Photodissociation of acetone in air: Dependence on pressure and wavelength. Behavior of the excited singlet state

Quantum yields of acetyl radicals resulting from the photodissociation of 0.1-1% acetone in air were studied in the wavelength range 280-330 nm and at total pressures ranging from 130 to 1000 hPa. Through the addition of small amounts of NO2 to the gas mixtures, acetyl peroxy radicals were scavenged to form peroxyacetyl nitrate (PAN). The PAN quantum yields obeyed a Stern-Volmer type pressure dependence at all wavelengths studied. The experimental data were combined with results from the literature to derive the rate of photodissociation from the excited singlet state of acetone and the competing intersystem crossing rate populating the triplet state, both relative to that of pressure quenching, as a function of energy above the dissociation threshold. The rate of photodissociation was found to increase with excitation energy, whereas that of intersystem crossing shows a maximum at intermediate energies. Previously reported quantum yields for CO formation were analyzed to allow comparison with acetyl radical production.