Nearside-farside analysis of differential cross sections: The reaction I + HI → IH + I

We have carried out quantum calculations of differential cross sections for the state selected reaction, I + HI(i, ji) → IH(f, jf) + I, where i, ji and f, jf are initial and final vibrational and rotational quantum numbers respectively. The extended London–Eyring–Polanyi–Sato potential energy surface A of Manz and Römelt (J. Manz and J. Römelt, Chem. Phys. Lett., 1981, 81, 179) was employed. The scattering matrix elements were computed by a quantum method which applies a Born–Oppenheimer type separation to the motion of the light and heavy atoms; a centrifugal-sudden approximation was also made. The scattering calculations were performed for three values of the total energy. Eighteen differential cross sections are reported, which display forward, sideward and backward scattering. Structure in the angular distributions is analysed using a semiclassical optical model. In addition, a nearside-farside (NF) decomposition is applied to the Legendre partial wave series representation of the scattering amplitude. The semiclassical optical model supplies a physical interpretation of the backward scattering in terms of simple classical concepts, but fails to interpret the sideward and forward scattering. In contrast, NF theory nearly always provides a physically clear explanation of the forward, sideward and backward scattering.