Spin-orbit ab initio study of the S2(B3Σ-u→X3Σ-g(ν′≥18,0)) diffuse bands

Abstract

The diffuse bands B３Σ-u→X３Σ-g(ν′≥18,0) of diatomic molecule S2 observed in the experiment are investigated. The electronic potential curves,including the spin-orbit coupling (SOC) effect of B３Σ-u and repulsive 1５Πu， 23Σ+u states are calculated. For the diffuse bands beginning at (18,0), a point of view different from others' results is presented in this work. Our results indicate that the SOC induced predissociation between B３Σ-u and 1５Πu， 23Σ+u plays the key role in the diffusion of spectra. Comparison with experimental results shows good agreement.

Predissociation of the B Σu−3 state of S2: A coupled-channel model

“…The 1 5 Π u0 state, through its outer-limb crossing with the B state and a large spin-orbit interaction matrix element, is the primary route for the predissociation of the B 3 Σ − u0 (v ≥ 18) levels. This conclusion is consistent with the results of Yan et al, 15,16 where an ab initio value of ∼160 cm −1 was found for the spin-orbit coupling at the crossing point, 9% less than the present value. The 1 1 Σ + u state, through a strong isoconfigurational spinorbit interaction of 409(15) cm −1 with B 3 Σ − u0 , 25 depresses its levels, thus making a significant second-order spin-orbit contribution to the effective spin-spin constant for the B state.…”

“…where V ∞ is the energy of the relevant dissociation limit, R x is the crossing point with the bound-state PEC, V x is the excess energy above dissociation at the crossing point, and M x is the magnitude of the slope of the exponential curve at the crossing point. The fitting process yielded R x = 2.816(1) Å and M x = 8800(100) cm −1 /Å for the 1 5 Π u0 PEC, which are in fairly good agreement with ab initio values deduced from the work of Yan et al, 15,16 R x = 2.829 Å and M x ≈ 8000 cm −1 /Å, respectively. The fitted interaction matrix elements for Ω = 0 contributing to the B 3 Σ − u0 -state predissociation are listed in the third column of Table I.…”

“…Thus, these three states, 1 1 Π u , 1 5 Π u , and 2 3 Σ + u , are certain to provide significant predissociation routes for the higher-v levels of the B 3 Σ − u state. It must be stressed here, as noted by Yan et al, 15,16 that the 5 Σ − u state invoked by Wheeler et al 14 to explain the B(v ≥ 18) predissociation cannot be responsible, due to symmetry selection rules, 18 and must be replaced by the 1 5 Π u state.…”

“…Further theoretical insight into the B-state predissociation has been provided by Yan et al, 15,16 who performed ab initio calculations of the relevant PECs, including scalar relativistic calculations of their mutual spin-orbit interactions, in three outer-limb crossing regions. 17 They found that the B and 1 1 Π u states crossed at an internuclear distance R = 2.583 Å, with H SO ≈ 65 cm −1 , the B and 1 5 Π u states crossed at R = 2.829 Å, with H SO ≈ 160 cm −1 (Ω = 0 + u ), and the B and 2 3 Σ + u states crossed at R = 3.025 Å, with H SO ≈ 115 cm −1 .…”

Fourier-transform-spectroscopic photoabsorption cross sections and oscillator strengths for the S2 BΣu−3−XΣg−3 system

“…Wheeler et al, 25 as part of their combined experimental and modeling study, calculated ab initio curves for the B and 1 1 Π u states. Yan et al 31 and Xing et al 32 present ab initio potential curves and evaluate spin-orbit coupling between the B state and the ungerade states that may contribute to its predissociation.…”