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 (≈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.
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