Ultraviolet photoabsorption in the $B\,{}^3{\Sigma}^- - X\,{}^3{\Sigma}^-$ and $C\,{}^3{\Pi} - X\,{}^3{\Sigma}^-$ band systems of SO sulphur isotopologues

TL;DR

This study provides detailed high-resolution ultraviolet spectra of sulfur monoxide isotopologues, identifying new vibrational levels and modeling their electronic states to aid understanding of atmospheric photodissociation processes.

Contribution

It presents the first experimental detection of certain excited states in sulfur monoxide isotopologues and develops an effective Hamiltonian model for their spectral features.

Findings

Detection of $C{}^3{\Pi}(v>2)$ levels in SO isotopologues

First spectral observation of these bands in S-substituted isotopologues

A detailed model of electronic states and dissociation widths for atmospheric applications

Abstract

High-resolution far-ultraviolet broadband Fourier-transform photoabsorption spectra of ${}^{32}{\rm S}^{16}{\rm O}$, ${}^{33}{\rm S}^{16}{\rm O}$, ${}^{34}{\rm S}^{16}{\rm O}$, and ${}^{36}{\rm S}^{16}{\rm O}$ are recorded in a microwave discharge seeded with SO$_2$ . The $B{}^3{\Sigma}^-(v=4-30) \leftarrow X{}^3{\Sigma}^-(v=0)$ and $C{}^3{\Pi}(v=0-7) \leftarrow X{}^3{\Sigma}^-(v=0)$ bands are observed or inferred in the 43000 to 51000 cm$^{-1}$ (196 to 233 nm) spectral range. This is the first experimental detection of a $C{}^3{\Pi}(v>2)$ level and of any of these observed bands in an S-substituted isotopologue. Additional measurements of $A{}^3{\Pi}(v=1-3) \leftarrow X{}^3{\Sigma}^-(v=0)$ provide a calibration of the SO column density. Measured band profiles are fitted to an effective-Hamiltonian model of coupled excited $B{}^3{\Sigma}^-$ and $C{}^3{\Pi}$ states along with their predissociation linewidths and absorption band strengths. Electronic-state potential-energy curves and transition moments are deduced. The end result is a list of line frequencies, $f$-values, and dissociation widths describing the far-ultraviolet photodissociation spectrum of SO that is accurate enough for computing atmospheric photolytic isotope-fractionation.