Ultrafast dynamics of iodide photodetachment

Electron transfer reactions are ubiquitous in many biological and chemical processes that occur in dense media; thus, controlling and mimicking these processes hinges on a fundamental understanding of electron transfer dynamics. Resonant photodetachment of an atomic anion in the condensed phase provides a prototype system for studying solvent-mediated electron transfer. In this thesis, the dynamics of electron ejection and survival following excitation of the iodide charge-transfer-to-solvent (CTTS) state were time-resolved using ultrafast pump-probe spectroscopy. Experiments employed state-of-the-art temporal resolution laser pulses in the UV (∼50 fs). Iodide was pumped with a 255 nm pulse to the CTTS state, which is unstable and results in the formation of unusual iodine-electron caged pairs. Transient absorption of the trapped electron was monitored with the probe pulse at visible and near IR wavelengths.; The results are presented in multiple solvent environments and contrasted with the electron dynamics arising from multi-photon ionization of the pure solvent. We find that the appearance time of iodine-electron caged pairs is invariant to the solvent environment, ∼200 fs, implicating solvent translational motions as the primary ejection mechanism. Over longer timescales, the pair lifetime is highly solvent dependent. A complete kinetic treatment of the longtime dynamics requires a model that simulates both the mutual diffusion and recombination of the caged pair and includes the potential of mean force between the iodine atom and electron. Data analysis with this model confirms that ejection of the electron from the CTTS state is short-range with the caged pair stabilized by a potential well; however, the pair reaction rate is sufficiently slow such that some escape recombination. Quenching experiments with protons show that the electron can be scavenged from the potential well, and comparisons with previous steady-state scavenging experiments suggest that the primary yield of electrons from the CTTS state is near unity. Further simulations suggest that the potential well is governed by the induced dipole interaction between the iodine atom and electron. The influence of solvent structure and distance-dependent rates of electron transfer are also considered to uncover the environment's role in determining the dynamics of CTTS systems in the condensed phase.