Many-body calculation of photoionization cross sections in CO.

Abstract

Total and partial photoionization cross sections are computed for CO at the fixed equilibrium internuclear separation. Finite basis sets were used, with no explicit recourse to continuum orbitals, and there is no need for input of empirical data such as the ionization threshold. By the present method the first step is to compute the (real) polarizability \ensuremath{\alpha}(i\ensuremath{\eta}) on the imaginary frequency axis. Many-body perturbation theory is used for that purpose, and the expansion is complete to second order in the Coulomb interaction. In the next step the cross sections are obtained from \ensuremath{\alpha}(i\ensuremath{\eta}) by inverting a simple integral equation of the Fredholm first kind (dispersion relation). For the partial cross sections, extra approximations may be necessary, as some diagrams to first and second order make contributions to more than one specific partial cross section. The computed total cross section is in very good agreement with experiment, and good agreement is also obtained for the 1\ensuremath{\pi} partial cross section. For the strongly interacting \ensuremath{\sigma} shells, the comparison with experiment is less certain. The present results, which include comprehensive correlation corrections, are also compared with existing theoretical results that are on the independent-particle level of accuracy.