Rapid solvent-induced recombination and slow energy relaxation in a simple chemical reaction: Picosecond studies of iodine photodissociation inCCl4

Abstract

The time scale for solvent "caging" in the classic I 2 photodissociation reaction appears to be ^15 ps, on the basis of examination of new picosecond absorption spectra. The previously observed slow recovery ( > 100 ps) of molecular absorption is shown to be due to both ground-state vibrational relaxation and excited-state trapping in the recombined molecule. In addition, newly observed absorptions in the 350-400-nm region provide direct information on solvent-induced predissociation and excited-state vibrational relaxation.PACS numbers: 82.20. Rp, 61.20.Lc, 82.40Js, 82.50.Et A central problem in liquid-phase chemical dynamics is determination of the time scale of reaction for species trapped together in a solvent "cage." The iodine photodissociation reaction has been the primary model system for studying cage recombination, 1 yet the cage recombination time for this system is still in controversy. Some experiments have indicated an ~~100-ps recombination time, 2 ' 3 in disagreement with other experiments, 4 and in contrast with theoretical predictions 5 of 1-10-ps recombination times. This Letter reports results which strongly suggest that cage recombination occurs in ^15 ps. The results also demonstrate that both solvent-induced vibrational relaxation and excited electronic-state relaxation are important in the understanding of this reaction.The basic processes in iodine photodissociation which are needed to explain these results can be outlined with reference to the potential-energy diagram (Fig. 1). The reaction is initiated by a transition from the ground (X) state to a vibrational^ excited level of the bound B state. The solvent induces both vibrational relaxation within the B state, as well as crossing to several repulsive states (only one shown). After initial separation, the two atoms may escape through the solvent to dissociate permanently, or they may recombine on either the A, A', or X potential. Those recombining on the X potential must dissipate their excess vibrational energy to the solvent before they return to the initial state. Those recombining on the A or A' potentials must undergo a solvent-induced crossing to the X potential before they can vibrational^ relax to the initial state.Chuang, Hoffman, and Eisenthal 2 first probed the loss of absorption near the peak of the ground-state molecular absorption (530 nm) upon photodissociation. The bleach of the ground-state absorption showed a recovery time of -140 ps in CC1 4 , which was associated with atomic recombination. Nesbitt and Hynes, 6 however, presented a contrasting interpretation. The Franck-Condon principle predicts that the maximum absorption for a molecule in a given vibrational state will occur at an energy corresponding to a vertical transition from the classical turning points. For the X -* B transition, this implies that the absorption from a vibrationally relaxing X-state population will initially be in the near ir and will move to shorter wavelengths as the population relaxes (Fig. 1). Thus, only recombining X-state molecules...

Nonadiabatic molecular dynamics simulation of ultrafast pump-probe experiments on I2 in solid rare gases

“…A very large body of work including both time independent, and time resolved spectroscopic experiments, as well as analytic theories and molecular dynamics simulations [1][2][3][4][5][6][7] ͓see Ref. ͑1͒ for other references to this theoretical and computational work͔, has been devoted to the study of diatomic molecular photodissociation, in particular the photodissociation of I 2 , in clusters, [8][9][10][11][12][13] solids, [14][15][16] liquids, 1,[17][18][19][20][21][22][23][24] and high pressure gases [25][26][27][28][29] composed of nonpolar molecules ranging from simple rare gas atoms to more complex polyatomic solvents such as cyclohexane and benzene. 30 The interpretation of much of this work is complicated by two related issues: First, despite the fact that many of the gas phase potential surfaces for I 2 are known quite accurately, the perturbation of these surfaces due to the presence of the solvent environment has only recently become ammenable to detailed study through the application of semiempirical electronic structure methods to these complex manybody interactions.…”

Visualizing Photochemical Dynamics in Solution through Picosecond X-Ray Scattering

“…wavelengths [17,21], highlighting the difficulties encountered when building a unique picture from spectroscopic observations alone. Diffuse x-ray scattering represents a spherical average of scattering contributions from all randomly oriented molecules within the sample.…”

Recombination Reactions of Atomic Chlorine in Compressed Gases. 2. Geminate and Nongeminate Recombinations and Photolysis Quantum Yields with Argon Pressure Up to 180 bar

“…Among the halogen elements, due to its easier accessibility in the generation and detection methods, iodine has been the most studied molecule. [1][2][3][4][6][7][8][9]13,[15][16][17][18][19][20][21][22][23][24][25][26][27][28][29] Followingups were the bromine 10,13,14 and chlorine 5 molecules. Except for the earliest studies, 1,2 in which the photochemical method was employed to follow the reactions, most studies have employed either the flash or laser pump-and-probe technique to monitor the chemical changes.…”