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, N₂, and CO₂). 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, <i>k</i>_1,<i>Ar</i>, aligns with previous studies and is given by <i>k</i>_1,<i>Ar</i> = 3.48 × 10^-10 exp (-257kJ/<i>RT</i>) cm³ molecule^-1 s^-1. The relative Chaperone efficiencies at 2 atm were determined as <i>k</i>_1,<i>He</i>/<i>k</i>_1,<i>Ar</i> = 2.68, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>k</mml:mi><mml:mrow><mml:mn>1</mml:mn><mml:mo>,</mml:mo><mml:msub><mml:mrow><mml:mi>N</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:msub></mml:math>/<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>k</mml:mi><mml:mrow><mml:mn>1</mml:mn><mml:mo>,</mml:mo><mml:mi>Ar</mml:mi></mml:mrow></mml:msub></mml:math> = 1.85, and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>k</mml:mi><mml:mrow><mml:mn>1</mml:mn><mml:mo>,</mml:mo><mml:mi>C</mml:mi><mml:msub><mml:mrow><mml:mi>O</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:msub></mml:math>/<i>k</i>_1,<i>Ar</i> = 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 <i>k</i>_1,<i>Ar</i>, ±12% for <i>k</i>_1,<i>He</i>, ±18% for <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>k</mml:mi><mml:mrow><mml:mn>1</mml:mn><mml:mo>,</mml:mo><mml:msub><mml:mrow><mml:mi>N</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:msub></mml:math>, and ±24% for <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>k</mml:mi><mml:mrow><mml:mn>1</mml:mn><mml:mo>,</mml:mo><mml:mi>C</mml:mi><mml:msub><mml:mrow><mml:mi>O</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:msub></mml:math>) 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).