A variant of cavity-enhanced Raman spectroscopy (CERS) is introduced, in which diode laser radiation at 635 nm is coupled into an external linear optical cavity composed of two highly reflective mirrors. Using optical feedback stabilisation, build-up of circulating laser power by 3 orders of magnitude occurs. Strong Raman signals are collected in forward scattering geometry. Gas phase CERS spectra of H(2), air, CH(4) and benzene are recorded to demonstrate the potential for analytical applications and fundamental molecular studies. Noise equivalent limits of detection in the ppm by volume range (1 bar sample) can be achieved with excellent linearity with a 10 mW excitation laser, with sensitivity increasing with laser power and integration time. The apparatus can be operated with battery powered components and can thus be very compact and portable. Possible applications include safety monitoring of hydrogen gas levels, isotope tracer studies (e.g., (14)N/(15)N ratios), observing isotopomers of hydrogen (e.g., radioactive tritium), and simultaneous multi-component gas analysis. CERS has the potential to become a standard method for sensitive gas phase Raman spectroscopy.

The kinetics of the reaction OH/OD + SO were studied using a laser flash photolysis/laser-induced fluorescence technique. Evidence for two-photon photolysis of SO at 248 nm is presented and quantified, and which appears to have been evident to some extent in most previous photolysis studies, potentially leading to values for the rate coefficient k that are too large. The kinetics of the reaction OH(v = 0) + SO (T = 295 K, p = 25-300 torr) were measured under conditions where SO photolysis was taken into account. These results, together with literature data, were modeled using a master equation analysis. This analysis highlighted problems with the literature data: the rate coefficients derived from flash photolysis data were generally too high and from the flow tube data too low. Our best estimate of the high-pressure limiting rate coefficient k was obtained from selected data and gives a value of (7.8 ± 2.2) × 10 cm molecule s, which is lower than that recommended in the literature. A parametrized form of k([N],T) is provided. The OD(v = 0) + SO (T = 295 K, p = 25-300 torr) data are reported for the first time, and master equation analysis reinforces our assignment of k.

Photoexcitation of glyoxal at wavelengths over the range of 395-414 nm was observed to initiate a chemical reaction that produces the HCO radical in addition to the photolytic production of HCO. The technique of dye laser flash photolysis coupled to cavity ring-down spectroscopy was used to determine the time dependence of the HCO radical signal, analysis of which suggests that the chemical source of HCO is the self-reaction of triplet glyoxal (HCO)2(T1) + (HCO)2(T1) --> 2 HCO + (HCO)2. As the photoexcitation wavelength increases, the production from the triplet glyoxal reaction increases relative to that of HCO from direct photolysis, and at 414 nm, the dominant source of HCO in the system is from the self-reaction of the triplet. The formation of HCO via this process complicates the assignment of the photolysis quantum yield at longer wavelengths and may have been overlooked in some previous glyoxal photolysis studies.

The photolysis of glyoxal has been investigated in the 355-414 nm region by dye laser photolysis coupled with cavity ring-down spectroscopy. Absolute quantum yields of HCO, ΦHCO, were determined using the reaction of chlorine atoms with formaldehyde as an actinometer. The dependence of the quantum yield on pressure was investigated in 3-400 Torr of nitrogen buffer gas and at four temperatures: 233 K, 268 K, 298 K and 323 K. For 355 nm ≤ λ < 395 nm the HCO quantum yield is pressure dependent with linear Stern-Volmer (SV) plots (1/ΦHCO vs. pressure). The zero pressure quantum yield, obtained by extrapolation of the SV plots, rises from 1.6 to 2 between 355 and 382 nm and remains at 2 up to 395 nm. For λ ≥ 395 nm ΦHCO shows a stronger pressure dependence and non-linear SV plots, compatible with formation of HCO by dissociation from two electronic states of glyoxal with significantly different lifetimes. These observations are used to develop a mechanism for the photolysis of glyoxal over the wavelength range studied.

The kinetics of the reaction OH/OD­(v = 1,2,3) + SO2 were studied using a photolysis/laser-induced fluorescence technique. The rate coefficients OH/OD­(v = 1,2,3) + SO2, k 1, over the temperature range of 295–810 K were used to determine the limiting high-pressure limit k 1 ∞. This method is usually applicable if the reaction samples the potential well of the adduct HOSO2 and if intramolecular vibrational relaxation is fast. In the present case, however, the rate coefficients showed an additional fast removal contribution as evidenced by the increase in k 1 with vibrational level; this behavior together with its temperature dependence is consistent with the existence of a weakly bound complex on the potential energy surface prior to adduct formation. The data were analyzed using a composite mechanism that incoporates energy-transfer mechanisms via both the adduct and the complex, and yielded a value of k 1 ∞(295 K) equal to (7.2 ± 3.3) × 10–13 cm3 molecule–1 s–1 (errors at 1σ), a factor of between 2 and 3 smaller than the current recommended IUPAC and JPL values of (2.0–1.0 +2.0) and (1.6 ± 0.4) × 10–12 cm3 molecule–1 s–1 at 298 K, respectively, although the error bars do overlap. k 1 ∞ was observed to only depend weakly on temperature. Further evidence for a smaller k 1 ∞ is presented in the companion paper.

The formation of HCO and of H in the photolysis of glyoxal have been investigated over the wavelength ranges 310-335 nm for HCO and 193-340 nm for H. Dye laser photolysis was coupled with cavity ring-down spectroscopy for HCO, and with laser induced fluorescence spectroscopy for H. Absolute quantum yields were determined using actinometers based on (a) Cl2 photolysis and the Cl + HCHO reaction for HCO and (b) N2O photolysis (and O(1)D + H2) and CH2CO photolysis (and CH2 + O2) for H. The quantum yields were found to be pressure independent in this wavelength region. Quantum yields for all product channels under atmospheric conditions were calculated and compared with literature values. Differences between this work and previously published work and their atmospheric implications are discussed.