Product Energy Distributions

Abstract

The measurement of product translational and rotational energies, and in some cases vibrational energy, is often more readily accomplished than the measurement of the dissociation rate. As a result there exists a considerable body of experimental information about product energy distributions (FED) for many classes of reactions. The only simple model for treating these FED is the statistical one; however, there is a considerable diversity in its application. In the dissociation of large molecules at moderate to large excess energies, the translational, rotational, and vibrational energy distributions can be treated as continuous functions. On the other hand, in the dissociation of triatomic molecules, it is often possible to measure the quantized rotational energy distribution for specific vibrational energy levels of the diatomic product. Just as in the determination of the dissociation rates, product energy partitioning is highly sensitive to the potential energy surface. If there is no reverse activation barrier, the product energies are often distributed statistically. That is, the distributions depend only upon the product phase space and are independent of the detailed shape of the potential energy surface. On the other hand, for reactions with a “tight” transition state located at the top of a reverse activation barrier, statistical redistribution of the product energies is often not possible. After passing through the transition-state region, the products move down the repulsive wall and rapidly dissociate with little chance to exchange and equilibrate the available energy. Often, such products are ejected with considerable translational energy. This happens in large as well as small molecules or ions. The resulting product energy partitioning is then highly nonstatistical, even though the dissociation rate is perfectly predicted by RRKM theory. That is, the dissociation rate and product energy partitioning are separate and uncoupled events. The rate is governed early in the reaction history by the structure of the transition state, while product energy partitioning is determined late in the reaction and is governed by the shape of the potential energy surface at large internuclear distances. The most effective model for treating product energy distributions (PEDs) of reactions with no reverse activation barriers is the statistical theory.