Exchange effects in laser-assisted electron-atom scattering

The electronic exchange in laser-assisted electron-atom scattering is considered. Two processes are studied: (i) the elastic 1s-1s transition in hydrogen, that, although not yet available experimentally, allows to get explicit results and insight into the problem; (ii) the excitation 11S-23S of helium, of particular experimental interest. For perpendicular geometry, the forward multiphoton cross-sections in hydrogen are dominated by the exchange; the exchange is instead reduced when the classical electron speed in a plane wave is much larger than the incident speed. This last feature is also found with the helium target both for parallel and perpendicular geometry, and leads to interesting predictions when applied to experimental situations recently achieved