STEVEN T. SHIPMAN AND BROOKS H. PATE, Department of Chemistry, University of Virginia, McCormick Rd., P.O. Box 400319, Charlottesville, VA 22904.
Vibrational lifetimes of a variety of C--H stretches in butyne, methylbutyne, and methylbutenyne have been measured in the gas and dilute solution phases with ultrafast mid-IR transient absorption spectroscopy. In all of these molecules, we monitor dynamics beyond the first stage of intramolecular vibrational relaxation (IVR) by exploiting the significant anharmonic coupling of the acetylenic C--H stretching and bending modes ( -20~\textrmcm-1). Due to this coupling, the absorption frequency of the acetylenic C--H fundamental redshifts as population eventually arrives in the low frequency ( 600~\textrmcm-1) bending mode after leaving the initially prepared bright state. By monitoring the redshifted acetylenic absorption as a function of time after the excitation pulse, we are effectively using the molecule's inherent anharmonic couplings to ``up-convert'' signals arising from the final stages of IVR, providing us with a sensitive probe of vibrational dynamics in a convenient spectral region.