Eletron-helium laser-assisted free-free scattering for incident energies from 30 - 200 eV: effects of polarization direction

We report on experiments that examine electron-helium scattering in the presence of an Nd:YAG laser field of 1.17 eV photons. At each incident electron energy (30, 60, and 200 eV), the laser polarization direction is varied within a plane perpendicular to the scattering plane. We compare our results with Kroll-Watson approximation (KWA)\footnote{N. M. Kroll and K. M. Watson, Phys. Rev. A 8, 804 (1973)} calculations. Of particular interest is the case where the polarization is perpendicular to the scattering plane for which the KWA predicts vanishing cross section; other workers have found that the KWA tends to be inaccurate for those cases where it predicts small cross sections.\footnote{M. O. Musa, A. MacDonald, L. Tidswell, J. Holmes, and B. Wallbank, J. Phys. B, 43 (17):175201, 2010} However, the KWA describes our results within the experimental uncertainties.