STATE-TO-STATE ROTATIONAL ENERGY-TRANSFER IN HIGHLY VIBRATIONALLY EXCITED ACETYLENE

Vibrational overtone excitation of single rovibrational eigenstates in acetylene, followed by state-resolved, laser-induced fluorescence (LIF) interrogation of the collisionally populated quantum states, permits a direct determination of both the pathways and rates of state-to-state rotational energy transfer in a polyatomic molecule containing about 10 000 cm-1 of internal energy. The data, which we acquire under single-collision conditions, demonstrate the importance of rotational energy transfer, even at high levels of vibrational excitation. The observed state-to-state rotational energy transfer pathways populate a wide range of angular momentum states and account for about 70% of the total relaxation rate. About one-third of the total relaxation occurs by \DELTAJ\ = 2 transitions, which are the smallest allowed, but there are also single-collision energy transfer pathways with \DELTAJ\ as large as 20 and \DELTAE\ as large as 600 cm-1 (almost-equal-to 3kT). The state-resolved rate constants for rotational energy transfer decrease monotonically as the energy difference between the initial and final states increases. Empirical exponential energy gap and combined power-exponential gap fitting relations recover the energy dependence of the state-to-state rate constants, but a simple power gap law does not. The discrepancy between the total observed rotational energy transfer rate and the total collisional relaxation rate suggests that rapid vibrational energy transfer, perhaps enhanced by Coriolis or anharmonic coupling, occurs as well.