On the importance of inner-shell contributions to multiphoton ionization of complex atoms in the VUV regime

Using the R -matrix-Floquet approach, we investigate two-photon ionization of the Ne + 1s 2 2s 2 2p 5 ground state irradiated by laser light with photon energies between 25 and 40 eV. We find that the ionization rates can increase by over an order of magnitude if, in addition to emission of a 2p electron, we also include emission of a 2s electron, and we use target states based on a considerable configuration-interaction expansion. The improved agreement with experiment demonstrates that the inclusion of inner-shell excitation and ionization processes is essential for theory to accurately describe the experimental multiphoton ionization cross sections in the VUV regime.