Discussion of Wednesday Afternoon Session More Dynamics of Larger Molecules

Abstract

The presentation by E.R. Bernstein of his work on the vibrational dynamics of aniline-Ar and aniline-CH4 Van der Waals complexes has raised two major questions. The first one deals with the actual singificance of the more and more common and widely used expressions: intramolecular vibrational redistribution (IVR) and vibrational predissociation (VP). These terms normally designates well-defined and often competing mechanisms: 1) IVR is the process governing within an isolated molecule the relaxation of excess energy into all the accessible vibrational degrees of freedom. When IVR is completed the system must be at equilibrium. When IVR is not completed, i.e. within a period of time which is such that kIVR τ ≤ 1 (kIVR = rate for IVR to process), the system must be found in a state intermediate between the initially prepared state and equilibrium. 2) VP is a process through which a molecular system, containing an excess vibrational energy larger than the energy required to break the system into two given fragments (dissociation energy of a given channel), evolves from the initially prepared state to a state made of these two fragments, one of which having less vibrational energy. Essentially VP must be accompanied by a change of at least one vibrational quantum in a particular mode. In most cases, VP is a multichannel process since the fragments can be found in various (vibrational or rotational) states. Each channel is characterized by a partial rate kiVP, such that: $$\mathop \Sigma \limits_i k_{VP}^i = 1/{\tau _{VP}}$$ where τVP is the global VP lifetime of the initially prepared state.