Influence of third-harmonic fields on multiphoton ionization of noble gases in unfocused laser beams

Unfocused linearly polarized laser beams, in single-pass and counterpropagating configurations, are used in conjunction with absolute ionization measurements in calibrated proportional counters to examine the nature of multiphoton ionization of xenon near three-photon resonances. Mixtures of xenon with other noble gases are also used to incorporate phase matching in order to elucidate additional characteristics of multiphoton ionization in the presence of third-harmonic fields. Point-by-point comparisons are made between theoretically predicted and experimentally observed features of the study. We also show how detailed information on multiphoton ionization involving phase-matched third-harmonic photons can be used to obtain optical constants in the vacuum-ultraviolet region.