The photochemistry of sulfur dioxide excited within its first allowed band (3130 Å) and the “forbidden” band (3700–;4000 Å)

The quantum yields of SO 3 formation have been determined in pure SO 2 and in SO 2 mixtures with NO, CO 2 , and O 2 using both flow and static systems. In separate series of experiments excitation of SO 2 was effected within the forbidden band, SO 2 ( 3 B 1 ) ← \documentclass{article}\pagestyle{empty}\begin{document}$$ {\rm SO}_2 (\tilde X,^1 A_1 ) $$\end{document} , and within the first allowed singlet band at 3130 Å. The values of Φ   SO   3were found to be sensitive to the flow rate of the reactants. These results and the apparently divergent quantum yield results of Cox [10], Allen and coworkers [24, 26, 29], and Okuda and coworkers [11] were rationalized quantitatively in terms of the significant occurrence of the reactions SO + SO 3 → 2SO 2 (2), and 2SO → SO 2 + S [or (SO) 2 ] (3), in experiments of long residence time. From the present rate data, values of the rate constants were estimated, k 2 =(1.2±0.7) × 10 6 ; k 3 =(5±4) × 10 5 l˙/mole · sec. Φ   SO   3values from triplet excitation experiments at high flow rates of NOSO 2 and CO 2 SO 2 mixtures showed the sole reactant with SO 2 leading to SO 3 formation in this system to be SO 2 ( 3 B 1 ); SO 2 ( 3 B 1 ) + SO 2 → SO 3 + SO( 3 Σ − ) (la); k   1   a=(4.2±0.4) × 10 7 l./mole · sec. With excitation of SO 2 at 3130 Å both singlet and triplet excited states play a role in SO 3 formation. If the reactive singlet state is 1 B 1 , the long‐lived fluorescent state, SO 2 ( 1 B 1 ) + SO 2 → SO 3 + SO ( 1 Δ or 3 Σ − ) (lb), then k   1   b=(2.2±0.5) × 10 9 l./mole · sec. From the observed inhibition of SO   3   −formation by added nitric oxide, it was found that the SO 3 ‐forming triplet state, generated in this singlet excited SO 2 system, had a relative reactivity toward SO 2 and NO which was equal within the experimental error to that observed here for the SO 2 ( 3 B 1 ) species. Either SO 2 ( 3 B 1 ) molecules were created with an unexpectedly high efficiency in 3130 Å excited SO 2 ( 1 B 1 ) quenching collisions, or another reactive triplet (presumably 3 A 2 or 3 B 2 ) of almost identical reactivity to SO 2 ( 3 B 1 ) was important here.