We report the time-resolved recombination of photodissociated IBr~-(CO_2)_n (n=5-10) clusters following excitation to the dissociative IBr~-A' ~2PI_(1/2) state of the chromophore via a 180 fs,795 nm laser pulse.Dissociation from the A' state of the bare anion results in I~-and Br products.Upon solvation with CO_2,the IBr~-chromophore regains near-IR absorption only after recombination and vibrational relaxation on the ground electronic state.The recombination time was determined by using a delayed femtosecond probe laser,at the same wavelength as the pump,and detecting ionic photoproducts of the recombined IBr~-cluster ions.In sharp contrast to previous studies involving solvated I_2~-,the observed recombination times for IBr~-(CO_2)_n increase dramatically with increasing cluster size,from 12 ps for n=5 to 900 ps for n=8,10.The nanosecond recombination times are especially surprising in that the overall recombination probability for these cluster ions is unity.Over the range of 5-10 solvent molecules,calculations show that the solvent is very asymmetrically distributed,localized around the Br end of the IBr~-chromophore.It is proposed that this asymmetric solvation delays the recombination of the dissociating IBr~-,in part through a solvent-induced well in the A' state that (for n=8,10) traps the evolving complex.Extensive electronic structure calculations and nonadiabatic molecular dynamics simulations provide a framework to understand this unexpected behavior.
展开▼
