Multiphoton ionization cross sections of neon and argon

We present the results of R-matrix Floquet calculations for generalized two- and three-photon ionization cross sections of neon and argon in the XUV region. Photoionization is strongly enhanced by singly- and doubly-excited resonances. We show that interference effects between different Rydberg states can strongly modify the photoionization cross sections. The observation of three- and four-photon resonances in the two- and three-photon spectra, respectively, demonstrates the influence from higher-order processes. The cross sections may be used to estimate the intensity attained by harmonic radiation in the XUV region.