Research - Ultrafast Chemical Processes

The Gaffney research group uses ultrashort duration pulses of light to generate stroboscopic movies of the atomic scale motions that lead to the physical and chemical transformations of condensed matter. We use femtosecond optical lasers to measure the ultrafast dynamics of electronic and vibrational degrees of freedom in a wide range of systems. We have also been instrumental in the development of femtosecond x-ray scattering at SLAC National Accelerator Laboratory. Our current research emphasizes the study of photo-initiated electronic relaxation dynamics in coordination chemistry, hydrogen bond dynamics in solution, and the dynamics of ion assembly in solution. We utilize steady state and time resolved optical spectroscopy, x-ray spectroscopy, x-ray scattering in our studies.

Femtosecond transient absorption spectrum measured for the Ir2[1,8-diisocyano-menthane]4 photo-catalyst. The signal centered at 710 nm corresponds to the stimulated emission from the excited state following excitation at 590 nm and the strong oscillations in the spectrum result from coherent metal-metal vibrational dynamics in the excited state. This oscillatory change in the Ir-Ir bond length provides an excellent test case for demonstrating the power of ultrafast x-ray scattering for the study of chemical dynamics.

Photochemical dynamics in solution: The initial stages of efficient photochemical reactions invariably occur on the femtosecond (fs) to picosecond (ps) time scale. Identifying the mechanisms for directed and efficient channeling of solar energy to chemical energy will be a principle objective of this research sub-task. The effective conversion of light to chemical energy necessitates directing the energy flow, which requires the suppression of the thermodynamic driving force to convert the light energy to heat and re-establish equilibrium. The effectiveness of molecular photo-catalysts depends critically on the excited state electronic structure and dynamics. Preserving the harvested energy within the electronic degrees of freedom represents a critical step to efficient light harvesting and depends intimately on the complex interplay between electronic and nuclear motion. While the investigation of non-adiabatic dynamics has been widely pursued with time resolved optical spectroscopy, the complexity of the phenomena and dual influence of nuclear and electronic arrangement on these optical signals has made unambiguous interpretation of experimental data unusual. We propose to disentangle this coupled evolution of the electrons and nuclei by probing the molecular structure with ultrafast x-ray scattering and the electronic structure with ultrafast x-ray spectroscopy.

Equilibrium chemical dynamics in solution: We will also investigate equilibrium chemical dynamics. The assembly and conformation of soft-matter depends critically on non-covalent interactions. These interactions, such as ion pairing, hydrogen bonding, and van der Waals attractions contribute to the assembly of nanostructures in a broad range of chemical and materials science applications as well. While the formation and folding of nanometer complexes generally involves the formation of numerous non-covalent interactions, the intrinsic interactions are generally well defined local interactions: hydrogen bond formation, ion pairing, and higher-order electrostatic interactions. We propose to study the thermal dynamics of H-bonding and ion pairing dynamics with the objective of generating a molecular-scale, mechanistic understanding conformational dynamics in solution. We will investigate these conformational dynamics with time resolved vibrational spectroscopy, x-ray photon correlation spectroscopy, and molecular dynamics simulations.

(A)Schematic of the H-bond switch from a water-water H-bond to a water-perchlorate H-bond. (B) H-bond switching occurs primarily via large, prompt angular jumps, Dq, as seen in molecular dynamics simulations. The average jump angle for water-water to water-anion H-bond jumps, or the reverse, equals 70°. We have used polarization resolved multidimensional vibrational correlation spectroscopy to confirm the prompt angle jump model of H-bond switching.