Many-photon ionization of atoms

The review reports the most important results of the investigation of a multiphoton ionization of atoms; Considered are specific features of the process, theoretical methods of description of the atomic ionization in a strong light field, basic principles of experimental procedures using intensive laser radiation, results of studies on direct and resonance processes of the multiphoton ionization. The most important result following from the investigation of the atomic ionization in a strong light field is that at γ > 1 (γ is the ratio of the tunneling time to the period of field γ=ω/ωsub(tun)) the ionization process is either of a direct or resonance character against the light frequency. The process of direct ionization, proceeding at the field intensity less than 107V/cm at γ > 10 and at a considerable difference between energies Khω (1 (<=) K (<=) Ksub(o); K0 is the number of photons absorbed by an atomic electron) and those of unperturbed bound states of an electron in the atom, is satisfactorily described by the nonstationary perturbation theory of the K0-th order, in which an unperturbed atomic spectrum serves as a basis. One of the most improtant conclusions from investigations of the resonance process, when the energy of a number K of quanta equals that of some quasi-stationary state of an electron in the atom, consists in the possibility to obtain information on quasi-stationary electron states in the atom. Namely in this direction data are obtained on constants of the quadratic perturbation of bound electron states in a light field. The interrelation between multiphoton processes (multiphoton excitation, multiphoton ionization) and different phenomena leading to a perturbation of a spectrum of bound electron states in an atom under a strong light field is discussed