Ph.D. Yale University, 1977
Professor of Chemistry and Biochemistry
Spectroscopic Studies of Atmospheric Molecules, Radicals, and Their Complexes
Professor Vaida's research interest focuses on issues of photoreactivity in the atmosphere. The approach employed to obtain the structure and dynamics of molecules, radicals, and their complexes involves a combination of spectroscopic, photofragment and theoretical techniques. Several lines of inquiry are being pursued:
Small predissociative molecules such as NH3, CS2, OCS, OClO, and O3 are under investigation to obtain the structure and dynamics of their reactive electronic states. The experimental techniques employed for these studies are direct absorption resonance, enhanced multiphoton ionization, laser-induced fluorescence and resonance Raman of supersonically cooled samples. The experimental data are modeled theoretically using ab initio electronic structure theory as well as calculations of chemical dynamics on the reactive potential energy surfaces.
Reactivity in solvent environments is studied using molecular complexes. Spectroscopic perturbations produced by intermolecular interactions are analyzed to bridge the gap between gas and condensed phases. This method employs molecular beam techniques where weakly bound molecular clusters of different composition and size can be produced in a controlled fashion and investigated spectroscopically.
The photochemistry of weakly bound clusters involving dissociative molecules is under investigation. Efficient covalent bond chemistry is observd to compete with energy flow to the weaker van der Waals bonds in molecular dimers containing highly dissociative samples.
The problematic thrust of the group is the investigation of photo-reactive molecules and clusters important in atmospheric chemistry. A recent example is the group's quantitative laboratory study of light-induced reactions important in the stratosphere. In collaboration with Dr. Susan Solomon from the Aeronomy Laboratory at NOAA [ About this Lab ] in Boulder, who has measured the concentration of chlorine dioxide over Antarctica, Professor Vaida's lab proposes, on the basis of its laboratory results, a new chemical reaction that contributes to polar ozone depletion. In parallel, a program to evaluate the photoreactivity of bimolecular complexes formed by molecules or radicals in the atmosphere is being developed. This work aims to obtain a laboratory data base needed for inclusion of the photochemistry or bimolecular complexes in atmospheric models.
Current Research: Multiphase atmospheric photochemistry
Our program explores water- and sunlight-mediated processes in planetary atmospheres including the contemporary and prebiotic Earth. My approach aims to provide new inputs for models of atmospheric chemistry and climate, using fundamental chemical physics to address complex multiphase chemistry. Using solar simulators in experimental studies in our lab, our group has been showing fundamental differences in the photochemistry of organic species involved in isoprene oxidation as a function of water in the environment. Water in all its phases is implicated in catalysis and/or suppression of atmospheric reactions. Especially interesting and novel is the establishment of the catalytic role of water surfaces—as available at the surface of oceans, lakes, and atmospheric aerosols—in promoting reactions not favorable in aqueous phase (see figure for illustration of chemistry occurring at water surfaces in the absence of enzyme catalysis). We have scaled fundamental processes investigated in our lab to realistic atmospheric conditions by using environmental chamber studies. Specifically we are using the CESAM (French acronym for Experimental Multiphasic Atmospheric Simulation Chamber) at the Université Paris–Est Créteil Val de Marne (UPEC) in collaboration with Professor Jean-Francois Doussin with UPEC. These combined laboratory and chamber studies resulted in multiphase aqueous photochemical mechanisms connecting volatile organic compounds to aerosol nucleation and growth. Properties of atmospheric aerosols are highly nonlinear, resulting in uncomfortably large uncertainties in aerosol effects on climate. Inspired by atmospheric measurements, which established that aerosols have a large organic content, we proposed that a significant population of organic aerosols consists of an aqueous core with an organic surface film and pointed to the profound consequences to their morphological, optical, and chemical properties. Our research group investigates the fundamental physical chemistry of interfacial organic films and, using theoretical models, explores the consequences of these results to Earth’s contemporary and ancient atmosphere. To bring this research to a broader community, I have included environmental chemistry topics in teaching chemistry at all levels. Simultaneously, I developed lectures that have been delivered to public and academic audiences nationwide during my tenure as Sigma Xi Distinguished Lecturer. Recent international research collaborations and student exchanges include Canada (University of Toronto), Brazil, France, and the Czech Republic.