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

Abstract

A coupled-channel Schrödinger equation model of predissociation in the B Σu-3 state of S is developed and optimized by comparison with recent photoabsorption spectra of the B Σu-3-X Σg-3(v,0) bands for 11 ≤ v ≤ 27, covering the energy range 35 800-41 500 cm. All bands in this range exhibit varying degrees of diffuseness, with corresponding predissociation linewidths Γ ≈ 4-60 cm full-width at half-maximum. Model comparisons with both low-temperature (T = 370 K) and high-temperature (T = 823 K) spectra indicate, for many bands, significant dependence of the linewidth on both the rotational quantum number J and the fine-structure component F. Just as in the analogous case of O, the B(v)-state predissociation in S is caused principally by spin-orbit interaction with Π, Π, Π, and Σu+3 states. The inner-limb crossing with BΠ is responsible for the predissociation of B(v = 11) and provides a significant slowly varying contribution for B(v ≥ 12). The outer crossings with the Π, Π, and 2 Σu+3 states are responsible for oscillatory contributions to the predissociation widths, with first peaks at v = 13, 20, and 24, respectively, and the Π contribution dominant. Prior to the photodissociation imaging study of Frederix et al. [J. Phys. Chem. A 113, 14995 (2009)], which redefined the dissociation energy of S, the prevailing paradigm was that only the Π interaction was responsible for the B(v = 11-16) predissociation: this view is not supported by our model.

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

“…The relative strengths of B − X(v, 1) hot bands in the predissociation region are consistent with the assignment of a single S 2 temperature. 33 Weak, sharp structure, originating from the S 2 metastable a 1 ∆ g state, was observed in the 38 800-40 850 cm −1 region. The features are associated with six vibrational bands, f 1 ∆ u (5-10) − a 1 ∆ g (0).…”

“…Two complementary sets of photoabsorption measurements were made on S 2 vapor at temperatures of 370 K and 823 K. Both sulfur samples (H 2 S precursor gas for the 370 K spectra; solid elemental sulfur for the 823 K spectra) were in natural abundance ( 32 S 94.93%, 33 S 0.76%, 34 S 4.29%), 34 resulting in S 2 relative abundances of 32 S 2 : 32 S 34 S: 32 S 33 S = 1.000:0.090:0.016.…”

“…The S 2 temperature in the discharge source was determined to be 370 K, with an estimated uncertainty of ∼10 K, 40 from measurements of rotational line strengths in the non-predissociated bands (Sec. III A) and modeling of the rotational structure in the predissociated bands (the details of the modeling procedure are presented in Paper II 33 ). The relative strengths of B − X(v, 1) hot bands in the predissociation region are consistent with the assignment of a single S 2 temperature.…”

“…In this report, we present the first direct measure of the photoabsorption cross sections and oscillator strengths for the strong, predissociating vibrational bands, v ≥ 11, in the B − X(v, 0) system. The cross sections, recorded at 370 K and 823 K, are used to inform a detailed coupled-channel Schrödinger-equation (CSE) predissociation model of the B(v) levels in Paper II by Lewis et al 33 For astrophysical and other applications, this model can be adjusted simply to provide isotopologue-specific cross sections across the full range of relevant temperatures.…”

Gas-phase UV cross sections of radicals

“…Fourier-transform absorption spectroscopy was use to measure cross sections between 240 and 285 nm. This data was used to constrain a coupled-channels model of the B 3 Σ − u state and its predissociation (Lewis et al (2018)). This predissociation is evident in the experimental cross section shown in Fig.…”