Intermolecular and intramolecular V→V transfer in the vibrational predissociation of some polyatomic van der Waals molecules

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Abstract
In this paper we present a theoretical study of vibrational predissociation (VP) on the ground state potential surface of van der Waals complexes consisting of polyatomic molecules. The intermolecular interaction was represented by separate contributions of atom–atom interactions, which were expressed in terms of Morse potentials. The VP process was handled utilizing the distorted wave approximation to describe a zero‐order Hamiltonian, which is separable in the intramolecular and in the intermolecular motion, while the residual interaction, which induces the reactive process, corresponds to the deviations of the intermolecular interaction from its value at the frozen intramolecular equilibrium configurations of the two constituents. Model calculations of the VP dynamics were performed for the He⋅⋅⋅N2O complex and for the (N2O)2 dimer, where the N2O unit is initially excited to the (001) vibrational state. These calculations were performed for the linear configuration as well as for the T‐shaped configuration of the complexes. We have considered three VP channels; (1) the V→T process, (2) the intramolecular V→V+T process and (3) the intermolecular V→V+T process. For He⋅⋅⋅N2O channels (1) and (2) prevail, while for (N2O)2 all three channels are open. For collinear He⋅⋅⋅N2O a second‐order process of type (2) dominates the VP, while for the T‐shaped He⋅⋅⋅N2O complex the first‐order process (1) is not efficient. In both cases the VP lifetimes, τ is τ∼10−5–10−6 sec. For the collinear (N2O)2 dimer a V→V+T process of type (2) dominates, with τ∼10−2–10−3 sec, the inefficiency of this process being attributed to a large mass effect. For the T‐shaped (N2O)2 dimer the most effective VP channel involves simultaneous intermolecular and intramolecular V→V+T process with low ΔE = 355 cm−1 translational energy, which is characterized by τ∼10−5–10−6 sec. These results provide guidelines for the analysis of the infrared spectra of the (N2O)2 complex.