A3Π and b3Σ–excited states of the so radical. Part 2.—Lifetimes of vibrational levels of the A state and quenching rates

Abstract

A novel frequency-doubled, scanning dye laser has been used to excite individual ro-vibrational levels of the excited A3Π state of the SO free radical. The vibrational level range 6 v′ 0 has been studied, using pulses at excitation wavelengths between 246 and 262 nm, provided from a lithium formate monohydrate frequency-doubling crystal. The rotationally resolved laser-excitation spectrum of the 5–0 band of SO (A3Π–X3Σ–) is reported.Collision-free lifetimes were determined, giving radiative lifetimes τR/µs (1σ) for SO (A, v′) as follows: 9.8 ± 0.8, v′= 6; 9.3 ± 1.0, v′= 5; 10.4 ± 0.8, v′= 4; 11.5 ± 1.3, v′= 3; 14.1 ± 1.1, v′= 2; 17.9 ± 1.4, v′= 1; and 35.9 ± 2.6, v′= 0. No dependence of τR upon rotational (J′) or spin–orbit (Ω) quantum number in the excited-state manifold was found. However, the results showed a strong trend for τR to decrease, and for the electric dipole moment |Re|2 to increase, as a function of increasing vibrational quantum number v′, for the A3Π–X3Σ– transition of SO. Possible reasons for these results are considered.Rate constants for electronic quenching, kQ, were determined for several collision partners, following excitation of SO (A3Π1), v′= 5, N′= 6, 7 (c-component of A-doublet). For O2 and CS2 as collision partners, excitation of the d-components of N′= 8 and 9 also was studied, but gave the same results as the blended N′= 6, 7 (c-) states. Mean values of kQ(2σ, cm3 molecule–1 s–1) at 298 K were as follows: (5.2 ± 0.4)× 10–10, Q = CS2; (1.0 ± 0.2)× 10–10, Q = O2; (9 ± 1)× 10–12, Q = N2; (7.5 ± 1)× 10–12, Q = Ar; and (2.6 ± 0.5)× 10–11, Q = SF6.