges of the corresponding excited-state multiplets are indicated. Relative energy scale with origin at respective L 3 edges is used.

at the E X A F S 3 beamline at L U R E , Orsay, with an Si(3 1 1 ) double-crystal m o n o c h r o m a t o r of 0.4 eV energy resolution. Absorption in the solvent was determined on equivalent anionic solutions. The I and Xe L 3 spectra were measured in a gas absorption cell at the E X A F S II station at HASYLAB, Hamburg. The beamline is equipped with a gold-coated mirror and a Si(1 1 1) double-

crystal m o n o c h r o m a t o r with 1 eV resolution at 5 keV: the crystals are detuned by stabilization feedback to eliminate beam harmonics. E X A F S spectra of the ions in solids (Cs ÷ in a silicate glass, Ce 2 ÷ in a C e - T i citrate xerogel, and Ba 2 ÷ in Y - B a - C u acetate xerogel) were measured on powder samples at the same station. Reference spectra without samples were taken in identical conditions. The superposition of the E X A F S signal and the M P E structure is shown in the clearest case of the L 3 spectrum of La 3÷ ion (Fig. 1). The E X A F S is well represented by a single sine-wave ansatz; the unrelated sharp structure spans the interval between the M C D F energy of the La 3 + [2p4d-]5d 2 state and the subsequent shakeup limit. A smaller [2p4p]6p5d resonance can also be recognized. In Fig. 2 the isolated M P E structures, after subtraction of the E X A F S ansatz, for the ions in the series are shown, together with the original finding in Xe. The peak size of the M P E almost doubles with each ion from Xe to La 3 +, following the intensity of the "white line" 2p ~ 4d at the L 3 edge. The simplest EXAFS spectra were used to extract the M P E structures. By their size, however, they should represent a nonnegligible pertubation also in more complex E X A F S spectra of the ions, and should be taken into account, especially in high-precision studies. In a given chemical environment, the strength of the M P E can be estimated from the strength of the "white line" of the ion.

Acknowledgements

Support by the Ministry of Science of Slovenia and by the IB Jfilich is acknowledged.

References

[1] S. J. Schaphorst et al., Phys. Rev. A 47 (1993) 1953. [2] S. Bodeur et al., Phys. Rev. A 39 (1989) 5075; G. Li et al., Phys. Rev. Lett. 68 (1992) 1609. [3] R. Frahm et al., in: Proc. 2nd Eu. Conf. on Progress in X-ray Synch. Rad. Res., vol. 5, Rome 2-6 October 19989, eds. A. Balerna, E. Bernieri and S. Mobilio (Bologna, 1990) p. 129; A. Kodre et al., Phys. Rev. A 45 (1992) 4682. [4] A. Kodre et al., HASYLAB Jahresbericht 1993, pp. 181-184. [5] A. Kodre et al., J. de Physique - Coll. (1994), in press. [-6] E. Dartyge et al., Phys. Rev. B 46 (1992) 3155; J. Chaboy et al., Chem. Phys. Lett. 174 (1990) 389.