Closed analytic expressions are derived for the probability of multiphoton
atomic and ionic ionization in a variable electric field E(t), which are ap
plicable for arbitrary Keldysh parameters gamma. Dependencies of the ioniza
tion probability and photoelectron pulse spectrum on the shape of a very sh
ort laser pulse are analyzed. Examples of pulse fields of various forms, in
cluding a modulated light pulse with a Gaussian or Lorentz envelope, are co
nsidered in detail. The interference effect in the photoelectron energy spe
ctrum during atomic ionization by a periodic field of a general form is exa
mined. The range of applicability of the adiabatic approximation in the mul
tiphoton ionization theory is discussed. The imaginary time method is used
in the calculations, which allows the probability of particle tunneling thr
ough oscillating barriers to be effectively calculated. (C) 2001 MAIK "Nauk
a/Interperiodica".