Photoacoustic Study of 280 nm Band of Acetone Vapor

Relaxation of the energy absorbed by acetone vapor (200 Torr) on excitation with UV (230–340 nm) was investigated by the photoacoustic technique. The presence of two relaxation paths was found by their difference in heat production rates: The faster path is S0→S1→T1→S0 and the slower path is S0→radicals→recombination products. The PA spectra corresponding to these paths were determined separately. The spectrum for the T1 path has a peak around 295 nm and extends to longer wavelengths beyond 310 nm. The spectrum for the decomposition path has a peak around 275 nm and overlaps with the spectrum for the T1 path in the region of λ<310 nm. The study of frequency dependences of PA signals showed the presence of faster and slower heat releasing steps in each path. In the T1 path, the faster and the slower steps are due to vibrational deactivation of hot T1 formed from S1 and of hot S0 formed from T1 respectively. In the radical path, the faster and slower steps are the deactivation of the hot radicals and the recombination of the thermalized radicals respectively. The addition of O2 to the system accelerated the heat release process via the relaxed T1 and also converted the slow heat release process of recombination into a fast process of addition reactions of O2 to the radicals.