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 compu ted ground-stare affinities are 1.350 and 1.227 eV for Si- and Ge-, respect ively, in reasonable agreement with the respective experimental values whic h are 1.389 51(20) and 1.232 712(15) eV. Photodetachment cross sections wer e 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 expli cit 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 sect ion, i.e. resonance, interference and background terms. The many-body appro ach 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 a re obtained in an independent-particle model. The present results are compa red with experiment for Si-, and with other theories.