Recent Aspects of Multiphoton Ionization of Atoms

Abstract

It is now more than twenty five years since the first demonstrations of multiphoton ionization (MPI) of atoms have been carried out. For quite a number of years, the natural theoretical framework to understand this process was the time-dependent perturbation theory in the lowest non-vanishing order (LOPT) in which the radiation field induces transitions between unperturbed atomic states. The atom absorbs photons and jumps into virtual intermediate states until the total energy absorbed is larger than the ionization potential. If the coupling between the initial (bound) state and the final (free) state requires the absorption of a minimum number of photons N, from elementary perturbation theory, the probability of the process scales as IN, the characteristic power law. There is however, no reason why N should be a maximum. As a matter of fact, the atom does, in general, absorb more photons than N in a process which is known as “Above-Threshold Ionization” (ATI) reaching final states with energies separated by the photon energy, as was observed in the photoelectron energy spectra (Agostini et al., 1979). As long as the energy of all the intermediate virtual states is far enough from the energy of any real state allowed by proper selection rules there is a clear distinction between a resonant process and a non resonant one. However, it was soon recognized that such a description is not sufficient because at the intensity required to observe the ionization, all the atomic energies are intensity-dependent and a non-resonant situation may become resonant.