Desorption Kinetics and Binding Energies of Small Hydrocarbons

Abstract

Small hydrocarbons are an important organic reservoir in protostellar and protoplanetary environments. Constraints on desorption temperatures and binding energies of such hydrocarbons are needed for accurate predictions of where these molecules exist in the ice vs. gas-phase during the different stages of star and planet formation. Through a series of temperature programmed desorption (TPD) experiments, we constrain the binding energies of 2 and 3-carbon hydrocarbons (C$_{2}$H$_{2}$ - acetylene, C$_{2}$H$_{4}$ - ethylene, C$_{2}$H$_{6}$ - ethane, C$_{3}$H$_{4}$ - propyne, C$_{3}$H$_{6}$ - propene, and C$_{3}$H$_{8}$ - propane) to 2200-4200 K in the case of pure amorphous ices, to 2400-4400 K on compact amorphous H$_{2}$O, and to 2800-4700 K on porous amorphous H$_{2}$O. The 3-carbon hydrocarbon binding energies are always larger than the 2-carbon hydrocarbon binding energies. Within the 2- and 3-carbon hydrocarbon families, the alkynes (i.e., least-saturated) hydrocarbons exhibit the largest binding energies, while the alkane and alkene binding energies are comparable. Binding energies are $\sim$5-20% higher on water ice substrates compared to pure ices, which is a small increase compared to what has been measured for other volatile molecules such as CO and N$_{2}$. Thus in the case of hydrocarbons, H$_{2}$O has a less pronounced effect on sublimation front locations (i.e., snowlines) in protoplanetary disks.