Characteristic features of the ionization of air by ultrashort ultraviolet laser pulses

An investigation was made of multiphoton ionization of air by fourth-harmonic radiation from a YAG:Nd³⁺ laser (λ = 266 nm) operating in the mode-locked regime. The degree of nonlinearity of the multiphoton ionization process was K = 1.5 ± 0.2. The dependence of the photoelectron density on the gas pressure, at a fixed laser radiation intensity, had a maximum at a pressure of 1–10 Torr. An interpretation of this dependence is based on the process of inelastic collisions of a photoelectron with neutral gas particles.