We report on the results of nonlinear ionization and dissociation of gas-phase oxygen molecules in an intense (${10}^{11\mathrm{-}13}$ W/${\mathrm{cm}}^2$) laser field. The investigations were performed with the second- and third-harmonic wavelengths of two different laser systems; a 50-ps Nd:YLF (where YLF denotes yttrium lithium fluoride) and a 10-ns Nd:YAG (where YAG represents yttrium aluminum garnet). Measurements include energy resolved photoelectron and mass spectroscopy. We report on an interesting observation made with excitation of the neutral with green (∼2.3 eV) photons, in which the vibrational distribution of the molecular ion shows a strong intensity dependence. We have analyzed this intensity-dependent behavior to be a result of a complex interplay between different resonant and nonresonant electronic channels, which to first approximation does not require dynamical Stark shifting of states. A model is presented that describes this behavior and results in the determination of the molecular constants for the intermediate states.

• Received 5 April 1991

DOI:https://doi.org/10.1103/PhysRevA.44.4493