Possible role of double scattering in electron - atom scattering in a laser field

By considering observations of double-scattering effects in the excitation of the level of He, gas density values are estimated for the laser-assisted elastic scattering experiments of Wallbank and Holmes (1993, 1994a, b) for which the Kroll - Watson approximation appears to fail. Using comparable densities for He and lower densities for Ar, and assuming the Kroll - Watson approximation for single-scattering events, differential cross sections are calculated including double scattering for laser-assisted scattering for a range of energies and scattering angles. Comparison with the observed values shows that double-scattering effects can give a semi-quantitative explanation of the apparent breakdown of the Kroll - Watson approximation in both He and Ar.