The Bessel uniform approximation developed by Stine and Marcus is applied to the collinear H f Hz reaction on Diestler's potential energy surface no. 3 to which we have' previously applied other orders of approximation of classicai S matrix theory. It appears that an accurate treatment of this system by classical S matrix theory requires interference of real and complex trajectories. Calculations were also performed on two other potential energy surfaces in older to mole clearly understand the interrelationships of previous semiclassical and quasiclassical studies of this reaction.
Recently we applied [ i] the classica! S matrix theory to a model collinear reaction with masses carresponding to H + Hz and a realisticzpotential energy surface, Diestler's surface no. 3(D3)1 121. This surface was chosen because the probabiiity of reaction for trajectories beginning with ground-state vibrational energy++ is unity over an extended energy region (16.5-23.5 kcal/mole total energy [2]). This makes application of the theory more straightforward (as discussed in ref.
[l] and below) and makes the comparison to quantum mechanical results particularly interesting. The reaction becomes classically allbwed (in the sense of classical S matrix theory) on the D3 surface at a total energy between 24.5 and 15.0 kcal/mole. In ref.
[l] we reported on the application of the Airy uniform approximation using realvalued trajectories in the eners range 15-30 kcal/ mole and the initial value representation in the range 17-23 kcallmole. These calculations were in poor agreement with the ex&t quantum [2] results. Here * Supported in part by the National Science Foundation.