Time-resolved study of solvent-induced recombination in photodissociated IBr−(CO2)n clusters

Abstract

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.

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

“…This paper is the third in a series 18,19 devoted to experimental and theoretical investigations of photodissociation and recombination of IBr − anions embedded in clusters of 1-16 CO 2 molecules. 18 This result is in dramatic variance with earlier investigations of the homonuclear dihalide cluster I 2 − ͑CO 2 ͒ n , where somewhat more kinetic energy was available to the photofragments and for which recombination times were of the order of 10 ps and decreased with increasing cluster size. For example, as the number of CO 2 solvents around IBr − was increased from 5 to 8, the recombination probability increased 19 from 90% to 100%, but the timescale for recombination increased from 12 to 900 ps.…”

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.…”

“…At both smaller and larger cluster sizes (n 7, n ! 11), recombination occurs on a picosecond timescale [139]. A detailed, timeresolved photoelectron spectroscopy and quantum dynamics study [142] of the dissociation dynamics of IBr À Á CO 2 shows that the solvent plays an important role in the charge transfer process that allows the formation of Br À .…”