A combined experimental/theoretical investigation of the near-infrared photodissociation of IBr−(CO2)n

Abstract

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.

The interaction of negative charge with carbon dioxide – insight into solvation, speciation and reductive activation from cluster studies

“…There has been an extensive body of work over more than two decades on the interaction of dihalide anions with CO 2 , experimentally mostly performed by Lineberger and coworkers [129][130][131][132][133][134][135][136][137][138][139][140][141][142][143], with theoretical work notably done by Parson and coworkers [135,[137][138][139][140][142][143][144][145][146][147][148][149][150] and by McCoy and coworkers [137,142,143,151,152]. In this series of papers, CO 2 had the role of a solvent that could react to electronic excitation of a solvated ion and modify the solute ion's photophysics as well as its vibrational characteristics.…”

“…Photodissociation of the bare IX À ion results in I À , but photodissociation of the CO 2 solvated ion can give rise to ionic photofragments based on I À , X À or IX À . For X = Br [138][139][140][141][142][143], the bromine atom is preferentially solvated in the ground state. The branching ratio for formation of I À based products upon excitation to the A 0 state decreases rapidly with the number of solvent molecules, n, whereas the caging fraction producing IBr À based ions increases and reaches unity at n ¼ 8 (see Figure 9).…”

“…Time-resolved photoelectron spectroscopy [138,139] and photodissociation [140] studies show that although recombination of the IBr À ion occurs with unit probability for n = 8-10 upon excitation to the A 0 state, the recombination time is very long Figure 9. Size evolution of the ionic photoproducts from photodissociation of IBr À Á(CO 2 ) n upon excitation to the A' state.…”

Solvent-mediated charge redistribution in photodissociation of IBr− and IBr−(CO2)

“…23,24 The presence of this single solvent molecule drastically changes the product distribution following excitation of IBr − to the B state. 23,24 The presence of this single solvent molecule drastically changes the product distribution following excitation of IBr − to the B state.…”

“…Unlike the A state, the B state correlates to I* + Br − photoproducts, 22,23 and therefore the TRPES probe accesses a different set of neutral IBr electronic states. The present experiment, inspired by the results of our earlier study, further tests our theoretical model by probing solvent-driven electron transfer from a different electronic configuration.…”