Collision-induced energy absorption and vibrational excitation by intense laser radiation in CH3F

Vibrational energy absorption and transfer in diatomic and polyatomic molecules from intense resonant laser pulses is studied in the regime where energy flow is determined by vibrational–vibrational (V–V) collisions. Simple theoretical models for single mode (diatomic) and two mode (polyatomic) oscillators, based on coupled rate equations with the addition of a laser pumping term, are presented. By invoking quasiequilibrium (temperature) assumptions on the vibrational level populations (confirmed with computer simulations) simple expressions are derived for the absorbed energy in the single mode case; and for two modes similar formulas based on a two-temperature model are derived and discussed in relation to the one mode model. For many cases of interest, CH3F included, the influence of a second mode on the total energy absorption is insignificant, with most of the energy flowing into the mode being pumped at nearly the same rate as for a single mode oscillator. The experimental work studies the absorption of the P(32) line of CO2 by 13CH3F, and the subsequent energy transfer among the several modes of the molecule. Approximately 1/4 J over 2 to 3 μsec delivers several quanta per molecule to the vibrational degrees of dreedom. Fitting the theoretical model to the measured absorption produces a V–V rate constant of γVV =1.0±0.5 μsec−1 Torr−1 for the ν3 mode. A determination of the energy partitioning among the several modes, by means of a technique for measuring the absolute energy stored in each, shows that roughly 50% of the absorbed energy is stored in the ν3 vibration with the remainder distributed among the other modes. Detailed knowledge of the partitioning allows confirmation of a specific energy flow path for CH3F.