Analytic formulae for occupation probabilities of atomic states in strong short laser pulses

Explicit analytic expressions, involving elementary functions, are given for occupation probabilities of atomic states after one or a few cycles of a strong short laser pulse. These formulae are strictly valid in the asymptotic limit of large momentum transfer from the field to the atom during a half cycle of the pulse. The cases of a hydrogenic atom, of a negative ion and of multielectron atoms in the independent-electron approximation are considered. The asymptotic expressions, derived in the limit of infinite momentum transfer from the field, have also the correct limit for vanishingly small momentum transfers, so that fortuitously they are valid for arbitrary momentum transfer