Vibrational overtone excitation of single rovibrational eigenstates in acetylene, followed by statehyphen;resolved, laserhyphen;induced fluorescence (LIF) interrogation of the collisionally populated quantum states, permits a direct determination of both the pathways and rates of statehyphen;tohyphen;state rotational energy transfer in a polyatomic molecule containing about 10thinsp;000 cmminus;1of internal energy. The data, which we acquire under singlehyphen;collision conditions, demonstrate the importance of rotational energy transfer, even at high levels of vibrational excitation. The observed statehyphen;tohyphen;state rotational energy transfer pathways populate a wide range of angular momentum states and account for about 70percnt; of the total relaxation rate. About onehyphen;third of the total relaxation occurs by Verbar;Dgr;JVerbar;=2 transitions, which are the smallest allowed, but there are also singlehyphen;collision energy transfer pathways with Verbar;Dgr;JVerbar; as large as 20 and Verbar;Dgr;EVerbar; as large as 600 cmminus;1(ape;3kT). The statehyphen;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 powerhyphen;exponential gap fitting relations recover the energy dependence of the statehyphen;tohyphen;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.
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