Multiphoton atomic ionization in the field of a very short laser pulse

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".