Electron-helium free-free scattering in the presence of a laser field

We report measurements of the scattering of electrons by helium atoms in the presence of 1.17 eV photons from a Nd:YAG laser. The incident energy of the electrons was in the range 50 – 350 eV and the polarization of the laser was arranged to be parallel to the scattered electrons. Energy-shifted peaks corresponding both to one- and two-photon emission were observed. Calculations using the Kroll-Watson approximation are perfectly consistent with the data.