Collision-induced three-photon ionization

A semiclassical treatment of collision-induced three-photon ionization of atoms with a near two-photon resonance is presented. The system is described by an effective two-level system. The wavelength dependence of the ionization yield does not obey a power law. When the first excited state is far from resonance, and with a $\frac{C_6}{R^6}$ interatomic potential, a first-order correction to the expected $-\frac{3}{2}$ power law obeys a - ½ power law. This three-photon process promises to be an extremely sensitive method for the study of collisional effects in the far wing of two-photon $S-S$ and $S-D$ transitions, and without the requirement of high densities.