Particle-atom ionising collisions in the presence of a laser radiation field

A theoretical treatment is presented of ionising collisions between a charged particle and a hydrogenic atom taking place in the presence of an intense laser radiation field. It consists of a reformulation, appropriate to the case, of the well known Coulomb wave Born (CWB) approximation. Two sets of calculations are presented for a coplanar geometry. In the first set (asymmetric case), the fast electron is taken at the fixed scattering angle theta 1 =20 degrees and the triple differential cross section (TDC) is calculated for different values of theta 2 , the angle of the slow electron ( theta 2 =40, 60, 140 and 260 degrees ). The energies of the incoming electron are epsilon (K i )=250, 300 and 400 eV, while those of the ejected electron are epsilon (q)=50, 75 and 100 eV. In the second set (symmetric case) the TDC is calculated for theta 1 = theta 2 =30, 45, 60 and 75 degrees and for equal energies of the outgoing electrons. The energies of the incoming electron are epsilon (K i )=300, 400 and 500 eV. For both cases the electric field of the laser is E L approximately=5*10 7 V cm -1 parallel to the direction of the incoming electron, and of frequency h(cross) omega =1.17 eV. Among other results, it is found that a sum rule, of the same kind as that known for laser-assisted free-free transitions, holds and exhibits a larger validity than expected. It is suggested that multiphoton effects predicted in ionising collisions could be directly observed by analysing the energy spectrum of the outgoing electrons.