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

✍ Scribed by Abraham Horowitz; Jack G. Calvert

• Publisher: John Wiley and Sons
• Year: 1978
• Tongue: English
• Weight: 784 KB
• Volume: 10
• Category: Article
• ISSN: 0538-8066
• DOI: 10.1002/kin.550100803

✦ Synopsis

The quantum efficiencies of the primary processes in formaldehyde photolysis (61 and &z) were determined as a function of wavelength in the range from 2890 to 3380 A and a t 25OC:

The estimates of 6 2 were derived from Q H ~ values obtained in photolyses of CHzO-isobutene mixtures at high isobutene concentrations where H-atom scavenging was essentially complete. Values of & + &, obtained from the Q.H~ values from the pure CH20 photolyses, were very near unity a t all but the longest wavelengths employed: ( P H ~ = $1 + 42 = 1.02 (2930 A); 1.12 (3130 A); 1.06 (3150 A); 1.01 (3250 A); 1.0 (3335 A); 0.75 (3380 A). Our results showed that the onset of photodissociation of CHzO by process (1) was a t 3370 f 10 A; this corresponds to D(H-CHO) = 84.8 f 0.3 kcal/mol. The values of $1 increased regularly with decreasing wavelength from 0 a t 3370 A to 4 . 7 at 3175 A. Little further variation in 41 occurred from 3175 to 2890 A. For experiments at X = 3300

A, the addition of COz (-300 torr) reduced 42, while the effect on 61 appeared to be small.

The present results coupled with the solar irradiance estimates of Peterson [24] and the extinction data for CHzO from McQuigg and Calvert [7] were used to make new estimates of the apparent first-order rate constants (min-I X lo3) of process (1) in the lower atmosphere a t various solar zenith angles (in parentheses): 2.31 (0"); 2.17 (20"); 1.71 (40O); 0.92 (60"); and 0.17 (78O). The corresponding first-order rate constants (min-l X lo3) for solar light absorption by CHzO in the lower atmosphere are 7.74 ( O O ) ; 7.38 (20O); 6.18 (40O); 3.80 (SO0); and 0.96 (78O).