Photodissociation dynamics of polyatomic molecules using degenerate four-wave mixing spectroscopy

Abstract

Two-photon degenerate four-wave mixing spectroscopy has been newly applied to detect photofragments formed by photodissociation. The photodissociation of methyl iodide has been studied at 266 nm by monitoring the atomic iodines nascent via two-photon degenerate four-wave mixing spectroscopy. The ground state $I(^2P_{3/2})$ was detected via the $5p^2P_{3/2}$ → $6p^2D_{5/2}$ two photon transition (304.67 nm), and the spin-orbit excited state $I^*(^2P_{1/2})$ via the $5p ^2P_{1/2}$ → $6p ^2D_{1/2}$ transition (304.02 nm). The signal line profiles were well characterized with Gaussian shaped functions. A cubic dependence of the signal intensity on the input laser was observed. The line center saturation intensities have been determined to be 3863 MW/㎠ for I and 1969 MW/㎠ for $I^*$. The signal ratios between I and $I^*$ has varied with the input laser intensity. In this study we have found that the $I^*$/I branching ratio has been extracted based on the various signal intensities. Two-photon degenerate four-wave mixing scheme is very useful for the measurement of the internal state distributions of the photofragments, which are formed in photodissociation.