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Published December 15, 2000 | Published
Journal Article Open

On the role of coherence in the transition from kinetics to dynamics: Theory and application to femtosecond unimolecular reactions


We consider the relation between observed pump–probe signals in the femtosecond regime and the kinetics of unimolecular reactions, that is, the exponential decay of reactants and the exponential rise of the product population, respectively. It is shown that the signals cannot be fully accounted for within standard approaches of unimolecular decay, conventionally used in the past, since interference effects between the quasi-bound vibrational states within the bandwidth of the pump laser cannot be neglected. When these effects are included, all features of the signals can be accounted for. We apply this theoretical treatment of coherent interference to examine the dynamics and kinetics of the quasi-bound transition configurations, and relate them to the decay rates of individual quasi-bound resonance states. The signals show multi-exponential behavior, reflecting the different decay rates of the resonance states, with an average rate constant (within the bandwidth of the pump laser) which can be extracted directly from the signals. The persistence of coherence is evident in the observed signals. The predissociation of NaI is used as a prototype for numerical illustration.

Additional Information

© 2000 American Institute of Physics. Received 17 August 2000 Accepted 19 September 2000. This work was supported by the National Science Foundation, and U.S. Air Force Office of Scientific Research. N.E.H. appreciates the hospitality he has experienced during his visit to Caltech and LMS.

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