title: Two-photon absorption in the relativistic four-component Hartree-Fock approximation.
abstract: A first implementation of the single residue of the quadratic response function in the four-component Hartree-Fock approximation is presented. The implementation is based on a Kramers paired molecular orbital basis and takes full advantage of time and spatial symmetry reductions in a quaternion formulation-in analogy with the previous work on the quadratic response function [J. Chem. Phys. 121, 6145 (2004)]. Sample calculations are given in terms of the monochromatic and coherent two-photon absorption cross sections in the noble gases. The relativistic two-photon selection rule DeltaJ = {0,+/-2} allows for nonrelativistically spin-forbidden transitions, and, even in neon, strong two-photon absorption is shown to occur for the X (1)S(0)-->2 (3)P(2) transition. It is argued that relevant comparisons between nonrelativistic and relativistic calculations must be performed at the level of integrated absorption cross sections.
authors.affiliation: Department of Physics and Measurement Technology, Linköping University, Sweden. johhe@ifm.liu.se
authors.forename: Johan
authors.initials: J
authors.lastname: Henriksson
authors.forename: Patrick
authors.initials: P
authors.lastname: Norman
authors.forename: Hans Jørgen Aa
authors.initials: HJ
authors.lastname: Jensen