The photochemistry of formaldehyde: Absolute quantum yields, radical reactions, and NO reactions

Absolute quantum yields have been measured for the gas phase photolysis of H2CO and D2CO as a function of excitation energy. Over the spectral region examined, 290 nm ?λ?360 nm, formaldehyde can dissociate into either molecular or free radical products: H2CO+hν→H2+CO or H2CO+hν→H+HCO. For H2CO the total photochemical quantum yield was Φtotal=1.0±0.1 for λ?326 nm and 0.68 at λ=353 nm. For D2CO, Φtotal=1.0±0.1 for λ?331 nm and drops to zero near the band origin. The mechanism of formyl radical recombination in pure formaldehyde was shown to be 2HCO→H2CO+CO. Addition of NO to the photolysis mixture allowed determination of the radical/molecule branching ratio by introducing the scavenging reactions: HCO+NO→HNO+CO, and H+NO+M→HNO+M. The threshold for production of radicals was found to be between 339 nm and 330 nm for H2CO, and between 326 nm and 320 nm for D2CO. Radical quantum yields increase steeply at the threshold and remain constant near 0.4 at higher energies.