In a chemical reaction, the shape of the potential energy surface (PES) dictates the reaction rate and energy disposal in the products. Not only does the dynamics depend crucially upon the features of the surface, but, ultimately one seeks to influence the course of the reaction by preparing selectively certain regions of the surface. For harpooning reactions, the propensity rules for energy disposal in the products (influence of the entrance kinetic energy, effect of the early or late barrier) have been established by Polanyi (1972) and have been used later as guidelines. Here, the surface may easily be modeled in simple terms using long-range electrostatic interaction in the entrance valley. There was, then, need of an experimental method which allows the possibility of observing directly the characteristic regions of this potential energy surface, but also to investigate precisely the surface in other types of reaction. The study of the reactivity of van der Waals complexes is intended to fulfil this purpose. In classical experiments, the surface is obtained by inversion of the experimental data which are differential cross sections and internal energy distribution of the products. This procedure is difficult and not unambiguous. The first step is to determine the correlation between the entrance channel's parameters (kinetic energy, internal energy, angular momentum) and the final states of the products (kinetic energy, internal energy, angular distribution). This requires a precise control of the entrance channel. Therefore, the goal of many experiments is to reduce the initial states to a small subset, and to measure the energy disposal in the products with the greatest accuracy. This was first achieved by controlling the kinetic energy of the reactants in crossed beam experiments. Later, a certain control of the collision geometry was obtained by orienting the molecules or the atomic orbitals in crossed beam experiments or by using prealigned systems in a van der Waals complex: this subject is discussed in Buelow et al. (1986).
Transport and Roaming on the Double van der Waals Potential Energy Surface
