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

The reported observation of two-photon ionization of He by $13\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ photons of the TESLA Facility in T. Laarmann, A. R. B. de Castro, P. G\urtler, W. Laasch, J. Schulz, H. Wabnitz, and T. M\oller, Phys. Rev. A72, 023409 (2005) is ambiguous, as it is not supported by the experimental data presented. We discuss the lack of evidence for the claimed two-photon ionization.