Experimental and Theoretical XANES Studies of Selected Vanadium Compounds

Abstract

As the region near an absorption edge is scanned in energy the ejected photoelectron seauentially probes the empty electronic levels of the material. The resulting spectra have long been known to be rich in chemical and structural information [1]. An attempt to correlate a large number of spectra, including vanadium compounds, was made by WHITE and McKINSTRY [2]. Reconciliation of molecular orbital theory with x-ray near-edge spectra as made by BEST [3] for tetra-oxo-bonded transition metals, by SEKA and HANSON [4] for octahedral complexes and by FISCHER and BAUM [5] for Ti compounds. All of these earlier efforts suffered from the (varying) poor resolution of their experiments. With the availability of intense synchrotron radiation sources, these spectra can now be gathered more quickly, simply and with greater resolution than ever before. A number of interesting papers have recently been published on K-edge and L-edge spectra [6] measured with synchrotron sources. For transition metals there are generally weak but distinct features in the spectra just before the onset of absorption. The so-called near-edge features have been attributed by SHULMAN et al. [7] to electronic transitions from ls to some d,s (non-dipole), and p-like (dipole-allowed) empty states. In this paper we report high resolution XANES spectra of vanadium in a series of selected compounds of well-defined coordination geometry and known bond distances. The intensity and position of these near-edge spectral features are analyzed within a quantum molecular orbital framework and a simple coordination charge concept.