Low-Temperature Rate Constants and Product-Branching Ratios for the C(1D) + H2O Reaction.

The gas-phase reaction between atomic carbon in its first electronically excited 1D state and water has been studied over the 50-296 K temperature range using a supersonic flow apparatus. C(1D) atoms were produced by pulsed ultraviolet multiphoton dissociation of carbon tetrabromide; a process that also generates ground-state atomic carbon C(3P). The reaction was followed by detecting product H-atoms by pulsed vacuum ultraviolet laser-induced fluorescence. Two types of experiment were performed. First, temperature-dependent rate constants were derived by recording H-atom formation curves at various gas-phase water concentrations at each temperature. Secondly, temperature-dependent H-atom yields were extracted by comparing the H-atom fluorescence intensities generated by the target C(1D) + H2O reaction with those of a reference reaction. The second-order rate constants are large and increase to low temperature, whereas the measured H-atom yields are close to the theoretical maximum value of 2 above 100 K. At 50 K, neither rate constants nor H-atom yields could be derived because of H-atom formation by quantum tunneling in the activated C(3P) + H2O reaction. The present results are discussed in the context of earlier work on the C(1D)/C(3P) + H2O reactions.