Absolute quantum yield measurements for the formation of oxygen atoms after UV laser excitation of SO2 at 222×4 nm

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 plus-minus 0,02)/(0,10 plus-minus .,01)/(0,02 plus-minus 0,01). Employing NO2 photolysis as a reference, a value of FO(3P) = 0,13 plus-minus 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( C ~ 1B2) (1, 2, 2) vibronic level to nearby continuum states of the electronic ground state.