Hydrated electrons are important in many areas of the physical sciences. They play a key role in the radiation chemistry in nuclear reactors, and they mediate the damage to DNA caused by ionizing radiation. In this award, funded by the Chemical Structure, Dynamics and Mechanisms Program of the Chemistry Division, Prof. Daniel Neumark of the University of California, Berkeley and his graduate student colleagues will carry out spectroscopic and dynamical studies of excess electrons in negatively charged clusters, which serve as useful model systems for bulk hydrated electrons. These studies will be done using time-resolved photoelectron imaging (TRPEI), a femtosecond pump-probe technique. The research will seek to characterize the gas phase reactivity of excess electrons in DNA subunits, the time evolution of electronic absorption in negatively charged water clusters, and Auger emission decay in mercury cluster anions. In addition, a project will develop a liquid jet technique to conduct both time-independent and time resolved photoelectron spectroscopy of hydrated electrons in bulk water.
The studies outlined will have a number of broader scientific impacts, since solvated electrons are ubiquitous in a number of scientific areas including atmospheric chemistry, biology, and medicine. The experiments on mercury cluster anions will shed light the photophysics of quantum dots and thus have relevance to solar energy conversion. The graduate students trained in this program will acquire significant skills in experimental and theoretical chemical physics research.
It is now established that low energy electrons can damage DNA, inducing mutations, cancer, and death. Our research has focused on "time-resolved radiation chemistry", in which we develop novel experiments with the ultimate goal of understanding the interaction of electrons with DNA constituents. These experiments involving using femtosecond laser pulses to generate low energy electrons in two different environments: mass selected, negatively charged clusters and liquid water microjets. In the cluster experiments, we form binary complexes in which an iodide anion is complexed to a nucleic acid constituent such as uracil, as shown in Fig. 1. An ultraviolet pump pulse ejects and electron from the iodide, and some of these electrons are temporarily trapped by the neighboring uracil. They are eventually re-emitted with very low kinetic energy, and our experiment probes the time-scale on which this occurs. In more complex nucleic acid components such as nucleosides or nucleotides, this excess electron should be able to induce bond cleavage, and planned experiments will probe the time-scale for this process, too. In the water jet experiments, we eject electrons into solution by photodetachment of aqueous iodide anions. These electrons can be photo-excited into highly reactive electronic states by means of a second laser pulse, and with third pulse we can monitor the lifetime of these states. The excited electronic states are relevant to radiation chemistry, and determining their lifetimes is an important step in understanding how these species interact with nucleic acid constituents.
