MULTIPLE-LASER SPECTROSCOPIES OF VIBRATIONALLY EXCITED MOLECULES
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Date
1992
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Ohio State University
Abstract
Over the last few years we have implemented several multiple-resonance schemes incorporating overtone excitation of light atom stretch vibrations to investigate the dynamics of highly excited molecules. In one scheme, we use infrared-optical double resonance to prepare excited reactant molecules in single rovibrational states at energies above the unimolecular dissociation threshold on the ground potential surface. Detection of the overtone excitation is achieved by monitoring dissociation fragments via laser induced fluorescence. Application of this approach to $H_{2}$$O_{2}$, $NH_{2}OH$, and $HN_{3}$ have provided detailed information on the dynamics of their dissociation. A second scheme uses direct vibrational overtone excitation to a level just below the dissociation threshold followed by infrared excitation to a level just above threshold. The infrared absorption is monitored by LIF detection of the dissociation fragments with a third laser. The infrared absorption spectrum of a molecule first excited to a high vibrational overtone level reveals the vibrational character of the cigenstate prepared by overtone excitation. Application of this approach to $H_{2}$$O_{2}$, $HN_{3}$, $HONO_{2}$, and $(CH_{3})_{3}COOH$ demonstrates the changing nature of the eigenstate composition with increasing density of states. Our most recent spectroscopic scheme permits the measurement of weak vibrational overtone transitions in a supersonic free jet. Subsequent to vibrational overtone excitation to a light atom stretch vibration, we dissociate the vibrationally excited molecules by $CO_{2}$ laser multiphoton excitation without dissociating the large excess of ground state molecules. Overtone absorption is ultimately monitored by laser induced fluorescence detection of the product fragments. Initial application of this approach to $CH_{3}OH$ reveals a strong Fermi resonance at the $5v_{OH}$ level that is totally obscured in a room temperature overtone spectrum. This talk will emphasize the most recent results using this new technique.
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This work has been supported by the Department of Energy, Office of Basic Energy Sciences
Author Institution: Department of Chemistry, University of Rochester
Author Institution: Department of Chemistry, University of Rochester