The kinetics of the reactions of the OH radical with ethene (k 1 ), propene (k 2 ), and 1-butene (k 3 ) are studied over a temperature range of T ) 96-296 K. The low-temperature environment is provided by a pulsed Laval nozzle supersonic expansion of nitrogen with admixed radical precursor and reactant gases. The OH radicals are produced by pulsed photolysis of H 2 O 2 at 248 nm. Laser-induced fluorescence of the OH radicals excited in the (1,0) band of the A 2 Σ + -X 2 Π i transition is used to monitor the OH decay kinetics to obtain the bimolecular rate coefficients. At T ) 296 K, the rate constants k 1 , k 2 , and k 3 are also measured as a function of total pressure. The room-temperature falloff parameters are used as the basis for extrapolation of the low-temperature kinetic data, obtained over a limited range of gas number density, to predict the high-pressure limits of all three rate coefficients at low temperatures. The temperature dependence of the measured high-pressure rate constants for T ) 96-296 K can be expressed as follows: k 1,∞ ) (8.7 ( 0.7) × 10 -12 (T/300) (-0.85(0.11) cm 3 molecule -1 s -1 ; k 2,∞ ) (2.95 ( 0.10) × 10 -11 (T/300) (-1.06(0.13) cm 3 molecule -1 s -1 ; k 3,∞ ) (3.02 ( 0.15) × 10 -11 (T/300) (-1.44(0.10) cm 3 molecule -1 s -1 . All three high-pressure rate constants show a slight negative temperature dependence, which is generally in agreement with both low-temperature and high-temperature kinetic data available in the literature. Implications to the atmospheric chemistry of Saturn are discussed.Incorporating the new experimental data on k 1 in photochemical models of Saturn's atmosphere may significantly increase the predicted rate of photochemical conversion of H 2 O into C-O containing molecules.

The kinetics of the reactions of the OH radical with propene and 1-butene are studied at T = 103 K. The low-temperature environment is provided by a pulsed Laval nozzle supersonic expansion of nitrogen with admixed radical precursor and reactant gases. The gas number density and temperature distributions in the flow are characterized by both dynamic pressure measurements and laser-induced fluorescence (LIF) spectroscopy of OH radicals excited in the (1,0) band of the A2Σ+−X2Πi transition. For the kinetic measurements, the OH radical decay profiles in the presence of reactants are monitored by LIF. The rate constants of the reactions of OH with propylene and 1-butene are measured at T = 103 K to be (0.81 ± 0.18) × 10-10 and (1.24 ± 0.27) × 10-10 cm3 molecule-1 s-1, respectively. The observed negative temperature dependences of the rate constants for both reactions studied by the pulsed Laval nozzle system show good agreement with both low-temperature and high-temperature kinetic data available in the literature.

A Michelson-type spectral interferometer that uses a common beam path for the reference and the sample arms is described. This optical arrangement is more compact and stable than the more commonly used dual-arm interferometer and is well suited for frequency-domain optical coherence tomography of biological samples. With a 16-bit CCD camera, the instrument has sufficient dynamic range and resolution for imaging to depths of 2 mm in scattering biological materials. Images obtained with this spectral interferometer are presented, including cross-sectional images in a Xenopus laevis tadpole.