Translational temperature dependence of mode-to-mode vibrational energy flow in 1B3u naphthalene induced by low energy collisions with Ar

Abstract

Mode-to-mode vibrational energy flow from the 8̄1 (b1g) level in the 1B3u state of naphthalene (C10H8) has been mapped in the energy regime εvib≤800 cm−1. Vibrational state changes are induced by low energy collisions with the carrier gas Ar in the warm to cool regions of a supersonic expansion. The pattern of energy transfer is estimated from time-resolved dispersed fluorescence spectra obtained following laser excitation of the absorption transition 8̄10. Propensities for particular transfer channels are found to be in qualitative accord with expectations based on studies of single-ring aromatics such as benzene and aniline. One-quantum changes are preferred over two-quantum changes and an energy gap law is evident. The competition between certain vibrational energy transfer channels is examined as a function of the translational temperature Ttrans at specific distances X/D from the nozzle aperture. At X/D≤1.5, evidence is found for both endoergic and exoergic transfer channels. Ttrans at X/D=1.5 is determined from our data to be 80±10 K. It is established that the collision energy here is sufficient to support vibrational relaxation out of the 8̄1 level via some endoergic transfers. The most efficient endoergic channel involves the addition of a single quantum of a low frequency out-of-plane mode with εvib=127 cm−1. At X/D≳2, endoergic channels appear to close and only exoergic transfer is observed. The dominant exoergic channel from 8̄1 is found to be transfer to the 1B3u zero point level, reached through the loss of the ν8̄ quantum (ΔE=435 cm−1). The collision energies at which endoergic transfer ceases to occur are found to agree with expectations based on an assumption of Maxwell–Boltzmann statistics and the estimation of Ttrans for particular values of X/D using our spectroscopic data.