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

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.