Coherent ultrafast MI-FROG spectroscopy of optical field ionization in molecular H/sub 2/, N/sub 2/, and O/sub 2/

Quantitative phase-sensitive measurements of ultrafast optical-field ionization rates in molecular H/sub 2/, N/sub 2/, and O/sub 2/ 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 sub-pulsewidth 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 (10/sup 14/ W/cm/sup 2/) (Siders et al, Phys. Rev. Lett.). We attempt to unify our studies in noble gases and molecules by performing experiments on N/sub 2/ and O/sub 2/, which have nearly identical ionization potentials to 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 H/sub 2/ and N/sub 2/. In the case of O/sub 2/, 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 O/sub 2/ ionization rate in ion mass spectroscopy studies (Guo, 2000). We attribute the suppressed ionization rate in O/sub 2/ to a multielectron screening effect and show that a modified version of the ADK formula, taking into account the electron screening as proposed by Guo, well approximates the MI-FROG O/sub 2/ ionization rate data.