Affinities and photodetachment cross sections for the negative ions Si- and Ge-

Affinities and photodetachment cross sections are computed for the negative atomic ions Si- and Ge- by use of many-body perturbation theory. The computed ground-stare affinities are 1.350 and 1.227 eV for Si- and Ge-, respectively, in reasonable agreement with the respective experimental values which are 1.389 51(20) and 1.232 712(15) eV. Photodetachment cross sections were obtained by inverting computed (real) dynamic polarizabilities alpha(in) on the imaginary frequency axis by use of a simple dispersion relation. In the inversion process the general Fano-profile formula was used in an explicit manner, treating the cross sections and parameters of the Fano formula as frequency-dependent quantities that were determined from the dispersion relation. The present inversion procedure was carried out in several steps, enabling separate studies of various contributions to the total cross section, i.e. resonance, interference and background terms. The many-body approach used to derive the cross section ii complete to second order, i.e. with two interactions with the electronic repulsion operator. This is important if the Fano profile is to be recovered, as only background contributions are obtained in an independent-particle model. The present results are compared with experiment for Si-, and with other theories.