Wavelength-Dependent Photolysis of i-Pentanal and t-Pentanal from 280 to 330 nm

Abstract

Photodissociation of i-pentanal ((CH3)2CHCH2CHO; 3-methylbutanal) and t-pentanal ((CH3)3CCHO; 2,2-dimethylpropanal) in the 280−330 nm region was investigated at 5 nm intervals by using dye laser photolysis in combination with cavity ring-down spectroscopy. Absorption cross-sections of i-pentanal and t-pentanal were obtained. Both aldehydes exhibited a broad absorption band peaking at 295 nm, with maximum absorption cross-sections of (5.90 ± 0.22) × 10-20 and (4.26 ± 0.07) × 10-20 cm2, respectively, for i-pentanal and t-pentanal. The formation yield of the HCO radical from photolysis of i-pentanal and t-pentanal was determined by monitoring HCO absorption at 613.8 nm. Except for 325 and 330 nm photolysis of i-pentanal, the HCO yield was found to be independent of aldehyde pressure (2−12 Torr) and total pressure (20−400 Torr). For the photodissociation of t-pentanal, the HCO yield was also determined in the 0.5−2 Torr range and was found to increase with t-pentanal pressure. The variation of HCO radical yield with photolysis wavelength was determined. The peak HCO yields from t-pentanal and i-pentanal photolysis were 0.92 ± 0.11 and 0.40 ± 0.08 at 315 nm, respectively, where uncertainty reflects experimental scatter only. The difference in the peak HCO yield from t-pentanal and i-pentanal photolysis was attributed to the opening up of the Norrish II channel for i-pentanal. End products from closed-cell photolysis of i-pentanal and t-pentanal were investigated at 290, 310, and 330 nm by using HPLC. Acetaldehyde was found to be a product from the photodissociation of i-pentanal/N2 mixtures. Photolysis rates of i-pentanal and t-pentanal to form HCO were calculated for two representative atmospheric conditions (noontime at sea level and 40° N latitude on January 1 and on July 1) and were compared to those obtained from acetaldehyde photolysis.