Transition state spectroscopy via infrared excitation of Li⋯HF and Li⋯DF van der Waals precursors

Abstract

The photoinitiated reactions after infrared excitation from the LiHF and LiDF complexes in the reactant valley are studied as an extension of a recent communication by Paniagua et al. [J. Chem. Phys. 109, 2971 (1998)]. For LiHF two broad bands, associated to Δv=1 and 2 transitions, are obtained at which the probability of forming LiF products is very high, >90%. For LiDF the Δv=1 band consists of several narrow resonances, and some of them are supported by the barrier separating reactant and product valleys. Even at these resonances the reaction probability is relatively high, starting at a value about 30% and increasing rapidly to >90% with increasing energy. This implies the tunneling through the barrier. The reason for the high efficiency in the photoinitiated reaction is that the main excitation corresponds to the HF (or DF) stretch within the complex, which is the “active” mode for the reaction in agreement with the presence of a late barrier. These results are very different from those obtained in Li+HF or Li+DF collisions at the same total energies, the reaction probabilities being much lower in these latter since the excitation of the HF (DF) mode is unlikely to occur during the collision.