Absolute Quantum Yields for HCO Production in the Photolysis of Aldehydes Measured by Cavity Ringdown Spectroscopy

Generation of most radicals in the atmosphere is initiated by photolytic processes. It is therefore important to know the efficiencies of the photolytic processes. The high sensitivity of cavity ringdown spectroscopy (CRDS) to detect certain free radicals with suitable absorption features offers a quantitative detection method for quantum yield measurements, especially for small quantum yields, say just a few percent. Because CRDS works at any pressure and temperature, it enables quantum yield measurements over the entire range of atmospheric conditions. Here we describe the first photolysis experiments carried out in our laboratory using a CRDS setup. We detected HCO radicals at 613.85 nm with a tunable dye laser and photolyzed aldehydes from 310-350 nm using a frequency doubled tunable dye laser. To derive absolute quantum yields, Φ , we used the photolysis of chlorine in the presence of chlorine nitrate as an actinometer by measuring the NO₃ produced from the very fast reaction of chlorine atoms with chlorine nitrate. Using the derived laser fluence we could calculate Φ . In a different set of experiments we measured the absolute HCO cross section by comparing the absorption due to NO₃ produced from the Cl + ClONO₂ reaction with the HCO absorbance from the Cl + H₂CO reaction under the same conditions. The NO₃ cross section at the detection wavelength of 613.85 nm (²A"(09⁰0) <- ²A'(00¹0) transition in HCO) is well known. Here we report our preliminary data on the wavelength dependence (315 - 340 nm) of quantum yields of the formyl radical, HCO, from propionaldehyde. Some preliminary data on HCO yield in chloral (Cl₃CCHO) photolysis at 325 nm will also be presented.