The reaction of IO radicals with dimethyl sulfide was studied using cavity ring-down laser spectroscopy. The reaction rate constant shows both a temperature and pressure dependence. At 100 Torr total pressure, the reaction has reached its high-pressure limit and has a rate constant of (2.5 ( 0.2) × 10 -13 molecule -1 cm 3 s -1 at 298 K. On the basis of the Arrhenius plot in the region of 273-312 K, the reaction has a negative activation energy (E a ) -18.5 ( 3.8 kJ mol -1 ). The atmospheric implications of these findings are discussed. In light of these new data, DMS oxidation by IO can compete with oxidation by the hydroxyl radical in the marine boundary layer. Quoted uncertainties are one standard deviation from regression analysis.

Cavity ring-down spectroscopy coupled with pulsed laser photolysis was used to study the visible absorption spectrum (490−535 nm, 2B1 ← 2A1 transition) of the phenyl radical, C6H5, in 10−50 Torr of argon diluent at 298 K. Absorption cross-sections were independent of total pressure over the range studied. At 504.8 nm, σphenyl = (3.6 ± 1.6) × 10-19 cm2 molecule-1 (base e). Spectral simulation of the rotational structure of an origin band was performed using a model for a type C vibronic band. The vibronic spectrum was analyzed using normal-mode information from quantum chemical calculations employing hybrid density functional theory (B3LYP/aug-cc-pVDZ). The a1 and b1 vibrations were confirmed in the vibronic spectrum. Cavity ring-down spectroscopy was used to follow the loss of phenyl radicals and measure k(C6H5+Cl) = (1.2 ± 0.8) × 10-10, k(C6H5+Br) = (7.0 ± 4.0) × 10-11, and k(C6H5+Cl2) = (2.96 ± 0.53) × 10-11 at 298K, and k(C6H5+Cl2) = ( ) × 10-12 exp[(1000 ± 470)/T] cm3 molecule-1 s-1. Relative rate techniques were used to measure k(C6H5+Cl2)/k(C6H5+O2) = 2.1 ± 0.4 in 10−700 Torr of N2 diluent at 296K. Combining the absolute and relative rate data gives k(C6H5+O2) = (1.4 ± 0.4) × 10-11 cm3 molecule-1 s-1. In 1 atm of air C6H5 radicals have a lifetime of approximately 1.4 × 10-8 s with respect to reaction with O2 to give C6H5O2 radicals. Results are discussed with respect to the spectroscopy and reactivity of C6H5 radicals. Quoted uncertainties are 2 standard deviations from regression analyses.

Buffer-gas pressure broadening for the (3 0(0) 1)(III)<--(0 0 0) band of CO(2) in the 1600 nm region was investigated with continuous wave cavity ring-down spectroscopy within the temperature range 263-326 K. The measured absorption profiles were analyzed with Voigt functions. Pressure broadening coefficient, gamma(gas), and the temperature dependent parameter (broadening exponent), n, were determined for a variety of buffer gases: N(2), O(2), He, Ne, Ar, Kr and Xe. gamma(air) values estimated subsequently are 0.096(2) for R(0), 0.085(5) for P(8), 0.075(2) for P(16), 0.070(4) for P(26), and 0.069(2) for P(38) in units of cm(-1) atm(-1), where numbers in parentheses are one standard deviation in units of the last digits quoted. n(air) values are 0.77(4) for R(0), and 0.73(11) for P(8).

The use of continuous wave cavity ring-down spectroscopy (cw CRDS) with near infra-red diode lasers is demonstrated for quantitative detection of trace levels of unsaturated volatile organic compounds (VOCs) at wavelengths that avoid overlapping absorptions by more abundant atmospheric constituents such as H 2 O and CO 2 . The current detection limit, with due allowance for pressure broadening by 1 atmosphere of air, is 6 parts per billion by volume (ppbv) for ethyne at an air wavelength of 1519.670 nm, and is sufficient for direct atmospheric detection of this molecule in many urban environments. Detection limits for alkenes are inferior, and, without incorporating the consequences of pressure broadening, include 78 ppbv for ethene and 900 ppbv for 1,3-butadiene. While the CRDS detection method offers several advantages over established gas chromatographic techniques for monitoring of small VOCs such as ethyne, it appears to be less well suited to study of larger organic compounds. Methods are discussed for improving the instrument to reach the sensitivities required to monitor the various alkenes and other C-H containing molecules in the troposphere.

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