Experimental Measurement of the Rate Coefficient for OCS + M, with M = Ar, He, N 2 , CO 2 in a Shock Tube Using Laser Absorption Spectroscopy

A comprehensive understanding of sulfur chemistry is crucial for the characterization and modeling of planetary atmospheres. The unimolecular decomposition of carbonyl sulfide (OCS + M = CO + S + M) is a critical reaction for the development of accurate photochemical models. In this study, we employ laser absorption spectroscopy (LAS)-based diagnostics in a shock tube to investigate the rate coefficient of OCS decomposition. Sensitive and interference-free diagnostics were developed to monitor OCS depletion at 2070.858 cm –1 and CO formation at 2115.628 cm –1 . The reaction rate coefficient of OCS decomposition was measured over a temperature range of 1800–2500 K. This work represents the first experimental determination of OCS decomposition rates in bath gases pertinent to several planetary environments (i.e., He, N 2, and CO 2 ). Additionally, we characterize the pressure dependence of the reaction rate through measurements at 1, 2, and 8 atm. The rate constant measured at 2 atm for argon, k 1, Ar, aligns with previous studies and is given by k 1, Ar = 3.48 × 10 –10 exp (−257kJ/ RT ) cm 3 molecule –1 s –1 . The relative Chaperone efficiencies at 2 atm were determined as k 1, He / k 1, Ar = 2.68, k 1, N 2 / k 1, Ar = 1.85, and k 1, C O 2 / k 1, Ar = 3.67 through our experiments. Our results provide new insights into OCS kinetics, marking the first systematic study of its pressure-dependent behavior in exoplanetary-relevant conditions. These findings, underscored by low experimental uncertainties (±9.0% for k 1, Ar, ±12% for k 1, He, ±18% for k 1, N 2, and ±24% for k 1, C O 2 ) reflect high-quality, repeatable measurements that will support sulfur chemistry atmospheric modeling and enhance the interpretation of spectroscopic observations from missions such as the James Webb Space Telescope (JWST).