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 (b_1g) level in the ¹B_3u state of naphthalene (C₁₀H₈) has been mapped in the energy regime ε_vib≤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 T_trans at specific distances X/D from the nozzle aperture. At X/D≤1.5, evidence is found for both endoergic and exoergic transfer channels. T_trans 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 ε_vib=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 ¹B_3u zero point level, reached through the loss of the ν_8̄ 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 T_trans for particular values of X/D using our spectroscopic data.