Dynamics of intramolecular vibrational redistribution in terminal acetylene molecules studies by time-resolved Raman spectroscopy

We report the first direct investigations of vibrational dynamics in isolated polyatomic molecules by means of Raman spectroscopy. A pump-probe experimental scheme is applied where laser pumping of one mode of a molecule is followed by probing its anti-Stokes Raman spectrum, so that a decrease ofRaman signal with the probe-pump delay time tells us about intramolecular vibrational redistribution (IVR) of the energy from the initially excited mode to the other modes. Experimental results for terminal acetylene molecules are presented and discussed. The principal observation is that IVR times in our studies are much shorter than those estimated previously for the same molecules from high-resolution spectroscopy ofthe cold molecular beams. We analyze this difference assuming that the rotational degrees of freedom, which are highly populated in our case of gas at room temperature, play an important role in mediating IVR in the molecules under consideration.