Studies of physisorbed ammonia overlayers adsorbed on graphite

An understanding of the interaction of ammonia (NH3) with dust grain surfaces is necessary in order to model the chemistry of the interstellar medium. With this in mind, a detailed reflection absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD) study of the adsorption of NH3 on highly oriented pyrolytic graphite (HOPG) has been performed at ∼90 K. Very sharp bands are observed in the RAIR spectrum, which are characteristic of the formation of crystalline NH3 on the HOPG surface. TPD spectra show that, unusually, the monolayer of NH3 desorbs at a lower temperature than the multilayer. This can be attributed to the extensive hydrogen bonding that occurs in the multilayer. Kinetic parameters, including the desorption order, desorption energy and pre-exponential factor, have been determined for this adsorption system for the first time.