FEMTOSECOND REAL-TIME PROBING OF REACTIONS .21. DIRECT OBSERVATION OFTRANSITION-STATE DYNAMICS AND STRUCTURE IN CHARGE-TRANSFER REACTIONS

This paper in the series gives our full account of the preliminary res ults reported in a communication [Cheng, Zhong, and Zewail, J. Chem. P hys. 103, 5153 (1995)] on real-time femtosecond (fs) studies of the tr ansition state of charge-transfer (CT) reactions, generally described as harpooning reactions. Here, in a series of experimental studies in a molecular beam, and with the help of molecular dynamics, we elucidat e the microscopic elementary dynamics and the structure of the transit ion states for the isolated, bimolecular reaction of benzenes (electro n donor) with iodine (electron acceptor). The transition state is dire ctly reached by fs excitation into the CT state of the complex Bz . I- 2, and the dynamics is followed by monitoring the product build up or the initial transition-state decay. We further employed the fs resolut ion in combination with the kinetic-energy resolved time-of-flight and recoil anisotropy techniques to separate different reaction pathways and to determine the impact geometry. Specifically, we have studied: ( 1) the temporal evolution of the transition state (tau(double dagger)) and of the final products (tau); (2) the product translational-energy distributions; (3) the recoil anisotropy (beta) in each channel; (4) the reaction time dependence on the total energy; (5) the dynamical an d structural changes with varying CT energy (ionization potential-elec tron affinity-Coulomb energy). Such a change is made by replacing the electron donor from benzene to toluene, and to xylenes and trimethylbe nzenes of different symmetries. We have also studied deutrobenzene as a donor. The reaction mechanism involves two exit channels. The first one (ionic) follows the ionic potential of the CT state. Following the harpooning (Bz(+). I-2(-)), the transition state [Bz(+).. I-.. I](do uble dagger) evolves on the adiabatic potential to produce Bz(+). I- a nd I products. The second channel (neutral) is due to the coupling of the transition state to neutral, locally excited, iodine repulsive sta tes and, in this case, the products are Bz . I+I. The latter process i s an intermolecular electron transfer and occurs on an ultrafast time scale of 250 fs, resulting in a greater yield for the neutral channel. Molecular dynamics simulations support this dynamical picture and pro vide the time scales for trajectories in the transition-state region a nd in the product valley. The geometry of the transition state is dete rmined from the anisotropy measurements and we found a nearly axial ge ometry with the iodine axis of recoil tilted 30 degrees-35 degrees awa y from the transition moment. These angular dependencies are related t o the molecular structure and the electronic structure with highest oc cupied molecular orbit-lowest occupied molecular orbit descriptions. B y increasing the level of solvation from the 1:1 complex structure to clusters, we address the dynamics of caging in small and large solvent structures. We also report studies in the liquid phase and compare ou r results with those from other laboratories in an attempt to unify th e nature of the dynamics and structure in going from the isolated gas phase complex to the liquid. (C) 1996 American Institute of Physics.