Laser-assisted electron impact ionisation of helium at 256.5 eV

Calculations on the triple differential ionisation cross section for the above process for laser energies of 0.12 and 1.17 eV, are reported. The two cases considered are when the laser field is (i) perpendicular to the momentum transfer, and (ii) parallel to it. The fast electrons are described by plane waves while the target initial and final states are described in 1s, 2s, 2p close coupling. The results are shown to be determined by the behaviour of the Bessel functions occurring in a Kroll-Watson-type formula. (author)