Theory of multilevel atom iomzation

We present the results of computer simulations on ionization of multilevel atom by femtosecond laser pulse in wide range of its intensity, from low intensities to intensities that overcome atomic one I0 = 3.5 upsilon . 10(16) W/cm2. The quantitative results are obtained for the case of silver atom ionization by the pulses of Ti:Sapphire ( lambda = 800 nm) and Nd:YAG ( lambda = 1.064 mu m) lasers. The main goal of our studies was to calculate the probability of ionization as a function of laser field strength as far as the photoelectron energy spectra in dependence on laser pulse parameters and here we present our results. In subatomic region of laser field strength the calculated probability of ionization is in good agreement with Keldysh formula. However, drastic difference between the calculated data and Keldysh formula appeared when the laser field strength approaches to near-atomic values. The calculated dependencies demonstrate stabilization, drop, and enhancement of ionization probability, which are not observed in the model of atom having only one bound state (Keldysh model). This discrepancy is due to the fact that the rate of transitions between the continuum and discrete spectrum states, i.e. the rate of ionization and recombination processes, depends significantly on their population difference. At the same time, in the absence of resonances between laser photon energy and interlevel energy spacings in atom, the rate of transitions from the ground state to the continuum states and higher-lying discrete spectrum states depends non-linearly on the laser field strength. As a result the ionization probability became the non-monotomc function of field strength.