Caged Contact Pair Formation in Cooled Triiodide-Ethanol Solutions

We have conducted spectrally-resolved optical pump-probe experiments following the photodissociation of triiodide (I₃^-) in cooled ethanol solutions from 300 K to 80 K. After being excited by a UV pump pulse at 400 nm, I₃^- dissociates into I₂^- and I fragments, the former of which is monitored by its broad absorption in the 660-870 nm range. Primary geminate recombination occurs within 4 ps over the temperature range studied and occurs in an increasing fraction of photofragments as the solution is cooled. Following an initial period of vibrational relaxation, the observed spectra continuously red-shifts with decreasing temperature before saturating near 800 nm below 160 K. The red-shifted band appears on a timescale similar to geminate recombination. These results are consistent with the increased formation of a caged contact pair species at low temperatures.