Multiphoton Dissociation of Polyatomic Molecules

Abstract

The dynamics of infrared multiphoton excitation and dissociation of SF/sub 6/ was investigated under collision free conditions by a crossed laser-molecular beam method. In order to understand the excitation mechanism and to elucidate the requirements of laser intensity and energy fluence, a series of experiments were carried out to measure the dissociation yield dependences on energy fluence, vibrational temperature of SF/sub 6/, the pulse duration of the CO/sub 2/ laser and the frequency in both one and two laser experiments. Translational energy distributions of the SF/sub 5/ dissociation product measured by time of flight and angular distributions and the dissociation lifetime of excited SF/sub 6/ as inferred from the observation of secondary dissociation of SF/sub 5/ into SF/sub 4/ and F during the laser pulse suggest that the dynamics of dissociation of excited molecules is dominated by complete energy randomization and rapid intramolecular energy transfer on a nanosecond timescale, and can be adequately described by RRKM theory. An improved phenomenological model including the initial intensity dependent excitation, a rate equation describing the absorption and stimulated emission of single photons, and the unimolecular dissociation of excited molecules is constructed based on available experimental results. The model shows that the energy fluencemore » of the laser determines the excitation of molecules in the quasi-continuum and the excess energy with which molecules dissociate after the laser pulse. The role played by the laser intensity in multiphoton dissociation is more significant than just that of overcoming the intensity dependent absorption in the lowest levels. 63 references.« less