Spin-orbit relaxation and recombination dynamics in I2−(CO2)n and I2−(OCS)n cluster ions: A new type of photofragment caging reaction

Abstract

We report a new type of photofragment caging reaction that is only possible because of the strong solvent-induced perturbation of the inherent electronic structure of the chromophore. The photoexcitation of I2− at 395 nm promotes it to a dissociative state correlating with I−+I*(2P1/2), the only near-ultraviolet dissociation channel for unsolvated I2−. In I2−(CO2)n and I2−(OCS)n clusters, interaction with the solvent is observed to result in extremely fast spin-orbit relaxation. In general, we detect three reaction pathways: (1) direct dissociation of the chromophore to I−+I*(2P1/2); (2) the I2−→I−+I* dissociation, followed by spin-orbit quenching leading to I−+I(2P3/2) products; and (3) the I2−→I−+I* dissociation, followed by spin-orbit quenching and I−+I(2P3/2)→I2− recombination and vibrational relaxation. We present experimental evidence of the spin-orbit relaxation and caging and discuss possible mechanisms. The results include: the measured translational energy release in 395 nm photodissociation of unsolvated I2−, indicating that solvation-free dissociation proceeds exclusively via the I−+I* channel; ionic product distributions in the photodissociation of size-selected I2−(CO2)n and I2−(OCS)n clusters at the same wavelength, indicating the above three reaction channels; and ultrafast pump-probe measurements of absorption recovery, indicating picosecond time scales of the caging reaction. We rule out the mechanisms of spin-orbit quenching relying on I*-solvent interactions without explicitly considering the perturbed electronic structure of I2−. Instead, as described by Delaney et al. (companion paper), the spin-orbit relaxation occurs by electron transfer from I− to I*(2P1/2), giving I(2P3/2)+I−. The 0.93 eV gap between the initial and final states in this transition is bridged by differential solvation due to solvent asymmetry. Favorable comparison of our experimental results and the theoretical simulations of Delaney et al. yield confidence in the mechanism and provide understanding of the role of cluster structure in spin-orbit relaxation and recombination dynamics.