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: Atmospheric photochemistry of organic molecules
Our program explores water- and sunlight-mediated processes in planetary atmospheres including the contemporary and prebiotic Earth. My approach aims to provide new input for models of atmospheric chemistry and climate, using fundamental chemical physics to address complex multiphase chemistry. Using solar simulators in laboratory studies, our group has been exploring the importance of water and the environment to the photochemistry of organic species involved in isoprene oxidation. Pyruvic acid is a small organic molecule found in the atmosphere in the gas phase, as well as in fogs, aerosols, and clouds. While the gas-phase photochemistry for pyruvic acid has been well-understood for decades, the sunlight- driven reaction pathways for the aqueous phase have remained more elusive. Our group has recently deduced a mechanism for the aqueous phase photolysis of pyruvic acid. This mechanism is not only fundamentally different from the gas-phase chemistry, but it is also dependent on the presence of oxygen in solution, highlighting the immense importance of water and the environment to photochemical processes.
Collaboration within CIRES allowed analysis of the importance of the aqueous phase photolysis of pyruvic acid in the atmosphere. Our laboratory measurements of the rate of the photochemical reaction in solution were used as input for an atmospheric model that compared the loss of pyruvic acid through aqueous photolysis to the loss from the gasphase photochemistry and oxidative processes. Results indicated that in an acidic environment, the aqueous photolysis is equally as important as the gas-phase photolysis, emphasizing the importance of developing a mechanistic understanding of aqueous- phase photochemical processes.
We have scaled the fundamental experimental work with pyruvic acid 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-François Doussin with UPEC. By combining these laboratory and chamber studies, we connect our understanding of the photolysis of pyruvic acid to mechanisms for aerosol nucleation and growth.