Temperature dependence of state-to-state vibrational relaxation from the 441(1B2u) state of naphthalene induced by very low energy collisions with argon

Abstract

State-to-state vibrational relaxation rate coefficients have been obtained for naphthalene colliding with argon in the very low energy collision range. A single vibronic level v′44=1(ν44 is an in-plane ring distortion) is prepared by laser excitation of naphthalene expanded in a supersonic free jet of argon. Relaxation to specific destination states is monitored using time resolved dispersed fluorescence spectroscopy. The observed state-to-state relaxation rate coefficients vary substantially among the available pathways but are explained satisfactorily by simple propensity rules when the destination state involves only out-of-plane vibrational motion. Transfer to the sole in-plane destination state (v24=1) is found to be less facile by more than an order of magnitude relative to that expected from simple propensity rules. Measured propensities are found to be relatively independent of collision energy for the temperature range studied here (3–20 K). This suggests that the translational energy dependence of the state-to-state cross section σif is similar to that for the total inelastic cross section σi, where from previous studies of polyatomic vibrational relaxation in this temperature range it has been demonstrated that σi scales with the Lennard-Jones cross section.