Vibrational Relaxation of SO(X3Σ-, v = 1−5) and Nascent Vibrational Distributions of SO Generated in the Photolysis of SO2 at 193.3 nm

Abstract

Rate coefficients for deactivation of SO(X 3 Σ - , v = 1-5) by collisions with SO 2 and nascent vibrational populations in v = 0-2 in the photolysis of SO 2 at 193.3 nm have been determined. A single vibrational level of SO(X 3 Σ - ) was detected with laser-induced fluorescence (LIF) excited via the B 3 Σ - -X 3 Σ - system. Time-dependent profiles of LIF signals were recorded as a function of the pressures of SO 2 . Observed profiles were analyzed by the integrated-profiles method and reproduced well by convolution calculations. Overall rate coefficients for vibrational relaxation of SO(v) by SO 2 have been determined to be (4.7 ′ 0.5) × 10 - 1 2 (v = 1), (6.7 ′ 0.4) x 10 - 1 2 (v = 2), (7.2 ′ 0.7) x 10 - 1 2 (v = 3), (6.1 ′ 1.0) x 10 - 1 2 (v = 4), and (8.6 ′ 0.7) × 10 - 1 2 (v = 5) in units of cm 3 molecule - 1 s - 1 (the quoted errors are 2σ). We have also found that 63% of the vibrational deactivations of v = 2 by SO 2 are governed by double-quantum relaxation: (4.2 ′ 0.9) x 10 - 1 2 cm 3 molecule - 1 s - 1 for v = 2 → v = 0 and (2.5 ′ 0.9) x 10 - 1 2 cm 3 molecule - 1 s - 1 for v = 2 → v = 1. Ab initio calculations enable us to find two stable complexes: OS-OSO and SO-SO 2 , indicating that attractive interactions play a significant role in the relaxation. The nascent vibrational distributions of SO have been measured to be 0.52 ′ 0.1/0.75 ′ 0.1/1.0 for v = 0/½. The differences in vibrational distributions reported by bulk and beam experiments are attributed to the difference in the temperature of parent SO 2 .