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 smal l 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 experimenta l data were combined with results from the literature to derive the rate of photodissociation from the excited singlet state of acetone and the compet ing intersystem crossing rate populating the triplet state, both relative t o that of pressure quenching, as a function of energy above the dissociatio n threshold. The rate of photodissociation was found to increase with excit ation energy, whereas that of intersystem crossing shows a maximum at inter mediate energies. Previously reported quantum yields for CO formation were analyzed to allow comparison with acetyl radical production.