Effect of changing reagent energy on reaction probability and product energy-distribution

Abstract

Experiment and theory have been used to ascertain the effect of various forms of reagent energy on the dynamics of three reactions; (a) F + HCl → HF + Cl, (b) F + D2→ DF + D and (c) H + Cl2→ HCl + Cl. The principal experimental technique was the arrested relaxation form of the infra-red chemiluminescence method, coupled with a quite wide variation in the temperature of the atomic or the molecular reagent. In addition, a novel approach was employed in which the “fluorescence depletion” of specified vibrational-rotational (v, J) states was measured, and hence the relative rate of reaction of these quantum states was obtained. The theoretical technique was the 3D classical trajectory method using LEPS (London, Eyring, Polanyi, Sato) potential-energy hypersurfaces. Some support was obtained from experiment and theory for the following generalisations. (1)ΔT and ΔV enhance the reaction rate-constant, but the former is more effective for these substantially-exothermic reactions. (ΔT and ΔV denote reagent translation and vibration enhancements, in excess of the activation-barrier). (2) On the average ΔT→ΔT′+ΔR′(the primed energies refer to reaction products; ΔR′ is enhancement in product rotational energy). (3) On the average ΔV→ΔV′. (1)–(3) are in accord with findings from earlier trajectory studies. The effect of substantial increase in collision-energy can be understood in terms of a contribution from “induced repulsive energy-release”, and that of increased reagent vibrational energy in terms of both induced repulsive energy-release and (more important)“induced attractive energy-release”. For the reaction F + HCl the increase in rate constant with reagent vibrational quantum number, k(v), was obtained for ν= 0–2, experimentally and theoretically. The fluorescence-depletion method was used for a preliminary measurement of the rotational dependence of reaction rate, k(J), in F + HCl(ν= 1). An estimate was obtained for the variation in threshold collision-energy, Ev′J′0, with the v′J′ state being formed in the reaction F + HCl(v= 0).