The research in the Bradforth group centers on understanding the ultrafast dynamics of chemical reactions in solution and in complex condensed phase systems. The common strand in our experiments is to explore the coupling of the dynamics of the nuclei (e.g. vibrations, bond formation/breaking and solvent motion) to a given non-adiabatic electronic process. The time scales for these non-adiabatic processes are on the order of 100 fs; likewise the timescales of nuclear motions are 10 fs - 1 ps. In a condensed environment, this allows only a short time window to explore the system evolution and catch the newly-formed products before complete relaxation. Femtosecond spectroscopy is therefore the appropriate experimental technique to elucidate the dynamics.
Most small molecule photochemistry that is well characterized in the gas phase takes place at wavelengths shorter than 350 nm. Our group follows reactions of such molecules when embedded in a liquid. However, the ultraviolet is traditionally a difficult area for ultrashort pulse generation. We have recently harnessed developments in hollow core fiber technology to achieve breakthrough tunable deep UV pulses of ~ 25 fs. When combined with fully dispersed spectral probing we carry out solution photochemistry experiments exploiting this unprecedented time resolution.
Experimental projects using ultrafast spectroscopy in the Bradforth group include (i) probing the primary pathways for electron ionization and detachment in liquid water (ii) investigating chemistry initiated by ejection of an ultrafast electron, (iii) the influence of solvent on bond dissociation dynamics and on rotational relaxation initiated by photodissociation, (iv) determining pathways of energy migration of an electronic excitation through an organized architecture of chromophores such as DNA. Strong connection with current theory is emphasized in all projects. In addition, we are engaged in several collaborative projects with theoreticians in the areas of electronic structure and quantum dynamics of solutes in bulk liquids.
Solution photodetachment and photodissociation
Photoionization and photodetachment are well-understood processes in vacuum. Our experiments build a molecular level picture of the dynamics of electron ejection in liquids. We have now firmly established mechanisms for the resonant detachment of many simple anionic systems. These experiments have allowed development of solution-based photodetachment spectroscopy for exploring reaction dynamics in liquids, such as descent from a bimolecular barrier and contact reactions of bimolecular partners who would normally meet by diffusion. In addition we are actively exploring the photophysics of liquid water, the nature of the conduction band in water and mechanisms of liquid photoionization. This fundamental work is of crucial importance in understanding the effects of radiation on animal tissue, and hazardous waste storage.
Photodissociation studies of benchmark systems such as ICN in polar liquids reveal unusually long-lived free rotor behavior in the very hot product fragment. This allows a new probe of liquid rotational friction. Our experiments characterize a reaction-induced change in the liquid structure which leads to a breakdown in the linear response type dissipative behavior of the solvent. In addition, curve-crossing dynamics in the photodissociation can be disentangled in a full liquid environment.
DNA and Aromatic photophysics: Energy Transfer in Chromophore Arrays
X-ray crystallography reveals that nature uses cyclic architectures of chlorophyll pigments to achieve phenomenal efficiency in photosynthetic light harvesting. Polymer analogs to the supramolecular pigment architectures employed in photosynthesis have been synthesized by Prof. Hogen-Esch of this department. These chromophore arrays successfully harvest electronic energy by efficient energy transfer between chromophores. In contrast, UV excitation of DNA – another ordered chromophore array – leads to damage and mutagenesis. Patterns of damage in oligonucleotides may be connected with energy transfer along base stacks in DNA and the increased excited state lifetime in the polymer. Using time-domain spectroscopy, our group is studying energy transfer dynamics in both macrocyclic polymers and in DNA. Fluorescence and time resolved absorption anisotropy and annihilation are being used to analyze energy migration pathways and timescales. We have recently observed via dispersed pump-probe spectroscopy that there are sub-100fs pathways for electronic relaxation in constituent DNA bases.