## Upper panel: rotationally averaged two-photon cross section of the O 1s orbital of CO

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**Figure 6.** Upper panel: rotationally averaged two-photon cross section of the O 1s orbital of CO. Central panel: β_{2} asymmetry parameter profile. Lower panel: β_{4} asymmetry parameter profile.

**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.