X-RAY EMISSION AND ABSORPTION SPECTRA OF Cr(CO)6: A COMPARISON OF QUANTUM-CHEMICAL AND FULL MULTIPLE SCATTERING CALCULATIONS WITH EXPERIMENT

摘要

The full multiple scattering procedure using the extended continuum approximation and the single semiempirical muffin-tin (MT) potential gave X-ray emission spectra and the CrK absorption spectrum of Cr(CO)6, which were in good agreement with experiment. The bound states above the MT zero become the resonance states corresponding to the poles of the Green function in the lower semi-plane of the complex plane of energy. A procedure for determining the parameters of the poles was developed. A group of poles close to the real line were isolated; their number equals the number of molecular orbitals generated by ellatomized atomic orbitals. These poles completely determine the fine structure of the emission spectra and the first narrow peak in the absorption spectrum. Their energies were close to those of molecular orbitals found at the DFT level of theory, but were in worse agreement with the results of Hartree–Fock calculations. The Hartree–Fock method gave worse agreement with the experimental emission spectra than full multiple scattering and DFT. The fine structure of the region after the first narrow peak in the absorption spectrum is also mainly determined by the contributions from the poles, but these poles are farther from the real line than the poles corresponding to molecular orbitals.