Absolute quantum yield measurements for the formation of oxygen atoms after UV laser excitation of SO2 at 222.4 nm

Abstract

The dynamics of formation of oxygen atoms after UV photoexcitation of SO2 in the gas-phase was studied by pulsed laser photolysis-laser-induced fluorescence 'pump-and-probe' technique in a flow reactor. SO2 at room-temperature was excited at the KrCl excimer laser wavelength (222.4 nm) and O(3Pj) photofragments were detected under collision-free conditions by vacuum ultraviolet laser-induced fluorescence. The use of narrow-band probe laser radiation, generated via resonant third-order sum-difference frequency conversion of dye laser radiation in Krypton, allowed the measurement of the nascent O(3Pj=2,1,0) fine-structure state distribution: nj=2/nj=1/nj=0 = (0.88 ± 0.02)/(0.10 ± 0.01)/(0.02 ± 0.01). Employing NO2 photolysis as a reference, a value of φO(3P) = 0.13 ± 0.05 for the absolute O(3P) atom quantum yield was determined. The measured O(3P) quantum yield is compared with the results of earlier fluorescence quantum yield measurements. A suitable mechanism is suggested in which the dissociation proceeds via internal conversion from high rotational states of the initially excited SO2(C̃ 1B2) (1, 2, 2) vibronic level to nearby continuum states of the electronic ground state.