Multiphoton effects in laser-assisted ionization of a helium atom by electron impact.

The dynamics of the electron impact multiphoton ionization of a He atom in the presence of an intense laser field ( n $\gamma _e$, 2 e) is studied theoretically for laser polarization ( $\vert\vert^l$) and perpendicular ( $\bot^r$) to the incident momentum. The triple differential (TDCS) as well as the double differential (DDCS) cross sections are studied for the coplanar asymmetric geometry. The results are compared with the only available kinematically complete experiment at high incident energy (1000 eV). Significant laser modification (enhancement) is noted due to multiphoton effects in the present binary and recoil peak intensities of the TDCS for both the geometries, in qualitative agreement with the experiment. In the single photon case, the net effect of the laser field is to suppress the field free (FF) TDCS as well as the DDCS in the zeroth order approximation of the ejected electron wave function (CV), while in the first order (MCV), the cross sections are found to be enhanced. The CV multiphoton cross sections obey the famous Kroll Watson (KW) sum rule while the latter does not hold good in the corresponding MCV approximation. [ABSTRACT FROM AUTHOR]