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

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.