A laser-assisted ($e, 2e$) [LA($e, 2e$)] process of Ar in an ultrashort (1.2 ps) near-infrared intense laser field, $\mathrm{Ar}+e^{-}+h\nu \to {\mathrm{Ar}}^{+}+e^{-}+e^{-}$, 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.

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• Electron & positron scattering
• Electronic structure of atoms & molecules
• Ultrafast phenomena