DIRECT MEASUREMENTS OF ROTATION-SPECIFIC, STATE-TO-STATE VIBRATIONAL-ENERGY TRANSFER IN HIGHLY VIBRATIONALLY EXCITED ACETYLENE

Vibrational overtone excitation followed by laser-induced fluorescence detection allows the direct measurement of rotationally resolved vibrational energy transfer rates in highly vibrationally excited acetylene molecules. We detect transfer from the initial, even rotational states J(i) = 0-22 of 3 nu(3) (nu ($) over tilde(0)-9640 cm(-1)) to the nearly isoenergetic final state J(f) = 4 of nu(1) + nu(2) + nu 3 + 2 nu(4), l = 0 (nu ($) over tilde(0) = 9668 cm(-1)). For these pathways, we observe changes in energy of up to \Delta E\ = 530 cm(-1) (approximate to 2.5 kT) and cm in angular momentum quantum number of up to \Delta J\ = 18 in a single collision, and we measure state-to-state rate constants of about 0.1 mu s(-1)Torr(-1) (160 collisions). Measurements under single collision conditions ensure that the vibrational relaxation is free of any rotational equilibration. By applying detailed balance and summing the resulting reverse rate constants, we obtain a total rate constant of 1.3 mu s(-1)Torr(-1) (13 collisions) for transfer from nu(1) + nu(2) + nu(3) + 2 nu(4), l = 0, J(f) = 4 to all final = 9668 cm(-1) rotational states in 3 nu(3). The energy transfer rate between two specific rovibrational states decreases exponentially with increasing energy difference. The vibrational relaxation does not have a strong angular momentum dependence in general, but transfer from the initial rotational states 3 nu(3), J = 16, and J = 20 is anomalously fast. The Fermi resonance of 3 nu(3) and nu(1) + nu(2) + nu(3) + 2 nu(4) l = 0 appears to enhance collisional transfer between the pair by a factor of 10 or more over that for uncoupled levels, and the anomalously fast transfer from initial states 3 nu(3), J = 16 and 20 is probably due to their relatively strong, rotation-specific intramolecular coupling with other nearby, unobserved vibrational states.