Laser-assisted ( e , 2 e ) collisions in helium

We have studied the influence of a strong laser on the dynamics of fast (e,2e) collisions in helium, in asymmetric, coplanar geometry. The interaction of the laser field with the unbound electrons is treated in a nonperturbative way. The wave functions of the incident and scattered electrons in the laser field are treated as Volkov waves, while that of the ejected electron moving in the combined field of the residual He{sup +} ion and of the laser is obtained by generalizing the ansatz proposed by Joachain {ital et al.} [Phys. Rev. Lett. {bold 61}, 165 (1988)] for the case of atomic hydrogen. On the other hand, the interaction of bound electrons with the laser field is treated by using first-order perturbation theory, assuming that the electric-field strength is much less than the atomic unit e/a{sub 0}{sup 2}{approx_equal}5{times}10{sup 9} Vcm{sup {minus}1}. The required scattering amplitudes are evaluated by using two different implementations of the Dalgarno and Lewis method. The first approach uses a technique proposed by Zernik and Klopfenstein, based on Laplace transforms and analytic continuation procedures. The second approach is based on a Sturmian basis expansion. The influence of the laser parameters (photon energy and intensity and direction of polarization) on the angular distribution of the ejected electron is analyzed, and several illustrative examples are discussed. We find that in general the triple-differential cross sections are strongly dependent on the dressing of the projectile and the target by the laser field. {copyright} {ital 1997} {ital The American Physical Society}