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 τ_R/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 τ_R upon rotational (J′) or spin–orbit (Ω) quantum number in the excited-state manifold was found. However, the results showed a strong trend for τ_R to decrease, and for the electric dipole moment |R_e|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, k_Q, 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 k_Q(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.