Vapor pressures of low-temperature C₂H_x hydrocarbon ices

Ethane (C₂H₆), ethylene (C₂H₄), and acetylene (C₂H₂) are cosmically abundant hydrocarbons with two carbon atoms. They play important roles in a variety of astrophysical settings from sublimation of cometary ices, to photolytic haze formation in the upper atmospheres of Titan and Pluto, to formation of Charon's dark reddish polar cap, to chemical evolution of protoplanetary nebulae. Temperature-dependent vapor pressures and latent heats of sublimation of these species at relevant temperatures are needed to accurately model the processes involved. We used a quartz crystal microbalance to make new measurements of the sublimation rates of these hydrocarbon ices at low temperatures and pressures relevant to diverse environments. Our method accounts for simultaneous sublimation from and condensation onto the quartz crystal. We find vapor pressures of C₂H₆ and C₂H₄ ices to be similar to previous estimates in the literature, while the vapor pressure of C₂H₂ is much lower, owing to the existence of a second, lower temperature solid phase stable below 133 K. We also provide updated parameters for ices of N₂, CO, CH₄, O₂, and NH₃ that we had previously published, applying a new correction to account for condensation from other cold parts of the apparatus.