I2- PHOTODISSOCIATION AND RECOMBINATION DYNAMICS IN SIZE-SELECTED I2-(CO2)N CLUSTER IONS

Pump-probe techniques are used in conjunction with a tandem time-of-flight mass spectrometer to investigate the I...I- cage recombination dynamics following I2- photodissociation in size-selected I2-(CO2)n cluster ions. The absorption recovery, which reflects the recombination and vibrational relaxation of the photodissociated I2-, 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 approximately 40 ps (n less-than-or-equal-to 12) to approximately 10 ps (n greater-than-or-equal-to 15). In addition, a recurrence is observed at almost-equal-to 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 I2- 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.