Publication Date:
2015-10-19
Description:
The thermal desorption of ammonia (NH 3 ) 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 NH 3 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 NH 3 desorption time-scale by 5.9 x 10 4 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.
Print ISSN:
0035-8711
Electronic ISSN:
1365-2966
Topics:
Physics
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