The Infrared Spectra of Neutral Dimethyl-Sulfide, -Disulfide and -Sulfoxide Biomarkers in Molecular Beams

The suggested detection of sulfur-based biomarkers in the atmosphere of exoplanet K2-18b has triggered intensive discussions about the use of observational data from the James Webb Space Telescope (JWST) to search for life on exoplanets. This methodology, however, requires precise infrared spectroscopic data of biomarker molecules, ideally without the influence of pressure broadening or contaminants. In this work, we use the light of the free-electron laser FELIX to record the infrared spectrum of three sulfur-based biomarkers, dimethyl sulfide (DMS), dimethyl disulfide (DMDS), and dimethyl sulfoxide (DMSO), in the isolated conditions of a molecular beam. The use of FELIX allows the sampling of a wide spectral range, spanning from 650 to 3300 cm^-1, where JWST is most sensitive. In addition, separate spectra of the (DMS)₂, (DMDS)₂ and (DMSO)₂ dimers are recorded, allowing for a clear assessment of the effect of complexation on the infrared spectra of the molecules. Density functional theory calculations and Born-Oppenheimer molecular dynamics simulations are performed to interpret the measured infrared spectra and to estimate absolute cross sections.