Time-Resolved Spectroscopic Studies of Chlorine and Bromine Atom Reactions in Solution

Ultrafast time-resolved pump-probe spectroscopy monitors the dynamics of bimolecular reactions of Cl and Br atoms in solution with sub-picosecond time resolution. A photolysis pulse dissociates a given precursor to generate either Cl or Br atoms, and a UV/VIS or mid-IR probe pulse monitors the evolution of the atom or a reaction product. The hydrogen abstraction reactions of Cl and Br atoms with organic molecules are exoergic and endoergic, respectively. In the case of Br atom reaction, we introduce an additional near-IR pump pulse to promote this endoergic reaction using vibrational excitation.;The reactions of Cl atoms with alkanes and alkenes exhibit rate constants that are diffusion-limited (or faster) and are ranked by the number of C=C bonds in the reactants. This suggests that an attractive intermolecular force between the Cl atom and the C=C bond influences the reaction rates. The reactions of Cl atoms with alkenes produce vibrationally excited HCl (v=1). The branching into HCl (v=1) shows a solvent viscosity dependence, owing to a dampening of the vibrational excitation. In contrast, the reaction rates are faster in a more-viscous solvent, implying a failure of simple diffusion theory in describing the reactions of Cl atoms with alkenes.;In the reactions of Br atoms, we seek to realize the first vibrationally driven bimolecular reaction. Our most promising prospects are the reactions of Br atoms with CH3OH and DMSO. We observe a difference signal in the decay of the Br--solvent complex when the near-IR pump is resonant with the C--H or O--H overtone stretches of CH3OH and DMSO. This difference signal contains a negative offset at long times, which signifies a loss of the Br--solvent following vibrational excitation. This could be due to vibrational predisocciation of the complex, followed by either geminate recombination to re-form the solvent and the Br precursor or reaction of the Br atoms with vibrationally excited solvent molecules.