Ultraviolet spectrum and reaction kinetics of the formyl radical

A transient ultraviolet absorption attributed to the formyl radical, HCO, was produced by the flash photolysis of water vapor in the presence of CO. For CO saturated with H/sub 2/O at a total pressure of 1 atm, the formation of HCO is complete within approx. 60 ..mu..s, after which it undergoes second-order decay, with k/epsilon(230 nm) = 1.5 +- 0.2) x 10/sup 7/ cm s/sup -1/. During the relatively slow formation reaction (H + CO(+M) ..-->.. HCO), the competing reaction H + HCO ..-->.. H/sub 2/ + CO is significant. By employing the H/sub 2/O + CH/sub 4/ system as an actinometer to determine the amount of H/sub 2/O dissociated by the flash (discussed elsewhere) and by computer modeling the observed formation and decay of HCO as a function of CO pressure, the following absolute rate constants were obtained: k/sub 3/(H + CO(+M) ..-->.. HCO) = (3.6 +- 0.9) x 10/sup 7/ M/sup -2/ s/sup -1/ for M = CO, (5.8 +- 0.6) x 10/sup 7/ M/sup -2/ s/sup -1/ for M = CH/sub 4/, and (3.8 +- 0.4) x 10/sup 7/ M/sup -2/ s/sup -1/ for M = H/sub 2/, k/sub 4/(HCO + HCO ..-->.. products) = (1.4 +-more » 0.3) x 10/sup 10/ M/sup -1/ s/sup -1/, and k/sub 5/(H +- HCO ..-->.. products) = (6.9 +- 1.7) x 10/sup 10/ M/sup -1/ s/sup -1/. The molar extinction coefficient epsilon(HCO, 230 nm) = 941 +- 171 M/sup -1/ cm/sup -1/.« less