Use of one-electron theory for the interpretation of near edge structure in K-shell x-ray absorption spectra of transition metal complexes

Abstract

We report the results of multiple scattered wave SCF X‐alpha calculations of the one‐electron cross section for K‐shell photoabsorption in the molecular complexes MoO₄⁻⁻, CrO₄⁻⁻, and MoS₄⁻⁻. We show that the method can successfully account for energy separations and relative cross sections of spectral features both below and above the K‐shell ionization threshold. Furthermore, we show: (a) that the first fairly intense peak on the low energy side of the rising edge for molybdate and chromate is due to a dipole allowed transition to a bound antibonding state of mainly nd character on the metal ion; this transition is possible because of the mixing with the ligand p orbitals having the proper T₂ symmetry induced by the tetrahedral molecular potential; (b) the shoulder on the rising absorption edge can be explained by the beginning of the steplike continuum absorption when convolved with a Lorentzian function of frequency to imitate lifetime and monochromator broadening: (c) the main absorption peak is due to the dipole allowed transition to a continuum state (positive energy) of T₂ symmetry, and the existence of this peak is the result of a ’’shape resonance’’ of the outgoing continuum electron in the molecular potential due to the ligands bound to the transition metal atom and bears only a vague resemblance to the (n+1)p atomic state on the isolated transition metal ion.