INTRA-MOLECULAR DYNAMICS OF SOME VANDERWAALS DIMERS

In this paper we consider the problem of intramolecular vibrational energy flow from a conventional bond to a van der Waals bond in a linear AB-CD van der Waals molecule, where AB and CD are conventional diatomics. A model for collinear vibrational predissociation of AB-CD on a single ground state potential surface was developed. The vibrational predissociation of a van der Waals heterodimer, consisting of a pair of distinct diatomics, can be described in terms of a simple exponential decay of a single discrete zero order state which corresponds to excited bond modes, into a dissociative continuum. The dependence on the vibrational predissociation rate of the heterodimer on the parameters of the molecular bonds and of the van der Waals bond is elucidated. The details of the intramolecular dynamics of a homodimer, consisting of a pair of identical diatomics, are determined by the excitation conditions. Optical infrared short time excitation is expected to result in a coherent, in phase, perposition of degenerate bond modes, which will subsequently exhibit a simple exponential time evolution, the decay rate being determined by the resonance width. Collisional excitation of the dimer is expected to result in an incoherent initial superposition of degenerate bond modes, the subsequent time evolution being determined by the discrete-discrete coupling and by the widths of the metastable states. Model calculations were performed for the nuclear dynamics of collisionally excited linear halogen dimers A2⋯ A2(A=F, Cl, Br, I). The linear (Cl2)2, (Br 2)2, and (I2)2 dimers are characterized by negligibly small discrete-discrete coupling terms whereupon the nuclear dynamics of these van der Waals molecules is determined by exponential decay due to vibrational predissociation, without direct energy exchange between the bond modes. For the (F2)2 linear dimer the discrete-discrete coupling terms overwhelm the widths of the resonances and the system will exhibit efficient direct energy exchange between the molecular bond modes before the occurrence of vibrational predissociation. Model calculations for the vibrational predissociation lifetimes for the halogen dimers reveal remarkably long lifetimes, which can be accounted for in terms of the energy gap law for vibrational predissociation. Our results account for the ineffective process of intramolecular vibrational energy flow discovered by Dixon and Herschbach in the (Cl2)2 dimer. © 1979 American Institute of Physics.