  Experimental results on the thermal desorption of benzene (C6H6) from
amorphous silica (SiO2) are presented. The amorphous SiO2 substrate was imaged
using atomic force microscopy (AFM), revealing a surface morphology reminiscent
of that of interplanetary dust particles (IDPs). Temperature programmed
desorption (TPD) experiments were conducted for a wide range of C6H6 exposures,
yielding information on both C6H6-SiO2 interactions and the C6H6-C6H6
interactions present in the bulk C6H6 ice. The low coverage experiments reveal
complicated desorption behaviour that results both from porosity and roughness
in the SiO2 substrate, and repulsive interactions between C6H6 molecules.
Kinetic parameters were obtained through a combination of direct analysis of
the TPD traces and kinetic modelling, demonstrating the coverage dependence of
both desorption energy and pre-exponential factor. Experiments were also
performed whereby the pores were blocked by pre-exposure of the SiO2 to water
vapour. C6H6 was observed to be adsorbed preferentially on the SiO2 film not
covered by H2O at the temperature at which these experiments were performed.
This observation means that intermolecular repulsion likely becomes important
at smaller C6H6 exposures on grains with a H2O mantle. Kinetic modelling of
C6H6 multilayer desorption yields kinetic parameters in good agreement with
previous studies, with the SiO2 having little impact on the desorption beyond
the first few layers.
