Calculation of near-edge x-ray-absorption spectra of molecules and polymers by a Green's-function method.

Abstract

We devise a method in which one-particle Green's-function calculations are used to obtain spectra of near-edge x-ray-absorption fine structures (NEXAFS spectra) for molecules and polymers. The x-ray rates are obtained as the residues of the retarded one-particle Green's function modulated by x-ray orbital transition moments. The spectral features are interpreted from the singularities of the spectral density of the one-particle Green's function and the site selectivity of the NEXAFS process. Utilizing this site selectivity and local-symmetry selection rules the transition moments are optionally decomposed into atomic contributions. Applications are performed for molecules in the sequence ethylene, butadiene, hexatriene, and polyacetylene. We find that the lowest bands in the discrete butadiene and hexatriene spectra are due to core excitations to the lowest ${\mathrm{\ensuremath{\pi}}}^{\mathrm{*}}$ levels pertaining to the different core-hole sites, that the second strong-intensity regions in these spectra are composed of several strong \ensuremath{\sigma} excitations, and that there is a significant reduction of the \ensuremath{\pi}-to-\ensuremath{\sigma}-intensity ratio with addition of ethylene subunits. We find that the building-block principle is inappropriate; the energy and intensity features are to a large extent determined by the different relaxation responses of the creation of different core holes. Comparison with experiment, which is possible for the smaller members of the series, encourages the use of the proposed method for interpretation of polymer NEXAFS spectra.