Condensed-Phase Photoionization

In this project, we use femtosecond spectroscopy to study photoionization in liquids. By using an aromatic solute such as indole, it is possible to remove an electron in a monophotonic process close to the condensed-phase photoionization threshold. This creates geminate ions with a well-defined initial distance. We showed that geminate recombination between the indole radical cation and its photoejected electron is considerably slower than the diffusion limit. To our knowledge, this is the first bimolecular reaction ever studied between solvated electrons and a transient organic radical. This result has attracted considerable interest in the radiation chemistry community, where it has been tacitly assumed that rapid, diffusion-limited recombination is the rule. Instead, our result demonstrates that the indole radical cation and photoejected electron form an unusually stable ion pair. This stable ion pair is reminiscent of the halogen atom / solvated electron ion pair studied theoretically by Peter Rossky’s Group and by Borgis and Staib. Those groups showed that there is a free energy barrier to adiabatic recombination that forces nonadiabatic charge recombination. Recombination is slow due to the large energy gap involved. We are currently pursuing this analogy with the indole system. The evidence is suggestive that recombination in our system occurs nonadiabatically in the Marcus inverted region. This work is providing a new perspective on electron transfer reactions in more conventional ion pairs.

In separate work, we have studied excess electrons in acetonitrile. This work has led to a much better understanding of excess electrons in this solvent, and provided fascinating comparison to negative ion spectroscopy of the excess electron in small acetonitrile clusters.

References

1. Xia, C.; Peon, J.; Kohler, B. "Femtosecond electron ejection in acetonitrile: Evidence for cavity electrons and solvent anions," J. Chem. Phys. 2002, in press.

2. Peon, J.; Hoerner, J. D.; Kohler, B. "Near-threshold Photoionization Dynamics of Indole in Water" in Liquid Dynamics: Experiment, Simulation, and Theory; Fourkas, J. T., Ed.; American Chemical Society: Washington, D.C., 2002, pp. 122-135.

3. Peon, J.; Hess, G. C.; Pecourt, J.-M. L.; Yuzawa, T.; Kohler, B., "Ultrafast Photoionization Dynamics of Indole in Water," J. Phys. Chem. A 1999, 103, 2460-2466.