Global reaction pathways in the photodissociation of I₃ ^- ions in solution at 267 and 400 nm studied by picosecond X-ray liquidography

The mechanism of a photochemical reaction involves the formation and dissociation of various short-lived species on ultrafast timescales and therefore its characterization requires detailed structural information on the transient species. By making use of a structurally sensitive X-ray probe, time-resolved X-ray liquidography (TRXL) can directly elucidate the structures of reacting molecules in the solution phase and thus determine the comprehensive reaction mechanism with high accuracy. In this work, by performing TRXL measurements at two different wavelengths (400 and 267 nm), the reaction mechanism of I₃ ^- photolysis, which changes subtly depending on the excitation wavelength, is elucidated. Upon 400 nm photoexcitation, the I₃ ^- ion dissociates into I ₂ ^- and I. By contrast, upon 267 nm photoexcitation, the I₃ ^- ion undergoes both two-body dissociation (I ₂ ^-+I) and three-body dissociation (I^-+2I) with 7:3 molar ratio. At both excitation wavelengths, all the transient species ultimately disappear in 80 ns by recombining to form the I₃ ^- ion nongeminately. In addition to the reaction dynamics of solute species, the results reveal the transient structure of the solute/solvent cage and the changes in solvent density and temperature as a function of time. An eye for an I: By performing time-resolved X-ray liquidography measurements at two different excitation wavelengths, the wavelength dependence of the reaction mechanism of I₃ ^- photodissociation is elucidated (see picture). In addition to the reaction dynamics of solute species, the results reveal the transient structure of the solute/solvent cage and the changes in solvent density and temperature as a function of time.