The thermal desorption characteristics of 16 astrophysically relevant species from laboratory analogues of the icy mantles on interstellar dust grains have been surveyed in an extensive set of preliminary temperature programmed desorption experiments. The species can be separated into three categories based on behaviour. Water‐like species have a single relevant desorption coincident with water. CO‐like species show the volcano desorption and co‐desorption of trapped molecules, monolayer desorption from the surface of water ice, and multilayer desorption if initially present in sufficient abundance in an outer layer separated from the water ice. Intermediate species show the two desorptions of trapped molecules, and may show a small monolayer desorption for molecules small enough to have a limited ability to diffuse through the structure of porous amorphous water ice. Methods by which the results obtained under laboratory conditions can be adapted for astrophysical situations are discussed.

Hot cores and their precursors contain an integrated record of the physics of the collapse process in the chemistry of the ices deposited during that collapse. In this paper, we present results from a new model of the chemistry near high‐mass stars in which the desorption of each species in the ice mixture is described as indicated by new experimental results obtained under conditions similar to those in hot cores. Our models show that provided there is a monotonic increase in the temperature of the gas and dust surrounding the protostar, the changes in the chemical evolution of each species due to differential desorption are important. The species H2S, SO, SO2, OCS, H2CS, CS, NS, CH3OH, HCOOCH3, CH2CO, C2H5OH show a strong time dependence that may be a useful signature of time evolution in the warm‐up phase as the star moves on to the main sequence. This preliminary study demonstrates the consequences of incorporating reliable temperature programmed desorption data into chemical models.

The adsorption and desorption of CO on and from amorphous H 2 O ice at astrophysically relevant temperatures has been studied using temperature programmed desorption (TPD) and reflection-absorption infrared spectroscopy (RAIRS). Solid CO is able to diffuse into the porous structure of H 2 O at temperatures as low as 15 K. When heated, a phase transition between two forms of amorphous H 2 O ice occurs over the 30-70 K temperature range, causing the partial collapse of pores and the entrapment of CO. Trapped CO is released during crystallization and desorption of the H 2 O film. This behavior may have a significant impact on both gas-phase and solid-phase chemistry in a variety of interstellar environments.

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Carbon monoxide (CO) is an important component of the icy mantles that accrete on interstellar dust grains. To develop a better understanding of the physicochemical basis of its infrared spectroscopy, we have studied the interaction of submonolayer coverages of CO with the surface of films of other astrophysically relevant species--(13)CO, carbon dioxide (CO2), ammonia (NH3), methanol (CH3OH) and water (H2O)--under ultrahigh vacuum and cryogenic (10 K) conditions using reflection-absorption infrared spectroscopy (RAIRS). In support of these measurements, we have performed ab initio calculations of gas phase dimer complexes, and made comparisons to experimental results of gas phase and matrix isolated complexes, which are extensively reported in the literature. The interaction of CO can be categorised as occurring via the C atom (C(CO) bonded), the O atom (O(CO) bonded) or in a π-bonded configuration. The C(CO) configuration is characterised by a blue shifted C≡O stretch frequency, and is observed for CO adsorbed on (13)CO, CO2 and H2O surfaces. From the absence of such a feature from the spectra of CO adsorbed on CH3OH it can be concluded that the dangling OH bonds required for this adsorption configuration are not present at the surface of the CH3OH film.

We present results from laboratory experiments on layered CO-H 2 O-ice systems, carried out from sub-monolayer to multilayer CO coverages, and review recent experimental data, as published by the authors. Under certain specific laboratory conditions the 2152 cm −1 feature, associated with CO molecules adsorbed at dangling-OH bonds at the ice surface, is 'missing'. A detailed analysis is used to understand why the same feature is not detected in spectra of interstellar ices. We conclude that the dangling-OH sites do exist in interstellar ices but that the sites are blocked by another species. The astronomical implications of this deduction are discussed.

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A b s t r a c t .Laboratory surface science under ultra-high vacuum (UHV) conditions allows us to simulate the growth of ices in astrophysical environments. Using the techniques of temperature programmed desorption (TPD), reflectionabsorption infrared spectroscopy (RAIRS) and micro-balance methods, we have studied binary ice systems consisting of water (H2O) and variety of other species including carbon monoxide (CO), at astrophysically relevant conditions of temperature and pressure. We present results that demonstrate that the morphology of water ice has an important influence on the behavior of such systems, by allowing processes such as diffusion and trapping that can not be understood through a knowledge of the binding energies of the species alone. Through an understanding of the implications of water ice morphology on the behavior of ice mixtures in the interstellar environment, additional constraints can be placed on the thermodynamic conditions and ice compositions during comet formation.A commonly applied description of the structure of the ice mantles accreted on interstellar dust grains is the 'onion layered' model (Allamandola et al 1999, Ehrenfreund et al 1998. In this model, t h e dust grain is surrounded by an inner layer of hydrogenated (polar) ice, which in t u r n is coated with an outer layer of non-hydrogenated (apolar) ice. Since the composition of each layer is dominated, respectively, by water (H2O) and carbon monoxide (CO), our laboratory representation of this model is a film of H2O upon which a film of CO is subsequently deposited.We have demonstrated previously t h a t the desorption characteristics of CO are strongly dependent on the morphology of the underlying water film (Collings et al 2003a, b). As the water ice undergoes an irreversible phase change from a high density to a lower density amorphous structure during thermal processing, CO molecules become trapped within pores as pathways to the surface are sealed off. Such CO entrapment can occur even when the CO is initially in a separate layer, since the molecules in the solid CO film become mobile and diffuse into the porous water film at less t h a n 15 K, well below the temperature at which the sublimation of solid C O becomes significant. Trapped C O is released as t h e water film crystallizes and again as the water desorbs.•Using a stochiastic integration software package, we have developed a kinetic simulation incorporating the phase changes of the water ice and diffusion of CO in laboratory experiments (Collings et al 2003b). The simulation can then be I adapted to suit astrophysical time scales, and hence provide a prediction of the i desorption behavior of ice mantles during the formation of the pre-solar nebula, j Current models of desorption in the pre-solar nebula tend to treat the non-| hydrogenated layer in isolation, allowing it to sublime entirely at roughly 20'K. | The results displayed in figure 1 predict that (at the film thickness simulated) | some 15 to 25 % of the CO in the outer layer will become trappe...