Coherent Ultrafast Mi-Frog Spectroscopy Of Optical Field Ionization In Molecular H2, N2, And O2


Diatomic molecules; Frequency-resolved optical gating (FROG); Ionization; Optical propagation in plasma media; Ultrafast optics


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.

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IEEE Journal on Selected Topics in Quantum Electronics





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0035410181 (Scopus)

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