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Theoretical two-, three- and four-photon ionization cross sections of helium in the XUV range

We present results of ab initio calculations of generalized cross sections for two-, three- and four-photon ionization of helium by VUV and XUV radiation. These cross sections can be useful in the estimation of the intensity required for observation of these processes. This work has been motivated by recent experimental results using high-order harmonic generation with which we find excellent agreement.