Photodissociation of ICN(-)(CO2)n, n = 0-18, with 500-nm excitation is investigated using a dual time-of-flight mass spectrometer. Photoabsorption to the (2)Π(1/2) state is detected using ionic-photoproduct action spectroscopy; the maximum absorption occurs around 490 nm. Ionic-photoproduct distributions were determined for ICN(-)(CO2)n at 500 nm. Following photodissociation of bare ICN(-) via 430-650 nm excitation, a small fraction of CN(-) is produced, suggesting that nonadiabatic effects play a role in the photodissociation of this simple anion. Electronic structure calculations, carried out at the MR-SO-CISD level of theory, were used to evaluate the potential-energy surfaces for the ground and excited states of ICN(-). Analysis of the electronic structure supports the presence of nonadiabatic effects in the photodissociation dynamics. For n ≥ 2, the major ionic photoproduct has a mass corresponding to either partially solvated CN(-) or partially solvated [NCCO2](-).

We report the collaborative experimental and theoretical study of the time-resolved recombination dynamics of photodissociated IBr(-)(CO(2))(n) clusters. Excitation of the bare anionic chromophore to the dissociative A(') (2)Pi(1/2) state yields only I(-) and Br products. Interestingly, however, the addition of a few solvent molecules promotes recombination of the dissociating chromophore on the X (2)Sigma(1/2)(+) ground state, which correlates asymptotically with Br(-) and I products. This process is studied experimentally using time-resolved, pump-probe techniques and theoretically via nonadiabatic molecular dynamics simulations. In sharp contrast to previous I(2)(-) studies where more kinetic energy was released to the photofragments, the observed recombination times increase from picoseconds to nanoseconds with increasing cluster size up to n=10. The recombination times then drop dramatically back to picoseconds for cluster sizes n=11-14. This trend, seen both in experiment and theory, is explained by the presence of a solvent-induced well on the A(') state, the depth of which directly corresponds to the asymmetry of the solvation about the chromophore. The results seen for both the branching ratios and recombination times from experiment and theory show good qualitative agreement.

We report the time-resolved recombination of photodissociated IBr-(CO2)n (n = 5-10) clusters following excitation to the dissociative IBr-A' 2Pi12 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 CO2, 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 I2-, the observed recombination times for IBr-(CO2)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.