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 applicable for arbitrary Keldysh parameters gamma. Dependencies of the ionization probability and photoelectron pulse spectrum on the shape of a very short laser pulse are analyzed. Examples of pulse fields of various forms, including a modulated light pulse with a Gaussian or Lorentz envelope, are considered in detail. The interference effect in the photoelectron energy spectrum during atomic ionization by a periodic field of a general form is examined. The range of applicability of the adiabatic approximation in the multiphoton ionization theory is discussed. The imaginary time method is used in the calculations, which allows the probability of particle tunneling through oscillating barriers to be effectively calculated. (C) 2001 MAIK "Nauka/Interperiodica".