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

We report a series of experiments that examine electron-helium scattering in the presence of an Nd:YAG laser field of 1.17~eV photons. The goal of these experiments is to span the range of incident electron energies from 50~eV to 350~eV, and compare the results to the Kroll-Watson approximation\footnote{N. M. Kroll and K. M. Watson, Phys. Rev. A 8, 804 (1973)} (KWA) calculations. Effects of an intense laser field on the elastic scattering of electrons from argon were first reported by Andrick in 1976\footnote{D. Andrick and L. Langhans, J. Phys. B 9, L459 (1976)}. In general, KWA calculations have been adequate to describe experimental results where the photon energy is significantly less than the incident electron energy -- a major exception being the case of small scattering angles where large discrepencies have been noted\footnote{e.g., B. Wallbank and J. K. Holmes, Phys. Rev. A 48, R2515 (1973)}. Our experiments test the KWA over a range of electron incident energies that has not been previously investigated.