Molecular K-shell photoionization cross sections in the relaxed-core Hartree-Fock approximation.

The relaxed-core Hartree-Fock (RCHF) approach to the calculation of E-shell photoionization cross sections is analyzed and applied to I(-shell single-hole ionization in CO. A direct method based on the Schwinger variational principle and single-center-expansion techniques is used to generate the continuum orbitals associated with the motion of the photoelectron in the direct and exchange potential of the relaxed ion. A method is presented for evaluating the 1lectron transition moment, a step that has posed a considerable computational obstacle due to the lack of orthogonality between the frozen and relaxed orbitals in the initial and final 1lectron states, respectively. Besides being very practical and efficient, this formulation establishes the distinction between the "direct" and "conjugate" part of the transition moment, introducing bound-free dipole and overlap integrals, respectively. Whereas for large photoelectron energies the conjugate terms can be neglected, they become important near threshold, contributing, for example, up to 30% to the 1s cross sections in CO. An analysis by means of low-order perturbation theory shows that the RCHF model correctly describes the e6'ect of ionic relaxation, that is, essentially the screening of the 1s hole by the valence electrons. As a consequence the 0 shape resonance is substantially shifted to higher energy and broadened compared with the frozen-core Hartree-Fock picture where the more attractive unscreened 1s-hole potentials are used. The remaining discrepancies with the experimental results are attributed to the neglect of target polarization in the RCHF model.