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 \pm 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 \times 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 .