ENERGY RELAXATION OF HIGHLY VIBRATIONALLY EXCITED MOLECULES - HOMOLOGOUS SERIES CNF2N-8) AND CNH2N+1F (N = 6, 8)(2 (N = 3)

Vibrational to translational (VT) energy-transfer rate constants (k(VT )) for two series of fluoroalkanes with argon as deactivator were meas ured by using time-resolved optoacoustics. A pulsed CO2 laser was used to excite the fluorinated alkanes; the average excitation energy, [E] , was in the range 15 000-40 000 cm-1. k(VT) was found to be independe nt of [E], indicating that the average energy transferred per collisio n, ((DELTAE)), is linear with [E]. It is observed that k(VT) decreases (by <30%) as the number of vibrational modes increases by a factor ap proximately 2.7 in this homologous series. This is contrary to that ob served for small molecules at low excitation energy, where k(VT) incre ases. Both the observed per-collision relaxation efficiency, beta(obs) , and [[DELTAE]]/[E] decrease with an increase in the number of carbon or fluorine atoms. These results are in qualitative agreement with a model in which the relaxation occurs via a single low-frequency doorwa y oscillator which is in statistical equilibrium with the remaining os cillators (bath) such that E(osc) = g[E]. This oscillator has an intri nsic efficiency, beta(int) = [[DELTAE]]/[E]osc, which is equal for all members of the series, so beta(obs) = gbeta(int). The decrease in bet a(obs) and [[DELTAE]] is due to the resulting decrease in [E]osc as th e number of effective vibrational modes increases for a given total en ergy.