Computational study of elastic and electronically inelastic scattering of Br by ground state I atoms: Role of potential curve crossing

Abstract

Two‐state, close‐coupled quantal computations of the elastic and inelastic scattering of ground‐state I atoms by ground‐state Br and spin–orbit excited Br* atoms have been carried out over a range of total energies E from 0.01 to 0.94 eV. The possibility of translational–electronic energy transfer arises from the ³Π_0⁺ potential curve crossing at E=0.25 eV, responsible for the well‐known IBr predissociation. The Y, B, and V₁₂(R) diabatic potentials have been obtained by judicious extension (and manipulation) of the spectroscopically derived ? and ?′ adiabats. At energies below the threshold for Br* formation (E_th=0.457 eV) collisions of I+Br are necessarily elastic, exhibiting both shape and compound‐state resonances. These produce interesting interference patterns in the differential cross sections, but no significant inverse‐predissociation resonance (which might have been anticipated for E?0.25 eV). The main features of the elastic scattering can be fairly well approximated considering only the lower adiabat, even at postthreshold energies where the inelastic process becomes important. An oscillatory pattern found in the total elastic cross section has been identified with a barrier effect associated with the maximum in this ? adiabat. Total inelastic cross sections are well reproduced by the closed‐form Landau–Zener–Stueckelberg (LZS) approximation. The Boltzmann‐averaged LZS rate constant for the collisional deactivation I+Br*→I+Br is calculated to be 1.8×10⁻¹¹ cm³ sec⁻¹ at 300 K (with only a slight temperature dependence over the range 300–1000 K). This large rate is due to the inverse predissociation mechanism, common