Reply to “Comment on ‘Photoionization of helium atoms irradiated with intense vacuum ultraviolet free-electron laser light. Part I. Experimental study of multiphoton and single-photon processes’ ”

Abstract

We do not agree with the conclusion of the Comment by Charalambidis et al. questioning our observation of two-photon ionization of helium by intense radiation with $13\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ photons from a vuv free-electron laser. Two-photon ionization is clearly established by the detection of low-energy photoelectrons at $\ensuremath{\sim}1.7\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, which agrees very well with the expected energy for a two-photon ionization process.