Kinematics of multiphoton ionisation in a laser pulse

Previous results for the shift of the ionisation potential of an atom in a laser field are used to investigate the kinematics of multiphoton ionisation in either a laser beam or a laser pulse. Previous results for a laser beam show essentially no shifts of the 'above-threshold ionisation' peaks in the electron energy distribution and only small widths. Results for a laser pulse show the possibility of large shifts and widths growing as the pulse becomes shorter. Electrons which absorb a larger number of photons are predicted to show less shifted and narrower peaks. 1. Introduction In a recent experiment (Kruit et a1 1983) the electron resulting from multiphoton ionisation of Xe was observed. The spectrum showed up to eight peaks with adjacent peaks separated by the energy of a laser photon. As the intensity was raised a novel phenomenon was observed which is the suppression of the lowest-energy peak. The explanation which was proposed (Muller et a1 1983) was that the effective ionisation potential is increased as the intensity is increased and for sufficiently high intensity, the lowest peak becomes energetically forbidden. The explanation was substantiated by an analysis of a model atom in a circularly polarised laser field (the Berson (1975) model). An expression for the ionisation potential as a function of laser intensity for an atom in a laser field of arbitrary polarisation was subsequently derived confirming this. The local ionisation potential, X (I) , as a function of the intensity was shown to be (Mittleman 1984a) X (I)-x(o) = U :) (I) + U!)(I) + A wp(I)-A wp(I) (1.1) where U:' and U!) are the ponderomotive potentials acting on the electron and final ion respectively. They are related by U:' = (Mi/ m) U!' (1.2) where m is the mass of the electron and Mi the mass of the ion. For example, for a linearly polarised laser U:'= e2E2/4mw2 (1.3) where E is the electric field amplitude of the laser and o its frequency. The energy shift (due to the laser) of the ground states of the atom and ion are given by A Wg'(I) and A Wg'(I) respectively.