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

摘要

Quantitative phase-sensitive measurements of ultrafastnoptical-field ionization rates in molecular H2,nN2, and O2 are obtained using a temporally gatednfrequency-domain interferometric pulse measurement technique: multipulseninterferometric frequency-resolved optical gating (MI-FROG). Bynmeasuring the pump-induced frequency change on a weak copropagatingnprobe pulse, the optical field ionization dynamics can be completelyntime-resolved with sub-pulsewidth time resolution. A one-dimensionalnnonrelativistic electromagnetic fluid code model is used to compute thenionization dynamics and optical field propagation through the plasma.nUsing the Ammosov-Delone-Krainov (ADK) tunnel ionization rate modelnoriginally developed for atoms, the relatively simple model proposednhere has been shown to compare favorably with the MI-FROG measurednionization rates in noble gases in the intermediate intensity regime (10n14 W/cm2) (Siders et al, Phys. Rev. Lett.). Wenattempt to unify our studies in noble gases and molecules by performingnexperiments on N2 and O2, which have nearlynidentical ionization potentials to Ar and Xe, respectively. For thenmolecules studied here, we show that an ADK-like description ofnmolecular ionization rates calculated from the model agree with thenexperimentally measured rates using the MI-FROG technique for H2n and N2. In the case of O2, however, thenexperimentally measured ionization rate is approximately two orders ofnmagnitude lower than that expected from the standard ADK formula. Thisnis in agreement with the previously observed suppressed O2nionization rate in ion mass spectroscopy studies (Guo, 2000). Wenattribute the suppressed ionization rate in O2 to anmultielectron screening effect and show that a modified version of thenADK formula, taking into account the electron screening as proposed bynGuo, well approximates the MI-FROG O2 ionization rate data