Scattering theory threshold effects

Whilst semi-classical theories have been successful in describing non-reactive processes such as elastic and inelastic scattering, they have had less impact on calculations for reactions. The majority of calculations have been for simple reactions such as H + H2, F + H2 or H + Cl2 which have been treated collinearly for the most part. The most successful use of semi-classical theory is for treating tunnelling and it has found use extending the scope of purely classical calculations by describing classically forbidden processes and threshold effects [156]. 

The simplest way of taking account of vibrational effects is to assume vibrational adiabaticity during the motion up to the critical dividing surface [27]. As mentioned already in the Introduction, much of the earlier work on vibrational adiabaticity was concerned with its relationship to transition-state theory, especially as applied to the prediction of thermal rate constants [24-26]. It is pointed out in [27] that the validity of the vibrationally adiabatic assumption is supported by the results of both quasiclassical and quantum scattering calculations. The effective thresholds indicated by the latter for the D + H2(v =1) and O + H2(v =1) reactions [37,38] are similar to those found from vibrationally adiabatic transition-state theory, which is a strong evidence for the correctness of the hypothesis of vibrational adiabaticity. Similar corroboration is provided by the combined transition-state and quasiclassical trajectory calculations [39-44]. For virtrrally all the A + BC systems studied [39-44], both collinearly and in three... 

Inelastic scattering of neutrons has recently received much attention because of its convenience for investigating reaction thresholds. The variation of yield of a reaction near threshold may be difficult to establish if there is an outgoing charged particle because of barrier effects, but with neutrons these are small and detailed comparison of the variation of yield may be made with theory. 

---