Photoionization of a gas by an ultrashort laser pulse with two-photon excitation of an intermediate level

Photoionization of a gas by a laser pulse propagating under conditions of a coherent two-photon interaction with the atoms (or molecules) of a medium is discussed. It is shown that the small yield of electrons for small photoionization cross sections remains small even in a strong field when the cross section is large. The reason is an ionization-induced broadening of an intermediate level. As a result, an extremum appears in the ionization yield as a function of the cross section. It is also shown that the two-photon self-induced transparency for the field of the pulse at this transition can significantly decrease the energy cost of ionizing the gas.