Angular distribution for reactive molecular collisions

Semiclassically, p exp(- / di y2/u.( V (R) E)) where the integral spans the region where the system moves under the barrier. We therefore expectp to scale exponentially down with the reduced mass. This type of momentum gap estimate provides a useful guide also in other examples of what we call predissociation. For tunneling, the momentum gap rule also tells us that states at energies just below the barrier height have the largest widths and so will be most amenable to observation in the frequency domain. 

Our agenda in this section is quite rich. We start with direct reactive collisions and the physically complementary case of compound collisions where the molecules stay for a while in the region of the chemical forces. Consistently with earlier sections of this chapter, we look at the angular distribution from a two-body point of view. Only later in the book will we leave the two-body simplicity and recognize thatthe products have different internal states that can be populated. The discussion will not be as quantitative as in earlier sections, and will blend physical 

1 The angular distribution as a probe of direct vs. compound collisions 

The observation of the very specific energy disposal in exoergic reactions (Section 1.2) is often accompanied by a strong preferential angular disposition of the products. For example, in reactions such as 

The Kl product molecules were observed in the backward hemisphere, with respect to the incident K atom. This is a rebound mode. On the other hand, for reactions such as 

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The study of quantum effects such as resonances In atom-molecule reactions has been largely confined to coupled-channel calculations for collisions constrained to colllnear geometries. Progress In quantum reactive scattering techniques Is reviewed periodically (1-4). A few 3D quantum calculations of simple reactions, some more approximate (5-17) than others (18-19), have been concerned with resonance features In the reaction dynamics, and with the Increasing sophistication and sensitivity of molecular beam experiments (20-23), It has become evident that the angular distribution of reaction products Is likely to be the most sensitive observable manifestation of resonant contributions to reaction mechanisms. 

Light-heavy-light reactions are also of experimental significance. For example, molecular beam measurements of angular distributions [2], and kinetic experiments yielding rate constants [3], have been carried out on the D+BrH- -DBr+H reaction. Furthermore, there has been much recent interest in the competition between non-reactive and reactive energy transfer in collisions such as H+C1H[4]. Vibrational quenching reactions like... 

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