MPE

 

 

 

L - EDGE SPECTRA of Cs and Xe

Related publications: J. Padežnik Gomilšek, A. Kodre, I. Arčon, M. Hribar Phys. Rev. A 68 (2003) 042505

Abstract
X-ray absorption coefficient in the vicinity of L edges of Cs has been measured on an alloy of Cs and Na. The absorption spectrum of the solid sample is virtualy free of stractural signal, similarly to spectra of monatomic gases. It exibits the same pattern of sharp multielectron photoexcitation features as in the L edge absorption spectrum of the adjacent element xenon.

Fig. 1: The L-edge absorption spectrum of Cs measured on a thin layer of the Na/Cs alloy.

 

Introduction
The study of x-ray atomic absorption, measured either directly on a monatomic gas sample [1-5] of an element or derived from an x-ray absorption spectrum of a compound sample after removal of the structural signal [6-10] (EXAFS = extended x-ray absorption fine structure), provides data on correlation in the atomic system. The tiny sharp features on the smoth energy dependence of the photoabsorption cross section are fingerprints of multielectron photoexcitations (MPE). These reaction channels arise from the change of the mean atomic field in the photoeffect but also from the correlated motion of atomic electrons. A detailed analysis can elucidate particularities in the coupling scheme and configuration interaction of the atom [1,2,4,5].

The atomic absorption spectrum is a very good approximation to the atomic absorption background (AAB) in the XAFS structural analysis. Since the structural signal and MPE occupy the same spectral region above a major absorption edge, the small MPE features, if unrecognized, interfere with the interpretation of the structural signal leading to errors in the structure parameters determined in the XAFS analysis [8,9].

In recent studies a parallel analysis of K-edge MPE in neighbor elements (Ar - K; Kr - Rb) was successfully introduced [4,5]. The method exploits the fact that the cores of the two neighbor elements (noble gas and alkali metal) are largely the same, apart from the unit difference in the nuclear charge, and an additional loosely bound electron in the outer shell of the alkali metal atom. From the comparison of MPE excitations it was possible to deduce that common features in the spectra follow from a specific interaction of core configurations, while the differences in the MPE features of the two elements stem from the additional coupling of the valence s electron.
In this study we compare L-edge MPE in neighbor elements Xe and Cs. Complete L-subshell MPE spectra have already been measured on Xe gas2). On the other hand, pure Cs atomic L-edge spectrum has not been measured yet. Only the most prominent resonant MPE features have been extracted from the Cs L-edge EXAFS spectra measured on amorphous materials [11,12,13]. A measurement of the full atomic Cs L-edge absorption spectrum, comparable in detail to that of Xe, promises another fruitful comparison MPE study.

The L-edge absorption spectrometry on monoatomic Cs metal vapor requires a very demanding and costly heat-pipe cell4). Instead we measured the absorption on a thin layer of Cs/Na alloy in the vicinity of its melting point (70° C), where the weak EXAFS signal can be removed numerically. Surprisingly, the experiment showed that the strong disorder in the alloy produces practically pure Cs atomic absorption spectrum almost without an EXAFS component.

Experiment
Cs/Na alloy with a concentration ratio of 1:9 was prepared. A small amount of the alloy was placed, together with a drop of paraffine oil to prevent oxidation, into a small lucite container between two kapton foils and squeezed into a thin layer. The container and the oil kept the metal perfectly stable for several hours of the experiment: no sign of oxidation was observed after demounting. The stability of the sample was confirmed also by the perfect reproducibility of the scans that were recorded in sequence at each of the L subshell edges.

The absorption experiment was performed at the E4 station of the DORIS ring at HASYLAB synchrotron facility, DESY (Hamburg, Germany). The beamline provides a focused beam from Au-coated mirror and a Si(111) double-crystal monochromator with 0.8 eV resolution at Cs L3-edge. Harmonics are effectively eliminated by a plane Au coated mirror and by detuning the monochromator crystal using a stabilization feedback control. Exact energy calibration was established with a simultaneous measurement on a Ti metal foil (EK = 4966 eV) between the second and the third ionisation cell.

Fig. 2: The comparison of the absorption in the L subshell regions of Cs and Xe after removal of the average trend to enhance the detail. Theoretical estimates of the energy of double excited states are shown by arrows. For each subshell, the origin of the energy scale is shifted to the respective ionization threshold.

Results
A compound picture of the L edges (Fig. 1) is obtained as a superposition of three scans per subshell region. Each of the edges is preceded by a resonance due to the excitation of the 2p or 2s electron to the unoccupied bound states just below the continuum.

The spectra above each of the edges are remarkably flat, almost without oscillatory EXAFS signal characteristic of solid samples. A slight convexity of the spectrum above the L3 and L2 edge, observed already in the L absorption spectra of some heavy elements, has been explained as a consequence of subshell polarization14).
The strongest of the sharp MPE features are just barely visible in the spectrum. To expose the details, the average trend of each subshell region is removed from the relative cross section (Fig. 2). Several groups of MPE can be clearly discerned above each absorption edge. The groups can be identified by their energy as multiple excitations involving electrons in consecutively deeper subshells from 6s to 4p, in complete analogy with the MPE groups in the neighboring Xe2), shown below. Hartree-Fock estimates [15] of the threshold energies of the corresponding double excitations are indicated by arrows.

Even at this level of magnification the oscillatory structural signal is visible only close to the edge. The amplitude of EXAFS oscillations is smaller than the MPE features and vanishes in the noise level about 80 eV above each edge.
The marked difference between MPE in Cs L1 and L2,3 spectra point to the same orbital-momentum-sensitive mechanism of multielectron coexcitation as already established in Xe2). Closer inspection of the Cs L2 and L3 absorption spectra shows similarities in [2p4d] and [2p4p] MPE features, and reveals differences in their amplitude and shape compared to those in Xe L3,2 spectra (Fig. 3). Differences may be ascribed to resonant and shake-up transitions into specific final states of a bound Cs atom.

Fig. 3: The comparison of the [2p4d] and [2p4p] MPE features above Cs L2 and L3 and Xe L3 edges spectra.

 

Acknowledgments
Support by the Ministry of Science and Technology of the Republic of Slovenia, and by Internationales Buero BMBF (Germany) is acknowledged. K. V. Klementiev of HASYLAB provided expert advice on beamline operation.

 

References

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