A New Method for Measuring Infrared Band Strengths in H₂O Ices: First Results for OCS, H₂S, and SO₂

Abstract

Infrared (IR) band strengths are needed to extract accurate molecular abundances from astronomical observations of interstellar and solar system ices. However, laboratory measurements of such intensities often have required multiple assumptions about ice composition and thickness. Here we describe a method that circumvents most of the common assumptions and uncertainties in IR band-strength determinations. We have applied the method to measure IR band strengths of OCS, H2S, and SO2 in the absence and presence of H2O ice at 10 K, the first measurements of their type. Our results show for the first time that the presence of H2O makes little difference in IR intensities for these three sulfur-containing molecules’ strongest IR features. The immediate application will be to laboratory studies of low-temperature chemistry of interstellar and cometary ices.

Thermal Reactions between H2S and O3: Implications for Europa Surface Chemistry

“…The OCS contamination is apparent from the small band centered at 2045 cm −1 (Ferrante et al 2008) and is likely introduced by the H 2 S source, which lists OCS as a possible contaminant at 0.2% by volume. Despite being fairly dilute, the band strength for the C = O stretch of OCS is comparable to CO 2 (Yarnall & Hudson 2022b), and minimal contamination from this compound is reasonable. Moreover, this feature can be seen in the spectra of mixtures of H 2 S and H 2 O ice throughout the literature, where similar qualities of H 2 S are used (Jiménez-Escobar & Caro 2011; Yarnall & Hudson 2022b).…”

“…Despite being fairly dilute, the band strength for the C = O stretch of OCS is comparable to CO 2 (Yarnall & Hudson 2022b), and minimal contamination from this compound is reasonable. Moreover, this feature can be seen in the spectra of mixtures of H 2 S and H 2 O ice throughout the literature, where similar qualities of H 2 S are used (Jiménez-Escobar & Caro 2011; Yarnall & Hudson 2022b). Recent work has demonstrated that OCS can undergo thermal nucleophilic addition in the presence of methylamine (CH 3 NH 2 ; Mahjoub & Hodyss 2018); however, the kinetics of a simple addition reaction between OCS and H 2 O are unfavorable (Ghosh et al 2010).…”

A systematic mid-infrared spectroscopic study of thermally processed SO2 ices

“…In this study, we have taken A n to be 7.3 Â 10 À18 and 4.2 Â 10 À17 cm molecule À1 for the n 1 and n 3 modes, respectively; and have taken r to be 1.395 and 1.893 g cm À1 for amorphous and crystalline SO 2 ice, respectively. 56,57 Uncertainties in the calculated column densities (typically AE12.5%) and ice thickness (on the order of AE0.002 mm) were determined through standard propagation-of-error calculations that assumed a 5% measurement error in the value of P and used published uncertainty values for the values of A n and r that were used. 56,57 It should be noted that these values for A n were calculated for amorphous SO 2 ices deposited at 10 K, 56 while the r values for amorphous and crystalline SO 2 were measured for ices deposited at 19 and 85 K, 56,57 respectively.…”

“…56,57 Uncertainties in the calculated column densities (typically ±12.5%) and ice thickness (on the order of ±0.002 μm) were determined through standard propagation-of-error calculations that assumed a 5% measurement error in the value of P and used published uncertainty values for the values of A ν and ρ that were used. 56,57…”

An Ice Age JWST inventory of dense molecular cloud ices

“…For CH 3 CN the value is less than 2% for both sources. The FWHM and band strengths for the pure ices are taken from 62,65,66 .…”