**Figure 7.** Two-photon cross section of the N 1s orbitals of nitrogen for both parallel and perpendicular orientations of the molecular axis with respect to the laser polarization vector.

**Abstract**

The two-photon core ionization dynamics of gas-phase methane, carbon monoxide and nitrogen have been studied with a recent implementation of the lowest order perturbation theory in the framework of density functional theory and a multicentric basis set expansion of bound and scattering states. Ionization cross sections and angular asymmetry parameters have been calculated for the case of a single radiation beam and for both linear and circular light polarizations in the fixed nuclei approximation. Expected resonances due to core valence excitations enhance the cross section by several orders of magnitude.