The probability of multilevel atom ionization by a femtosecond laser pulse

The article examines the dependence of the total probability of multilevel atom ionization on the field strength of the Ti:Sapphire-laser femtosecond pulse for different values of its length. The laser field strength varies in a wide range from subatomic to near-atomic values. It is shown that within the subatomic value area the dependence under consideration is in a good agreement with the Keldysh formula. However, when the pulse amplitude increases, both the decrease and the increase of the ionization rate are observed. These effects are due to the dependence of the discrete atomic level population dynamics on the laser pulse amplitude.