I−2 photodissociation and recombination dynamics in size-selected I−2(CO2)n cluster ions

Abstract

Pump–probe techniques are used in conjunction with a tandem time-of-flight mass spectrometer to investigate the I...I− cage recombination dynamics following I−2 photodissociation in size-selected I−2(CO2)n cluster ions. The absorption recovery, which reflects the recombination and vibrational relaxation of the photodissociated I−2, exhibits a strong cluster size dependence in the range of n=13–15. Over this limited cluster size range, the absorption recovery time decreases from ∼40 ps (n≤12) to ∼10 ps (n≥15). In addition, a recurrence is observed at ≊2 ps in the absorption recovery of the larger clusters (n=14–17). This feature results from coherent I...I− motion following photodissociation. Measurement of the absorption recovery with both parallel and perpendicular pump–probe polarizations demonstrates that the pump and probe transition dipoles lie in the same direction. Analysis of the I−2 transition dipole directions shows that the coherent motion takes place on the first two repulsive excited potential surfaces. The two-photon photofragment distribution reflects the solvent cage structure as a function of pump–probe delay time.