MolecularK-shell photoionization cross sections in the relaxed-core Hartree-Fock approximation

J. Schirmer, M. Braunstein, V. McKoy
1990 Physical Review A. Atomic, Molecular, and Optical Physics  
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
more » ... n 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.
doi:10.1103/physreva.41.283 pmid:9902869 fatcat:u4z66zolbjasfjrbf55nyx7tii