Wavelength dependence of the quantum efficiencies of the primary processes in formaldehyde photolysis at 25°C

The quantum efficiencies of the primary processes in formaldehyde photolysis (ϕ 1 and ϕ 2 ) were determined as a function of wavelength in the range from 2890 to 3380 Å and at 25°C: CH 2 O + h ν → H + HCO (1); CH 2 O + h ν → H 2 + CO (2). The estimates of ϕ 2 were derived from Φ H2 values obtained in photolyses of CH 2 O‐isobutene mixtures at high isobutene concentrations where H‐atom scavenging was essentially complete. Values of ϕ 1 + ϕ 2 , obtained from the Φ H2 values from the pure CH 2 O photolyses, were very near unity at all but the longest wavelengths employed: Φ H2 = ϕ 1 + ϕ 2 = 1.02 (2930 Å); 1.12 (3130 Å); 1.06 (3150 Å); 1.01 (3250 Å); 1.0 (3335 Å); 0.75 (3380 Å). Our results showed that the onset of photodissociation of CH 2 O by process (1) was at 3370 ± 10 Å; this corresponds to D (H‐CHO) = 84.8 plusmn; 0.3 kcal/mol. The values of ϕ 1 increased regularly with decreasing wavelength from 0 at 3370 Å to ∼0.7 at 3175 Å. Little further variation in ϕ 1 occurred from 3175 to 2890 Å. For experiments at λ = 3300 Å, the addition of CO 2 (∼300 torr) reduced ϕ 2 , while the effect on ϕ 1 appeared to be small. The present results coupled with the solar irradiance estimates of Peterson [24] and the extinction data for CH 2 O from McQuigg and Calvert [7] were used to make new estimates of the apparent first‐order rate constants (min −;1 × 10 3 ) of process (1) in the lower atmosphere at various solar zenith angles (in parentheses): 2.31 (0°); 2.17 (20°); 1.71 (40°); 0.92 (60°); and 0.17 (78°). The corresponding first‐order rate constants (min −1 × 10 3 ) for solar light absorption by CH 2 O in the lower atmosphere are 7.74 (0°); 7.38 (20°); 6.18 (40°); 3.80 (60°); and 0.96 (78°).