Thermal desorption of ammonia from crystalline forsterite surfaces

Abstract

The thermal desorption of ammonia (NH3) from single crystal forsterite (010) has been investigated using temperature-programmed desorption. The effect of defects on the desorption process has been probed by the use of a rough cut forsterite surface prepared from the cleaved forsterite sample. Several approaches have been used to extract the desorption energy and pre-exponential factor describing the desorption kinetics. In the sub-monolayer coverage regime, the NH3 desorption shows a broad distribution of desorption energies, indicating the presence of different adsorption sites, which results in an apparent coverage-dependent desorption energy. This distribution is sensitive to the surface roughness with the cut forsterite surface displaying a significantly broader distribution of desorption energies compared to the cleaved forsterite surface. The cut forsterite surface exhibits sites with desorption energies up to 62.5 kJ mol-1 in comparison to a desorption energy of up to 58.0 kJ mol-1 for the cleaved surface. Multilayer desorption is independent of the nature of the forsterite surface used, with a desorption energy of (25.8 ± 0.9) kJ mol-1. On astrophysically relevant heating time-scales, the presence of a coverage-dependent desorption energy distribution results in a lengthening of the NH3 desorption time-scale by 5.9 × 104 yr compared to that expected for a single desorption energy. In addition, the presence of a larger number of high-energy adsorption sites on the rougher cut forsterite surface leads to a further lengthening of ca. 7000 yr.