Experimental and theoretical study of free-free electron-helium scattering in a CO2 laser field

Free-free transitions during the scattering of electrons by helium in the presence of a linearly polarized CO2 laser field are investigated both experimentally and theoretically. Signals for laser-assisted scattering at 22 eV with absorption or emission of up to two photons are measured at scattering angles between 20 degrees and 70 degrees, and are compared to the values obtained from an 11-state R-matrix Floquet calculation and from the low-frequency approximation of Kroll and Watson. The two sets of theoretical results are found to be in very good agreement for the scattering geometries considered in the experiment. The order of magnitude of the experimental results is reproduced by calculations with intensities in the region of 10(7) W cm(-2). Agreement is improved by averaging the theoretical results over the spatial distributions of the three beams as well as the temporal intensity profile of the laser pulse, and by allowing for some misalignment of the three beams in the experiment.