Ultraviolet photoabsorption in the B³Σ^- − X³Σ^- and C³Π − X³Σ^- band systems of SO sulphur isotopologues

Abstract

High-resolution far-ultraviolet broadband Fourier-transform photoabsorption spectra of ³²S¹⁶O, ³³S¹⁶O, ³⁴S¹⁶O, and ³⁶S¹⁶O are recorded in a microwave discharge seeded with SO₂. The $B{}^3{\mathrm{\Sigma }}^{-}(v=4-30)\leftarrow X{}^3{\mathrm{\Sigma }}^{-}(v=0)$ and $C{}^3\mathrm{\Pi }(v=0-7)\leftarrow X{}^3{\mathrm{\Sigma }}^{-}(v=0)$ bands are observed or inferred in the $\mathrm{43,000}$–51,000 cm⁻¹ (196 to 233 nm) spectral range. This is the first experimental detection of a $C{}^3\mathrm{\Pi }(v>2)$ level and of any of these observed bands in an S-substituted isotopologue. Additional measurements of $A{}^3\mathrm{\Pi }(v=1-3)\leftarrow X{}^3{\mathrm{\Sigma }}^{-}(v=0)$ provide a calibration of the SO column density. Measured band profiles are fitted to an effective-Hamiltonian model of coupled excited B ³Σ⁻ and C ³Π 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. Linelists, and photoabsorption and photodissociation cross sections are available as supplementary data.

GRAPHICAL ABSTRACT

Keywords:

• Ultraviolet spectroscopy
• photoabsorption
• predissociation
• sulphur monoxide
• isotopologues

Acknowledgements

The authors are very pleased to contribute to this special issue in honour of Prof. Wim Ubachs. We have benefited enormously from his pioneering work in high-resolution and time-resolved spectroscopy and his insightful and energised collaboration on numerous projects. We thank Bérenger Gans of the Institut des Sciences Moléculaires d'Orsay for permitting the use of the radio-frequency discharge source.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Correction Statement

This article has been corrected with minor changes. These changes do not impact the academic content of the article.

Additional information

Funding

AH was funded by the NASA Postdoctoral Program through the NASA Astrobiology Institute, by grant number 19-03314S of the Czech Science Foundation, and the European Regional Development Fund (ERDF/ESF) ‘Centre of Advanced Applied Sciences’ [grant number CZ.02.1. 01/0.0/0.0/16_019/0000778]. JRL acknowledges support from the NASA Exobiology program [grant #80NSSC19K0475 to Arizona State University].