Laboratory infrared spectra and fragmentation chemistry of sulfur allotropes

Sulfur is one of six life-essential elements, but its path from interstellar clouds to planets and their atmospheres is not well known. Astronomical observations in dense clouds have so far been able to trace only 1 percent of cosmic sulfur, in the form of gas phase molecules and volatile ices, with the missing sulfur expected to be locked in a currently unidentified form. The high sulfur abundances inferred in icy and rocky solar system bodies indicate that an efficient pathway must exist from volatile atomic sulfur in the diffuse interstellar medium to some form of refractory sulfur. One hypothesis is the formation of sulfur allotropes, particularly of the stable S8. However, experimental information about sulfur allotropes under astrochemically relevant conditions, needed to constrain their abundance, is lacking. Here, we report the laboratory far-infrared spectra of sulfur allotropes and examine their fragmentation pathways. The spectra, including that of cold, isolated S8 with three bands at 53.5, 41.3 and 21.1 mu m, form a benchmark for computational modelling, which show a near-perfect match with the experiments. The experimental fragmentation pathways of sulfur allotropes, key information for astrochemical formation/destruction models, evidence a facile fragmentation of S8. These findings suggest the presence of sulfur allotropes distributions in interstellar space or in the atmosphere of planets, dependent on the environmental conditions. Sulfur allotropes are proposed to be an important sulfur reservoir in molecular clouds, but spectroscopic data to test this hypothesis are limited. Here the authors measure laboratory far-infrared spectra of cold isolated S8 molecules, which show a near-perfect match with calculations, and examine their fragmentation pathways.