Vibration Dynamics in Solute/Solvent Clusters: Aniline (Ar)1 and Aniline (CH4)1

Abstract

The first excited electronic state (S1) vibrational dynamics of aniline(Ar)1 and aniline(CH4)1 Van der Waals (vdW) clusters have been studied using molecular jet and time resolved emission spectroscopic techniques. The rates of intramolecular vibrational energy redistribution (IVR) and vibrational predissociation (VP) as a function of excess vibrational energy are reported for both clusters. For vibrational energy in excess of the cluster binding energy, both clusters are observed to dissociate. The dispersed emission spectra of these clusters demonstrate that aniline(Ar)1 dissociates to all energetically accessible bare molecule states and that aniline(CH4)1 dissociates selectively to only the bare molecule vibrationless state. The emission kinetics show that in the aniline(Ar)1 case, the initially excited states have nanosecond lifetimes, and intermediate cluster states have very short lifetimes. In contrast, the initially excited aniline(CH4)1 states and other vibrationally excited cluster states are very short lived (<100 ps), and the intermediate cluster 00 state i s observed. These results can be understood semiquantitatively in terms of an overall serial IVR/VP mechanism which consists of the following elements: 1. the rates of chromophore to vdW mode IVR are given by Fermi’s golden rule, and the density of vdW vibrational states is the most important factor in determining the relative [aniline(Ar)1 vs. aniline(CH4)1] rates of IVR; 2. IVR among the vdW modes is rapid; and 3. VP rates can be calculated by a restricted vdW mode phase space RRKM theory. Since the density of vdW states is three orders of magnitude greater for aniline(CH4)1 thananiline(Ar)1 at 700 cm-1 of excess energy in S1, the model predicts that IVR is slow and rate limiting in aniline(Ar)1, whereas VP is slow and rate limiting in aniline(CH4)1. The agreement of these predictions with the experimental results is very good and i s discussed in detail.