Anomalous intensity-dependent vibrational distributions of oxygen molecules in a nonresonant laser field: A molecular perspective

We report on the results of nonlinear ionization and dissociation of gas-phase oxygen molecules in an intense (1011-13 W/cm2) 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