The kinetics of elementary reactions involving the oxides of sulphur III. The chemiluminescent reaction between sulphur monoxide and ozone

Abstract
The chemiluminescent reaction between sulphur monoxide (SO) and ozone has been studied in a fast flow system at pressures between 0·3 and 3·0 mmHg, These species undergo a rapid bimolecular reation (1) SO + O 3 = SO 2 + O 2 + 106 kcal/mole (1) to yield ground state products, where k 1 = 1·5 x 10 12 exp ( –2100/ RT ) cm 3 mole -1 s -1 . This reaction also yields electronically excited SO 2 molecules in the 1 B and 3 B 1 states. The 1 B SO 2 molecules are produced with up to 16 kcal/mole vibrational energy. Emission from the longer lived 3 B 1 state is vibrationally relaxed and provides no information about the initial energy distribution. Comparison with fluorescence studies shows that the 3 B 1 SO 2 molecules are produced mainly by collisional quenching of SO 2 molecules formed in the 1 B state. The formation of electronically excited SO 2 is also a simple bimolecular process, but it involves a higher energy barrier than formation of ground state SO 2 . Our measurements on the chemiluminescence, when combined with data on the quenching of the SO 2 fluorescence, yield the rate constants k 1a = 10 11 exp ( – 4200/ RT ) and k lb ≯ 3 x 10 10 exp ( –3900/ RT ) cm 3 mole -1 s -1 for the bimolecular reactions SO + O 3 = SO 2 ( 1 B ) + O 2 + 21 kcal/mole, (1 a ) SO + O 3 = SO 2 ( 3 B 1 ) + O 2 + 35 kcal/mole (1 b ) which form electronically excited SO 2 . No electronically excited O 2 appears to be formed. It is deduced that electronically excited SO 2 is produced by crossing to a separate potential surface at or near the transition state rather than by the formation of a highly vibrationally excited SO 2 molecule which crosses to the excited electronic state.