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Paul J. Dagdigian
Experimental Chemical Physics Johns Hopkins University Phone: 410.516.8420 PhD - University of Chicago Laser spectroscopy is a powerful tool for the study of molecules and their interactions in the gas phase. We are employing various laser spectroscopic techniques for the study of the dynamics of gas-phase collisional processes and for the sensitive detection of trace concentrations of molecules such as explosives. Molecular free radicals are important chemical intermediates in various environments, including combustion media and the atmosphere. We are interested in the dynamics of molecular collisional processes, including rotational and vibrational energy transfer, photodissociation, and chemical reactions, involving diatomic and small polyatomic free radicals. The techniques of laser-induced fluorescence (LIF) and resonance enhanced multiphoton ionization (REMPI) allow us to study these processes with resolution of the internal quantum states of the radical. Recently, we have been turning to the study of the spectroscopy and kinetics of polyatomic free radicals, which are important in the early stages of the decomposition of energetic materials. We have implemented the technique of cavity ring-down spectroscopy (CRDS) in this work, since LIF and REMPI are not applicable to these polyatomic species, since they generally dissociate upon electron excitation. CRDS is seeing wide application and is a sensitive absorption-based laser detection method which offers effective absorption path lengths of a kilometer or more in a table-top apparatus. We are employing CRDS in both the UV and IR for CRDS studies on both electronic and vibrational transitions. There is a continuing need for the development of rapid detection of explosives and explosive-related compounds in trace quantities. As part of a DoD multi-university research initiative (MURI), our group has explored the use of UV CRDS for the detection of TNT and related compounds in the vapor phase. This technique displays very good sensitivity of detection, but poor selectivity. In recent work, we are partners in another MURI, in which femtosecond laser-matter interactions are being explored. Our principal role in this project is to develop an understanding of the chemical and physical processes which lead to the observed optical emission from laser-induced plasmas. This has practical applications in the analytical technique of laser-induced breakdown spectroscopy (LIBS), in which the optical emission is being used for materials identification, most notably in the detection of explosives. In service to the chemical physics community, Professor Dagdigian has been Chair of the Division of Chemical Physics, of the American Physical Society (APS), and is a Fellow of the APS. |
