Spiers Memorial Lecture: the dynamics of elementary reactions

Abstract

Comparing the reaction F + Na₂→ NaF + Na(i)(where i are electronically excited states) in the thermal and hyperthermal regimes one sees evidence of a transition from near-statistical to a deterministic regime; the increased collision energy has narrowed the range of intermediate configurations. Ordering can be achieved more precisely by photoinducing reaction in an aligned condition, either in a van der Waals cluster (Wittig et al.) or in surface-aligned photoreaction. Both methods are reviewed. In the latter case the alignment can be demonstrated by studying photofragment angular distributions. The surface can enhance the photodissociation cross-section by 10³–10⁴ times. Theory indicates that reaction product angular distribution relative to the surface will be sensitive to the geometry of reactant adsorption. Photodetachment from surfaces could be used in a crossed ablated beam (CAB) variant of the crossed molecular-beam approach, employing beams which are purely free radicals, or (by ‘photoejection’) accelerated molecules. An adsorbate-covered surface with rectangular grooves that align photoreagents would constitute a ‘morphological catalyst’. We note pitfalls and progress in the realm of transition-state spectroscopy. Emission from sodium iodide vapour in the process of photodissociation, i.e. in an unbound ‘transition state’ NaI⁺, is obscured by the small fraction of NaI* that is present in a shallow bound state. For the case of H + D₂→ HD + D it has proved possible to detect HD₂^{[graphic omitted]} in the transition state by two-photon ionization.