Low-Temperature Gas-Phase Rate Coefficients: Measurements, Theory, and Astrochemical Modeling.

Abstract:

In low-temperature astrochemical environments such as in dense molecular clouds and in regions of stellar winds, the relative abundance of molecules can be greatly affected by rate coefficients of particular gas-phase neutral-neutral reactions that become large at these temperatures. However, relatively few of the reaction rate coefficients at these temperatures have been measured due to the experimental difficulty of keeping the reactant molecules from condensing out as they are cooled at pressures near 0.001 Atm. At the University of Leeds, low-temperature gas environments between 31 and 133 K were generated with a Laval nozzle apparatus to avoid condensation of reactants, and reaction rate coefficients were measured with the pulsed laser photolysis-laser induced fluorescence (PLP-LIF) technique. In recent experiments, rate coefficients for CH + CH₂O were measured down to 31 K and were large enough to follow the positive temperature dependence of the collision limit below ∼130 K.[1] When utilized in astrochemical models the newly measured rate coefficients produced up to a factor of two increase in ketene, H₂CCO, under certain model conditions. Similar experiments studying OH + CH₂O yielded rate coefficients that increased by ∼10 times with decreasing temperature below ∼300 K and for CN + CH₂O rate coefficients increased by ∼3 times with decreasing temperature below ∼100 K. OH + CH₂O rate coefficients were previously explained theoretically through quasi-classical trajectory and ring polymer molecular dynamics calculations.[2] However, a high-level potential energy surface is currently being calculated at the University of Leeds for CN + CH₂O in order to explain the behavior of the rate coefficients at low temperatures. Additionally, measurements of C₂H + NH₃ are currently underway.

References

• West, N. A.; et al., ApJ, 885 (2), 134 (2019)
• del Mazo-Sevillano, P.; et al., JPhysChem, 10, 1900 (2019)