On the photodissociation of alkali-metal halides in solution

Gas-phase alkali-metal halide dissociation is influenced by the crossing of the covalent and ionic potential-energy surfaces at a certain internuclear separation, leading to interesting dynamical effects. The dissociation fragments for e.g. NaI may be trapped in a well formed by the avoided crossing of the covalent and ionic surfaces, and then undergo a non-adiabatic curve crossing transition to form atomic products. On the other hand, ionic products are stabilized by a polar environment and might be energetically accessible in solution. More generally, the photodissociation dynamics could be influenced by the solvent. A theoretical study of NaI photodissociation in a weakly polar solvent is presented here to explore the mechanism and timescale by which the ions are produced subsequent to photoexcitation. A solution-phase valence-bond resonance theory predicts that the diabatic ionic and covalent solution Gibbs free energy curves do not cross in the equilibrium solvation regime, such that atomic products would result. When considering non-equilibrium solvation and dynamical effects, the theory indicates the short-time dissociation products in solution to be atoms, but that on the ms timescale they could convert to ions by activated inverted regime electron transfer (ET). However, the radiative lifetime is estimated to be much shorter (approximate to ns) than this timescale, so that in fact no excited state ET is expected. Instead, the formation of ions proceeds by radiative deactivation of the photoexcited NaI and is followed by ionic recombination on the ground-state surface. Nevertheless it is estimated that the photodissociation of NaI in small clusters may proceed via activated ET and lead to some ionic dissociation products.