Wavelength-dependent photolysis of n-butyraldehyde and i-butyraldehyde in the 280-330-nm region

We have investigated the gas-phase photolysis of n-butyraldehyde (CH3(CH2)(2)CHO) and i-butyraldehyde ((CH3)(2)CHCHO) at 5 nm intervals in the 280-330 nm region using dye laser photolysis in combination with cavity ring-down spectroscopy. Absorption cross sections of both aldehydes have been measured. The quantum yields of radical/molecular photolysis products have been determined as functions of photodissociation wavelength (A), aldehyde pressure, and nitrogen buffer-gas pressure. The HCO radical is a photodecomposition product of both aldehydes. The HCO quantum yields, determined by monitoring HCO absorption at 613.8 nm, decrease with increasing aldehyde pressure in the 1-10 Torr range because of the increasing HCO + HCO, HCO + R, and HCO + RCHO reactions (R = n-C3H7 and i-C3H7) at higher aldehyde pressures and because of quenching by ground-state aldehydes. After separation of the contribution of HCO radical reactions, the aldehyde pressure quenching effect was observed only at lambda greater than or equal to 310 nm. The HCO quantum yields (at all lambda values) and the ratios of quenching to unimolecular decay rate constants of excited aldehydes (lambda greater than or equal to 310 nm) are,given. The HCO quantum yields from n-butyraldehyde photolysis are 0.52 +/- 0.05, 0.74 +/- 0.08, 0.84 +/- 0.04, and 0.77 +/- 0.08 at 305, 310, 315, and 320 nm, respectively, where the uncertainty (1sigma) represents experimental scatter only. The corresponding HCO quantum yields from i-butyraldehyde photolysis are 0.92 +/- 0.08, 1.06 +/- 0.07, 1.06 +/- 0.13, and 1.10 +/- 0.10. The differences in the peak HCO quantum yields are attributed to the opening up of the Norrish II channel (formation of C2H4 + CH3CHO) from n-butyraldehyde photolysis. The dependence of the HCO quantum yield on the nitrogen buffer-gas pressure was examined between 8 and 400 Torr; no dependence was observed. The end products from the photolysis of both aldehydes were analyzed by mass spectrometry. The occurrence of the Norrish II channel is confirmed for n-butyraldehyde.