The third experimental approach employed is high-pressure discharge flow, a proven technique for the measurement of radical-molecule rate constants [Donahue et al., 1996a] as well as reaction product yields [Donahue et al., 1996b]. Diameter of the flow tube is large (12.4 cm), so that reactions may be initiated in the center of the flow, and reactants or products may be monitored before reaching the reactor walls. Thus, heterogeneous reactions may be categorically eliminated from the observed chemistry. Because wall loss is negligible, studies may be performed at very low temperatures.
We currently use two High-Pressure Flow (HPF) systems in our laboratory; a long (10 m) tube in which velocity is well-developed and temperatures as low as 180 K have been achieved, and a shorter one (3.5 m) that allows for the rapid exchange among different radical detection schemes. Presently, only the long tube has an FTIR spectrometer coupled with a multipass White cell for measurement of molecular species. In carrying out the proposed work, we plan to extend the capabilities of both systems. This includes modification of the detection zone of the larger flow tube, allowing for interchangeable radical detection schemes; this modification will also allow for a wider solid angle, and thus increased sensitivity, for laser-induced fluorescence (LIF) and resonance fluorescence (RF) measurements. The smaller flow tube will be extended, so that flow velocity in the detection zone will be well-developed, and temperatures may be lowered by cooling of the walls upstream. Because of the smaller thermal mass of the system, temperatures as low as 150 K should be attainable. Addition of a White cell for high-resolution FTIR detection of molecular species will also be useful, particularly for low-temperature studies.
At present, radical detection schemes used on the HPFs have been limited to LIF (hydroxyl kinetics [Sprengnether et al., 2002b]) and ARF (atom kinetics [Clarke et al., 2000b]). The range of species detected will be expanded considerably with the addition of infrared cavity ringdown spectroscopy (IR CRDS) axes. This will allow for the detection of a variety of reactive species that absorb in the IR and near-IR, such as O3, HO2, RO2, and RO. Light from a single tunable diode laser may be split, amplified, and fiber-coupled into each detection axis, allowing for simultaneous measurements at different reaction times from a single laser. Unlike emission techniques such as LIF and ARF, an absorption technique such as CRDS does not specifically image the center of the flow; this will not affect kinetic measurements so long as velocity in the reaction zone is well-developed. Reactive species may be generated not only thermally or by microwave discharge but also photolytically; using excimer lamps that provide high flux, wavelength-specific emission in selected UV transitions, a new class of photolytic sources of radicals is possible.