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 2and 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 ∼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.

A major ice component in Pluto’s haze

“…Our simulations suggest that heterogeneous processes mainly affect the C2H2 and C2H6 profiles, while they have smaller impact on the C2H4 profile. Desorption studies 17 of these organic ices do demonstrate that C2H4 desorbs at lower temperature than C2H2 and C2H6 consistent with the qualitative picture we find here from fitting the New Horizons observations. At its maximum contribution between 100 and 200 km (Fig.…”

Infrared spectra and optical constants of astronomical ices: III. Propane, propylene, and propyne

“…Studies of the chemistry of worlds on which solid hydrocarbons exist, such as Pluto, Titan, and trans-Neptunian objects (TNOs), can benefit from our results in the quantification of ice composition (e.g., Brown et al, 2015;Sasaki et al, 2005). Similarly, laboratory studies of the reaction chemistry of small molecules of planetary and interstellar interest find our work relevant for measuring molecular abundances and reaction yields (e.g., Dartois et al, 2017;Fleury et al, 2019;Behmard et al, 2019).…”

Same-Energy UV Photons and Low-Energy Electrons Activating Methane and Ammonia Frozen in Amorphous Solid Water

“…61 We have introduced a TPD-based analysis of complex molecules embedded within ASW films that has a GC-MS like character to it, being able to characterize weakly and more strongly interacting molecules with the amorphous solid water host matrix. In addition to our previous studies employing this methodology, 36,44 recent studies from the groups of McCoustra 62 and O ̈berg 63 have applied the TPD method as an identification method of various small molecules desorbing from crystalline water-ice and ASW substrates, demonstrating the strength of this method for model studies of molecules that have been accumulated in solid water matrixes in protostellar and protoplanetary environments. 41,61 ■ ASSOCIATED CONTENT * sı Supporting Information…”