Nonresonant Multiphoton Ionization

Ionization is perhaps the most common multiphoton phenomenon that occurs when a strong enough radiation field, virtually of any frequency, is made to interact with gas atoms. As the sum of energies of the absorbed photons exceeds the ionization threshold-energy, the continuum states of the atom allow for both the necessary energy conservation and the satisfaction of the angular-momentum selection rules which give rise to a nonvanishing ionization probability. The degree of ionization, however, depends in significant and characteristic ways on the strength, polarization, and frequency of the field.