Satellite intensities in the K-shell photoionization of CO.

Abstract

A recently proposed scheme for calculating K-shell ionization cross sections in atoms and molecules using the relaxed-core Hartree-Fock (RCHF) approximation is extended to the treatment of shakeup processes. As an example, this scheme is applied to the \ensuremath{\pi}-${\mathrm{\ensuremath{\pi}}}^{\mathrm{*}}$ satellites in the C 1s photoelectron spectrum of CO. These results for the (S0, S1) pair of \ensuremath{\pi}-${\mathrm{\ensuremath{\pi}}}^{\mathrm{*}}$ satellite states reveal strong interference effects between the ``direct'' (bound-free dipole integral) and ``conjugate'' (bound-free overlap integral) contributions in the transition moment at near-threshold energies. These effects are particularly dominant in the k\ensuremath{\pi} subchannel and, for the triplet-coupled satellite (S1), lead to a drastically increasing cross section with decreasing photon energy. For the singlet-coupled satellite (S0) destructive interference leads to a comparatively small cross section near threshold. While these findings may explain the strikingly different behavior of the S0 and S1 satellite intensities in recent experiments, the present results also show distinct \ensuremath{\sigma}-type shape resonances in both satellite channels at about 10 eV photoelectron energy that are apparently at variance with the experimental evidence. An analysis of the RCHF transition moment via perturbation theory shows that already in first order some potentially important dynmical contributions are missing. In these studies a direct method based on the Schwinger variational principle and single-center expansion techniques is used to obtain the photoelectron orbitals.