Closed analytical expressions for the probability of multiphoton ionization
of atoms and ions by a time-varying electric field E(t) are obtained by th
e imaginary time method. These expressions apply for arbitrary values of th
e Keldysh parameter gamma. The dependence of the ionization probability and
the photoelectron momentum spectrum on the shape of an ultrashort laser pu
lse is considered. (C) 2001 MAIK "Nauka/Interperiodica".