Research Interests

We are currently focusing our efforts on three main areas of research:

Charge- and proton-transfer phenomena in molecular clusters
Heterogeneous atmospheric chemistry mediated by solid aerosol particles
Real-time environmental mass spectrometry

An important facet of our work is the development of new experimental approaches using a variety of instrumentation, including laser sources. We often employ computational methods to supplement the experimental results. We are particularly interested in systems that are relevant to environmental and atmospheric chemistry. The diverse nature of our work calls for a multidisciplinary approach and much of the research is carried out in collaboration with colleagues in chemistry, physics and engineering. We also benefit from our association with the Optical Science and Technology Center (OSTC) and the Center for Global and Regional Environmental Research (CGRER).

Charge- and proton-transfer in clusters

Clusters, consisting of two or more molecules bound together by intermolecular forces, represent an ideal system to investigate the details of various chemical processes. The cluster environment can also result in unique, supramolecular chemistry that is vastly different from the behavior of the isolated molecules. We form isolated cluster species in a low temperature molecular beam that is coupled to a specialized time-of-flight mass spectrometer (TOFMS). We have developed an array of powerful laser analysis methods that couple multiphoton ionization probes with mass-specific detection to study cluster photochemistry. We are investigating clusters that exhibit an intermolecular charge-transfer (CT) absorption band due to the transfer of an electron from a donor to an acceptor molecular orbital. Electron transfer is a fundamental concept in chemistry with significant implications for both basic and applied science. We are especially interested in the identification of new processes that may play a role in atmospheric chemistry. For instance, photofragmentation of O₂ clustered with an alkene to produce O atoms, an important atmospheric radical species, is enhanced by at least three orders of magnitude compared to the isolated oxygen molecule;

alkene-O₂

	alkene⁺-O₂^-

		alkene-O₂^*

			alkene + O + O

We are also investigating proton-transfer systems that exhibit a so-called low barrier hydrogen-bond, which is of great importance in the structure of biomolecules and in enzyme kinetics.

Heterogeneous atmospheric chemistry on aerosol particles

We are interested in chemistry that is mediated by aerosol particulates suspended in the troposphere, particularly soot from combustion processes and mineral aerosols from windborne soil. Our unique experimental approach is to study the chemistry of the isolated aerosols, as opposed to more traditional bulk studies. We have constructed a large volume atmospheric reaction chamber to monitor gas-aerosol interactions at pressure, temperature and relative humidity conditions found in the troposphere. We utilize a FTIR, a fiber-optic UV/Vis spectrometer, a fast-sampling mass spectrometer, and lasers to probe the chemistry. As part of our collaborative effort, the experimental information we obtain regarding reaction probabilities and product branching serves as input to sophisticated computer models of atmospheric chemistry. The results of the models provide feedback so that we may target the most sensitive and critical systems in our experiments.

Real-time mass spectrometry

We are actively developing real-time mass spectrometric methods for field and laboratory studies in environmental and atmospheric chemistry. For example, airborne biological particulates, such as microorganisms and microbial products, are sampled into a modified TOFMS. The isolated particles are size-selected using a laser scattering method and then probed using a combination of laser wavelengths. The resultant mass spectrum is used to identify and classify the particle with fast time resolution. Our approach will greatly aid studies of inhalation toxicology in contaminated settings, which require knowledge of the size-correlated chemical speciation as well as details regarding the distribution of airborne microorganisms. A similar instrument will be used to detect species absorbed onto the surface of aerosol particles in our atmospheric reaction chamber. We are also applying a novel mass sensitive technique to the study of aqueous photochemistry in rain and surface waters.