Coherent ultrafast MI-FROG spectroscopy of optical field ionization in molecular H₂, N₂, and O₂

Quantitative phase-sensitive measurements of ultrafast optical-field ionization rates in molecular H₂, N₂, and O₂ are obtained using a temporally gated frequency-domain interferometric pulse measurement technique: multipulse interferometric frequency-resolved optical gating (MI-FROG). By measuring the pump-induced frequency change on a weak copropagating probe pulse, the optical field ionization dynamics can be completely time-resolved with subpulsewidth time resolution. A one-dimensional nonrelativistic electromagnetic fluid code model is used to compute the ionization dynamics and optical field propagation through the plasma. Using the Ammosov-Delone-Krainov (ADK) tunnel ionization rate model originally developed for atoms, the relatively simple model proposed here has been shown to compare favorably with the MI-FROG measured ionization rates in noble gases in the intermediate intensity regime (10¹⁴ W/cm²) [1]. We attempt to unify our studies in noble gases and molecules by performing experiments on N₂ and O₂, which have nearly identical ionization potentials as Ar and Xe, respectively. For the molecules studied here, we show that an ADK-like description of molecular ionization rates calculated from the model agree with the experimentally measured rates using the MI-FROG technique for H₂ and N₂. In the case of O₂, however, the experimentally measured ionization rate is approximately two orders of magnitude lower than that expected from the standard ADK formula. This is in agreement with the previously observed suppressed O₂ ionization rate in ion mass spectroscopy studies [2]. We attribute the suppressed ionization rate in O₂ to a multielectron screening effect and show that a modified version of the ADK formula, taking into account the electron screening as proposed in [2], well approximates the MI-FROG O₂ ionization rate data.