Knowledge Bank

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 ($\sim -20cm-1$). Due to this coupling, the absorption frequency of the acetylenic C--H fundamental redshifts as population eventually arrives in the low frequency ($\sim 600cm-1$) 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.