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

The photoinitiated reactions after infrared excitation from the LiHF and Li DF complexes in the reactant valley are studied as an extension of a recent communication by Paniagua [J. Chem. Phys. 109, 2971 (1998)]. For LiHF two broad bands, associated to Delta upsilon = 1 and 2 transitions, are obtaine d at which the probability of forming LiF products is very high, > 90%. For LiDF the Delta upsilon = 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 hi gh, starting at a value about 30% and increasing rapidly to > 90% with incr easing energy. This implies the tunneling through the barrier. The reason f or the high efficiency in the photoinitiated reaction is that the main exci tation corresponds to the HF (or DF) stretch within the complex, which is t he "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 L i+DF collisions at the same total energies, the reaction probabilities bein g much lower in these latter since the excitation of the HF (DF) mode is un likely to occur during the collision. (C) 1999 American Institute of Physic s. [S0021-9606(99)00539-5].