Surface science studies of the coverage dependent adsorption of methyl acetate and methyl propanoate on graphite

Complex organic molecules (COMs) have been detected in a wide range of astrophysical environments, including the interstellar medium, comets and proto-planetary disks. The icy mantles that form on dust grains in these environments are thought to be the chemical nurseries that allow the formation of many of the COMs that have been identified. As such, the adsorption, thermal processing and desorption of COMs from dust grain surfaces are important in understanding the astrochemical networks as a whole. To study these processes, surface science techniques (temperature programmed desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS)) have been used to investigate ices of the simple esters, methyl acetate and methyl propanoate, adsorbed on a graphitic dust grain analogue surface (highly oriented pyrolytic graphite, HOPG) at 28 K. From the TPD experiments, kinetic parameters have been determined for the desorption of the esters from graphite. The data show a clear coverage dependence for the desorption energies and pre-exponential factors in the sub-monolayer regime. For methyl acetate, the desorption energies and pre-exponential factors range from 57.1 ± 0.4 to 47.2 ± 0.3 kJ mol^-1 and 3.1 × 10^19±0.2 to 1.6 × 10^19±0.1 s^-1 respectively. For methyl propanoate the same parameters range from 57.0 ± 0.1 to 51.0 ± 0.1 kJ mol^-1 and 7.7 × 10^19±0.1 to 4.4 × 10^19±0.1 s^-1. As expected, neither ester shows coverage dependent values for multilayer ices. The determined desorption energies and pre-exponential values for the multilayer ices are 43.5 ± 0.9 kJ mol^-1 and 4.2 × 10^32±0.4 molecules cm^-2 s^-1 for methyl acetate and 45.7 ± 0.9 kJ mol^{- 1} and 8.7 × 10^29±0.4 molecules cm^-2 s^-1 for methyl propanoate. Experimental RAIRS data were also recorded, showing that the ices undergo an irreversible phase change from an amorphous to a crystalline structure when thermally processed. This study provides fundamental data for use in astrochemical models as well as the basis for a future investigation of methyl acetate and methyl propanoate adsorbed in mixed ice environments with water ice.