Rate Constant of the Reaction between CHO Radicals and OH Radicals Revisited.

The reaction between CHO and OH radicals has been studied in a laser photolysis cell using the reaction of F atoms with CH and HO for the simultaneous generation of both radicals, with F atoms generated through 248 nm photolysis of XeF. An experimental setup combining cw-Cavity Ring Down Spectroscopy (cw-CRDS) and high repetition rate laser-induced fluorescence (LIF) to a laser photolysis cell has been used. The absolute concentration of CHO was measured by cw-CRDS, while the relative concentration of OH(v = 0) radicals was determined by LIF. To remove dubiety from the quantification of CHO by cw-CRDS in the near-infrared, its absorption cross section has been determined at 7489.16 cm using two different methods. A rate constant of k = (1.60 ± 0.4) × 10 cm s has been determined at 295 K, nearly a factor of 2 lower than an earlier determination from our group ((2.8 ± 1.4) × 10 cm s) using CHI photolysis as a precursor. Quenching of electronically excited I atoms (from CHI photolysis) in collision with OH(v = 0) is suspected to be responsible for a bias in the earlier, fast rate constant.