Knowledge Bank

Vibrational lifetimes of the acetylenic C-H stretch for eight molecules in dilute carbon tetrachloride $(CCl4)$ solutions have been measured at room temperature using femtosecond infrared pump/probe spectroscopy. The vibrational population relaxation in solution is driven by two factors: intramolecular vibrational energy redistribution (IVR) and collision induced relaxation. The solvent-induced relaxation rate has been determined by measuring the vibrational lifetimes of propyne, propargyl chloride, and propargyl bromide in $CCl4$ solutions. High-resolution gas-phase infrared studies of these molecules using an electric-resonance optothermal molecular-beam spectrometer (EROS) show no evidence of IVR. The solvent-induced relaxation rate is approximately $2\times 1010s-1$ in dilute $CCl4$ solutions. The femtosecond pump/probe study shows that lifetimes are independent of the vibrational state densities of molecules, where the state densities range from $100$ to $105$ states/$cm-1$ around $3330cm-1$, with propyne $(HCCCH3)$ being the smallest and trimethylsilylacerylene $(HCCSi(CH3)3)$ being the largest. The overall trend of the relaxation rates, however, strongly resembles the trend of molecules' IVR rates studied with EROS, which demonstrates the solvent-induced energy relaxation is unvarying for different molecules in the same solvent.