Observation of laser-assisted electron-impact ionization in ultrashort intense laser fields

Abstract

A laser-assisted ($e, 2e$) [LA($e, 2e$)] process of Ar in an ultrashort (1.2 ps) near-infrared intense laser field, $\mathrm{Ar}+{e}^{\ensuremath{-}}+h\ensuremath{\nu}\ensuremath{\rightarrow}{\mathrm{Ar}}^{+}+{e}^{\ensuremath{-}}+{e}^{\ensuremath{-}}$, was observed by coincidence measurements of the fast electron scattered forward within the small-angle range and the slow electron emitted to the wide-angle range. The spectrum of the sum of the kinetic energies of fast and slow electrons exhibited a shoulder structure at the one-photon energy shift from the ($e, 2e$) peak for the Ar $3p$ orbital, which was unambiguously assigned to the LA($e, 2e$) process with the one-photon energy gain. The determined triply differential cross section of the LA($e, 2e$) process with one-photon gain from the laser field was approximately twice as large as a theoretical estimate and this enhancement was regarded as an evidence of the formation of light-dressed states of Ar.