Kinetics of fluorescence decay of SO₂ excited in the 2662–3273 Å region

Abstract

Abstract The kinetics of fluorescence decay of SO 2 excited in the 2975–3273 Å region was studied using a powerful, frequency doubled, tunable dye laser system. The existence of two emitting species, first observed by Brus and McDonald, was confirmed. The collision‐free lifetimes of the long‐lived (L) species ranged from 100 to 300 μsec and the short‐lived (S) species from 17 to 43 μsec over the wavelength range employed. The magnitude of the bimolecular quenching rate constant for the L state was a function of the excitation energy; the data show that about 1 kcal/mol of internal energy is lost per collision of the SO 2 (L) species excited in the range of 2998–3107 Å. Studies of the relative initial fluorescence intensity of the S to that of the L state ( I S °/ I L °) were made in experiments which extended to 0.11 mtorr. The pressure dependence of the I S °/ I L ° ratio for experiments at 3107, 3211, and 3225 Åproved that the S and L states do not decay independently. Either efficient bimolecular S → L conversion occurs or bimolecular S ⇄ L interconversion of both states is important. These data coupled with spectroscopic studies of Hamada and Merer and Shaw and coworkers favor the designation of the S and L states as SO 2 ( 1 A 2 ) and SO 2 ( 1 B 1 ), respectively. However, if the assignment is correct, then the band origin of the 1 B 1 state must be at a somewhat longer wavelength, λ> 3273 Å than tentative spectroscopic assignments suggest. Bimolecular quenching rate constants for the L and S components with various atmospheric gases were determined in 3130‐ and 2662‐Å studies.