Photolysis of Acetaldehyde in Air: CH4, CO and CO2 Quantum Yields

✍ Scribed by Dr. Geert K. Moortgat; Dr. Horst Meyrahn; Prof. Dr. Peter Warneck

• Publisher: John Wiley and Sons
• Year: 2010
• Tongue: English
• Weight: 364 KB
• Volume: 11
• Category: Article
• ISSN: 1439-4235
• DOI: 10.1002/cphc.201000757

✦ Synopsis

Abstract

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.