Ponderomotive force and AC Stark shift in multiphoton ionisation

Abstract

A non-perturbative approach to the photoionisation of a model atom in arbitrarily strong fields is described. An explanation is given for the disappearance of the lowest energy electrons in a multiphoton ionisation experiment at very high intensities, in terms of the AC Stark shift of the ionisation potential of the atom, followed by ponderomotive acceleration of the photoelectrons.