15min:

HYUN S. YOO AND BROOKS H. PATE, Department of Chemistry, University of Virginia, McCormick Rd., P.O. Box 400319, Charlottesville, VA 22904.

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 2x10¹⁰ s^-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 3330 cm^-1, with propyne (HCCCH₃) being the smallest and trimethylsilylacetylene (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.