Binding Energies and Vibrational Spectral Features of S$_n$ Species on Amorphous Water-ice Mantles: A Quantum Mechanical Study

TL;DR

This study uses quantum mechanical calculations to determine the binding energies and spectral features of sulfur species on water ice, aiding their identification and understanding their role as sulfur reservoirs in the interstellar medium.

Contribution

It provides new quantum mechanical data on binding energies and spectral features of S$_n$ species on amorphous water ice, which were previously unknown or assumed.

Findings

S$_n$ species have lower binding energies than previously thought.

Spectral features of S$_n$ are minimally shifted when adsorbed on water ice.

IR band intensities of S$_n$ are very low, requiring sensitive instruments for detection.

Abstract

In the denser and colder regions of the interstellar medium (ISM), gas-phase sulfur is depleted by 2 or 3 orders of magnitude with respect to its cosmic abundance. Thus, which species are the main carriers of sulfur is an open question. Recent studies have proposed S$_n$ species as potential sulfur reservoirs. Among the various sulfur allotropes, the most stable one is the S$_8$ ring, detected in the asteroid Ryugu and Orgueil meteorite. Shorter species, namely S$_3$ and S$_4$, have been found in the comet 67P/C-G, but their presence in the ISM remains elusive. In this study, we compute the binding energies (BEs) of S$_n$ (n = 1-8) species on an amorphous water-ice surface model and analyze their infrared (IR) and Raman spectral features to provide data for their identification in the ISM. Our computations reveal that these species exhibit lower BEs than previously assumed and that their spectral features experience minimal shifts when adsorbed on water ice, because of the weak and nonspecific S$_n$-ice interactions. Furthermore, these species display very low IR band intensities and, therefore, very accurate instruments operating in the mid-IR range are required for detecting the presence of these species in dense interstellar environments.