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 vibr ational energy transfer rates in highly vibrationally excited acetylen e molecules. We detect transfer from the initial, even rotational stat es J(i) = 0-22 of 3 nu(3) (nu ($) over tilde(0)-9640 cm(-1)) to the ne arly isoenergetic final state J(f) = 4 of nu(1) + nu(2) + nu 3 + 2 nu( 4), l = 0 (nu ($) over tilde(0) = 9668 cm(-1)). For these pathways, we observe changes in energy of up to \Delta E\ = 530 cm(-1) (approximat e 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 con stants of about 0.1 mu s(-1)Torr(-1) (160 collisions). Measurements un der 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 r ate 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 increas ing energy difference. The vibrational relaxation does not have a stro ng angular momentum dependence in general, but transfer from the initi al 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 facto r of 10 or more over that for uncoupled levels, and the anomalously fa st 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.