# Desorption Kinetics and Binding Energies of Small Hydrocarbons

## TL;DR

This study measures the binding energies of small hydrocarbons on different ice substrates using temperature programmed desorption, providing key data for understanding their phase distribution in star and planet formation environments.

## Contribution

It presents new experimental constraints on the binding energies of 2- and 3-carbon hydrocarbons on various ice substrates, including pure, compact, and porous amorphous water ice.

## Key findings

- Binding energies range from 2200 to 4700 K depending on substrate and molecule.
- 3-carbon hydrocarbons have higher binding energies than 2-carbon hydrocarbons.
- Hydrocarbon binding energies are slightly higher on water ice compared to pure ices.

## 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.