Reflection absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD) have been used to perform a detailed investigation of the adsorption of water on highly oriented pyrolytic graphite (HOPG) at 90 K. RAIRS shows that water is physisorbed on HOPG at all coverages, as expected. Experiments at higher surface temperatures show marked changes in the O-H stretching region of the spectrum which can be assigned to the observation of the amorphous to crystalline ice phase transition. The infrared signature of both phases of solid water has been determined on HOPG and can be used to identify the phase of the ice. TPD spectra show the desorption of multilayers of crystalline ice. At high exposures a small bump appears in the TPD spectrum, on the low temperature side of the main peak, which is attributed to the amorphous to crystalline phase transition. At very low exposures of water, it is possible to distinguish the desorption of water from two-and three-dimensional islands and hence to determine the growth mode of water on the HOPG surface. Isothermal TPD studies have also been performed and show that the desorption of water does not obey perfect zero-order kinetics. Desorption orders, derived directly from the TPD spectra, confirm this observation. Desorption energies and preexponential factors have also been determined for this adsorption system.

Reflection absorption infrared spectroscopy and temperature programmed desorption investigations of the interaction of methanol with a graphite surface Copyright and reuse:Sussex Research Online is a digital repository of the research output of the University.Copyright and all moral rights to the version of the paper presented here belong to the individual author(s) and/or other copyright owners. To the extent reasonable and practicable, the material made available in SRO has been checked for eligibility before being made available.Copies of full text items generally can be reproduced, displayed or performed and given to third parties in any format or medium for personal research or study, educational, or not-for-profit purposes without prior permission or charge, provided that the authors, title and full bibliographic details are credited, a hyperlink and/or URL is given for the original metadata page and the content is not changed in any way. Reflection absorption infrared spectroscopy ͑RAIRS͒ and temperature programmed desorption ͑TPD͒ have been used to investigate the adsorption of methanol (CH 3 OH) on the highly oriented pyrolytic graphite ͑HOPG͒ surface. RAIRS shows that CH 3 OH is physisorbed at all exposures and that crystalline CH 3 OH can be formed, provided that the surface temperature and coverage are high enough. It is not possible to distinguish CH 3 OH that is closely associated with the HOPG surface from CH 3 OH adsorbed in multilayers using RAIRS. In contrast, TPD data show three peaks for the desorption of CH 3 OH. Initial adsorption leads to the observation of a peak assigned to the desorption of a monolayer. Subsequent adsorption leads to the formation of multilayers on the surface and two TPD peaks are observed which can be assigned to the desorption of multilayer CH 3 OH. The first of these shows a fractional order desorption, assigned to the presence of hydrogen bonding in the overlayer. The higher temperature multilayer desorption peak is only observed following very high exposures of CH 3 OH to the surface and can be assigned to the desorption of crystalline CH 3 OH.

The desorption of molecular ices from grain surfaces is important in a number of astrophysical environments including dense molecular clouds, cometary nuclei and the surfaces and atmospheres of some planets. With this in mind, we have performed a detailed investigation of the desorption of pure water, pure methanol and pure ammonia ices from a model dust‐grain surface. We have used these results to determine the desorption energy, order of desorption and the pre‐exponential factor for the desorption of these molecular ices from our model surface. We find good agreement between our desorption energies and those determined previously; however, our values for the desorption orders, and hence also the pre‐exponential factors, are different to those reported previously. The kinetic parameters derived from our data have been used to model desorption on time‐scales relevant to astrophysical processes and to calculate molecular residence times, given in terms of population half‐life as a function of temperature. These results show the importance of laboratory data for the understanding of astronomical situations whereby icy mantles are warmed by nearby stars and by other dynamical events.

RAIRS experiments have been performed to investigate the adsorption of NO on Pt{211}. Results show that adsorption is complex and strongly temperature dependent. At 307 K, three bands are seen at saturation with frequencies of 1801, 1609, and 1576 cm-1. However, at 120 K only two bands, at 1688 and 1620 cm-1, are observed. To help with the assignment of these vibrational bands, DFT calculations were also performed. The calculations show that a bridged NO species, bonded to the step edge, is the most stable species on the surface and gives rise to the band observed at 1610−1620 cm-1. The calculations also suggest that the temperature dependence of NO adsorption on Pt{211} can be assigned to NO dissociation which occurs at room temperature but not at 120 K. In particular, the RAIRS band observed at 1801 cm-1, which is observed on adsorption at 307 K but not at 120 K, is tentatively assigned to the formation of an O−NO complex. This species forms when a NO molecule bonds on top of an O atom, which results from the dissociation of NO on the Pt{211} surface at room temperature.

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