Many-body calculation of total and partial photoabsorption cross sections in N2

Abstract

Total and partial photoabsorption cross sections are computed for N2 at the fixed equilibrium internuclear separation. By the present method the basic step is to compute the (real) polarizability alpha (i eta ) on the imaginary frequency axis, and the cross sections are then in turn obtained from the alpha (i eta ) values by inverting a dispersion relation. Many-body perturbation theory to second order in the Coulomb interaction is used to compute alpha (i eta ), and the continuum is represented by a finite set of discrete virtual states. In this way the difficult problem of obtaining continuum orbitals for a two-centre system is avoided, and a high degree of electron correlation will be included if there is reasonable convergence of the perturbation expansion. The present results are compared with experiment and with other calculations. For the total cross section there is good agreement with experiment, and the same conclusion applies to the 1 pi u-1 partial cross section. Problems related to the 1 pi u to k pi g partial channel cross section that are typical for other investigations, do not arise with the present method. However, for the 3 sigma g-1 and 2 sigma u-1 partial cross sections problems are encountered due to slow convergence of the perturbation expansion. The poor convergence seems to be related to strong interactions between the valence shells.