Resonances in heavy + light–heavy atom reactions: influence on differential and integral cross-sections and on transition-state photodetachment spectra

Abstract

The effect of resonances on the observable properties of chemical reactions is studied theoretically, in particular for I + HI → IH + I and Cl + HCl → ClH + Cl. All of our calculations use hyperspherical coordinates and accurate coupled-channel solutions to the Schrödinger equation for reactive scattering in three dimensions. For I + HI, we investigate the effect of potential surface variation on transition-state IHI^– photodetachment spectra for total angular momentum quantum number equal to zero. Four different I + HI London–Eyring–Polanyi–Sato (LEPS) surfaces are used, with classical barrier heights varying from 0.048 to 0.243 eV. The low-energy portion of the photodetachment spectra are examined in detail; it is found that peaks in the spectra arising from resonances and from direct scattering move up and broaden in energy as the barrier height is increased. An approximate match of the theoretical peak widths and spacings with the experimental ones is obtained for one of the surfaces. However, the peak intensities differ, suggesting that LEPS surfaces may not be adequate to characterize fully the observed spectra. For Cl + HCl, we perform a centrifugal-sudden hyperspherical calculation in order to examine the scattering properties of the single isolated resonance with transition-state quantum numbers (ν₁, ν₂, ν₃)=(0, 0, 2)(where ν₁= symmetric stretching, v₂= bending, V₃= asymmetric stretching quantum numbers). This relatively narrow (width 4 meV) resonance produces substantial peaks in certain state resolved reaction probabilities, which leads to a smooth step-like behaviour in the integral cross-sections, and Breit–Wigner (or Lorentzian) peaks in the differential cross-sections. Simple dynamical models are developed which explain these results.