FEMTOSECOND PUMP/PROBE STUDY OF VIBRATIONAL LIFETIMES OF THE FUNDAMENTAL ACETYLENIC C-H STRETCH IN DILUTE SOLUTIONS

Abstract

Vibrational lifetimes of the acetylenic C-H stretch for eight molecules in dilute carbon tetrachloride $(CCl_{4})$ 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 $CCl_{4}$ 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 $2 \times 10^{10} s^{-1}$ in dilute $CCl_{4}$ solutions. The femtosecond pump/probe study shows that lifetimes are independent of the vibrational state densities of molecules, where the state densities range from $10^{0}$ to $10^{5}$ states/$cm^{-1}$ around $3330 cm^{-1}$, with propyne $(HCCCH_{3})$ being the smallest and trimethylsilylacerylene $(HCCSi(CH_{3})_{3})$ 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.