Photolysis of acetaldehyde in air: CH4, CO and CO2 quantum yields.

The quantum yields for CO and CH(4), formed in the photolysis of about 200 μmol mol(-1) of acetaldehyde in air at atmospheric pressure, were determined at 15 wavelengths in the range: 250-330 nm. The quantum yields for CO(2) were determined at nine wavelengths in the range: 250-315 nm. The products are mainly assigned to three primary processes: I) CH(3)CHO*→CH(3)+HCO, II) CH(3)CHO*→CH(4)+CO, III) CH(3)CHO*→CH(3)CO+H, with dissociation occurring from the initially populated S(1) singlet state of acetaldehyde. The pressure dependence of the quantum yields at wavelengths of 270, 304.4, and 313 nm exhibits a Stern-Volmer behavior in all cases. Collisional quenching is shown to deactivate the singlet state S(1) by pressure-induced intersystem crossing to the neighboring triplet state, which is subsequently rapidly quenched by oxygen. Quenching coefficients as a function of excitation energy are obtained by comparison with literature data; the wavelength dependence of individual primary quantum yields is also derived. CO(2) quantum yields at 304.4 and 313 nm were found to increase with rising CH(3)CHO concentrations due to secondary reactions. The reaction mechanism responsible for this effect was explored by means of computer simulations.