Knowledge Bank

We report the ultrafast recombination dynamics of large $\backslash chemIBr-(CO2)n$ (n=11-14) clusters. Excitation of the bare $\backslash chemIBr-$ chromophore $via$ a 180 fs, 795 nm laser pulse leads to dissociation on the $\backslash chemA\prime$ $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Pi 1/2$ state resulting in $\backslash chemI-$ and Br products. Recombination of the dissociating chromophore on the ground state is induced by solvation of the dihalide. The recombination time is determined by using a delayed femtosecond probe laser at the same wavelength to monitor the population of recombined $\backslash chemIBr-$-based products. Previously observed long recombination times for n=8 and 10, $\sim$1 ns, have been explained by a solvent-induced well that increases in depth with increasing asymmetry of the solvent molecules about the chromophore. Confirming a theoretically predicted pattern, we find that the recombination times decrease for larger cluster sizes, beginning at n=11. The increased symmetry of larger clusters (n $>$10) causes a decrease in the depth of the $\backslash chemA\prime$ well, resulting in a rapid recombination time, $\sim$10 ps for n=11. Subsequent addition of $\backslash chemCO2$ molecules to the cluster results in the further decrease of the recombination time such that simple exponential transients are no longer observed for n=13 and 14.