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Published April 4, 1996 | metadata_only
Journal Article

Femtosecond Real-Time Probing of Reactions. 19. Nonlinear (DFWM) Techniques for Probing Transition States of Uni- and Bimolecular Reactions


Degenerate four-wave mixing (DFWM), using ∼60 femtosecond (fs) laser pulses, is introduced to study transition-state dynamics of chemical reactions in the gas phase. The ultrafast techniques are applied to a range of systems, atomic, unimolecular, and bimolecular. It is shown how fs DFWM can be incorporated in different temporal pulse schemes to extract the dynamics. The DFWM beams are configured in a folded boxcar geometry, producing a spatially separated, background-free, femtosecond signal pulse. Aspects of the technique, such as absorption, are investigated. We have taken advantage of the relatively broad spectral width of the fs pulses and extended the techniques to two-color grating experiments in the gas phase. The unimolecular system, NaI, provided a means of testing this new approach. Our experimental observations of the wave packet motion are in excellent agreement with results obtained using laser-induced fluorescence (LIF). A control experiment was also performed on this system, demonstrating the advantages of the nonlinear technique. Bimolecular reactions were initiated for the system Na+H_2. Atomic sodium was investigated with fs DFWM and the oscillatory wave packet behavior (2 ps period) was observed, corresponding to the fine structure splitting of the 3p level (17.2 cm^(-1)). We also explored the application of fs DFWM to the reactive and nonreactive collisions of the Na+H_2 system, which serves as a good model for studying dynamics of nonadiabatic quenching processes and collision complexes.

Additional Information

© 1996 American Chemical Society. Received: January 26, 1996. This work was supported by the U.S. Air Force Office of Scientific Research. We wish to thank Professor P. D. Kleiber and Professor W. C. Stwalley for providing us with the heat-pipe oven and for helpful discussions. One of us (M.M.) gratefully acknowledges a postdoctoral fellowship from the Deutsche Forschungsgemeinschaft and a fellowship from the Max-Planck-Institut für Quantenoptik, Garching.

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August 20, 2023
August 20, 2023