Glycolaldehyde, methyl formate and acetic acid adsorption and thermal desorption from interstellar ices

We have undertaken a detailed investigation of the adsorption, desorption and thermal processing of the astrobiologically significant isomers glycolaldehyde, acetic acid and methyl formate. Here, we present the results of laboratory infrared and temperature programmed desorption (TPD) studies of the three isomers from model interstellar ices adsorbed on a carbonaceous dust grain analogue surface. Laboratory infrared data show that the isomers can be clearly distinguished on the basis of their infrared spectra, which has implications for observations of interstellar ice spectra. Laboratory TPD data also show that the three isomers can be distinguished on the basis of their thermal desorption behaviour. In particular, TPD data show that the isomers cannot be treated the same way in astrophysical models of desorption. The desorption of glycolaldehyde and acetic acid from water-dominated ices is very similar, with desorption being mainly dictated by water ice. However, methyl formate also desorbs from the surface of the ice, as a pure desorption feature, and therefore desorbs at a lower temperature than the other two isomers. This is more clearly indicated by models of the desorption on astrophysical time-scales corresponding to the heating rate of 25 and 5 M⊙ stars. For a 25 M⊙ star, our model shows that a proportion of the methyl formate can be found in the gas phase at earlier times compared to glycolaldehyde and acetic acid. This has implications for the observation and detection of these molecules, and potentially explains why methyl formate has been observed in a wider range of astrophysical environments than the other two isomers

Trapping and desorption of complex organic molecules in water at 20 K

The formation, chemical and thermal processing of complex organic molecules (COMs) is currently a topic of much interest in interstellar chemistry. The isomers glycolaldehyde, methyl formate and acetic acid are particularly important because of their role as pre-biotic species. It is becoming increasingly clear that many COMs are formed within interstellar ices which are dominated by water. Hence the interaction of these species with water ice is crucially important in dictating their behaviour. Here we present the first detailed comparative study of the adsorption and thermal processing of glycolaldehyde, methyl formate and acetic acid adsorbed on and in water ices at astrophysically relevant temperatures (20 K). We show that the functional group of the isomer dictates the strength of interaction with water ice, and hence the resulting desorption and trapping behaviour. Furthermore, the strength of this interaction directly affects the crystallization of water, which in turn affects the desorption behaviour. Our detailed coverage and composition dependent data allow us to categorize the desorption behaviour of the three isomers on the basis of the strength of intermolecular and intramolecular interactions, as well as the natural sublimation temperature of the molecule. This categorization is extended to other C, H and O containing molecules in order to predict and describe the desorption behaviour of COMs from interstellar ices

Thermally induced mixing of water dominated interstellar ices

Despite considerable attention in the literature being given to the desorption behaviour of smaller volatiles, the thermal properties of complex organics, such as ethanol (C2H5OH), which are predicted to be formed within interstellar ices, have yet to be characterized. With this in mind, reflection absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD) have been used to probe the adsorption and desorption of C2H5OH deposited on top of water (H2O) films of various thicknesses grown on highly oriented pyrolytic graphite (HOPG) at 98 K. Unlike many other molecules detected within interstellar ices, C2H5OH has a comparable sublimation temperature to H2O and therefore gives rise to a complicated desorption pro. le. RAIRS and TPD show that C2H5OH is incorporated into the underlying ASW film during heating, due to a morphology change in both the C2H5OH and H2O ices. Desorption peaks assigned to C2H5OH co-desorption with amorphous, crystalline (CI) and hexagonal H2O-ice phases, in addition to C2H5OH multilayer desorption are observed in the TPD. When C2H5OH is deposited beneath ASW films, or is co-deposited as a mixture with H2O, complete co-desorption is observed, providing further evidence of thermally induced mixing between the ices. C2H5OH is also shown to modify the desorption of H2O at the ASW-CI phase transition. This behaviour has not been previously reported for more commonly studied volatiles found within astrophysical ices. These results are consistent with astronomical observations, which suggest that gas-phase C2H5OH is localized in hotter regions of the ISM, such as hot cores

The effects of methanol on the trapping of volatile ice components

The evaporation of icy mantles, which have been formed on the surface of dust grains, is acknowledged to give rise to the rich chemistry that has been observed in the vicinity of hot cores and corinos. It has long been established that water ice is the dominant species within many astrophysical ices. However, other molecules found within astrophysical ices, particularly methanol, can influence the desorption of volatile species from the ice. Here we present a detailed investigation of the adsorption and desorption of methanol-containing ices, showing the effect that methanol has on the trapping and release of volatiles from model interstellar ices. OCS and CO2 have been used as probe molecules since they have been suggested to reside in water-rich and methanol-rich environments. Experiments show that methanol fundamentally changes the desorption characteristics of both OCS and CO2, leading to the observation of mainly codesorption of both species with bulk water ice for the tertiary ices and causing a lowering of the temperature of the volcano component of the desorption. In contrast, binary ices are dominated by standard volcano desorption. This observation clearly shows that codepositing astrophysically relevant impurities with water ice, such as methanol, can alter the desorption dynamics of volatiles that become trapped in the pores of the amorphous water ice during the sublimation process. Incorporating experimental data into a simple model to simulate these processes on astrophysical timescales shows that the additional methanol component releases larger amounts of OCS from the ice mantle at lower temperatures and earlier times. These results are of interest to astronomers as they can be used to model the star formation process, hence giving information about the evolution of our Universe

Glycolaldehyde formation via the dimerization of the formyl radical

Glycolaldehyde, the simplest monosaccharide sugar, has recently been detected in low- and high-mass star-forming cores. Following our previous investigation into glycolaldehyde formation, we now consider a further mechanism for the formation of glycolaldehyde that involves the dimerization of the formyl radical, HCO. Quantum mechanical investigation of the HCO dimerization process upon an ice surface is predicted to be barrierless and therefore fast. In an astrophysical context, we show that this mechanism can be very efficient in star-forming cores. It is limited by the availability of the formyl radical, but models suggest that only very small amounts of CO are required to be converted to HCO to meet the observational constraints

On the formation of glycolaldehyde in dense molecular cores

Glycolaldehyde is a simple monosaccharide sugar linked to prebiotic chemistry. Recently, it was detected in a molecular core in the star-forming region G31.41+0.31 at a reasonably high abundance. We investigate the formation of glycolaldehyde at 10 K to determine whether it can form efficiently under typical dense core conditions. Using an astrochemical model, we test five different reaction mechanisms that have been proposed in the astrophysical literature, finding that a gas-phase formation route is unlikely. Of the grain-surface formation routes, only two are efficient enough at very low temperatures to produce sufficient glycolaldehyde to match the observational estimates, with the mechanism culminating in CH3OH + HCO being favored. However, when we consider the feasibility of these mechanisms from a reaction chemistry perspective, the second grain-surface route looks more promising, H3CO + HCO

Ice in space: surface science investigations of the thermal desorption of model interstellar ices on dust grain analogue surfaces

More than 140 different molecules have been identified in the interstellar medium (ISM) to date. Dust grain particles are also found in the ISM, and some of these molecules freeze out at the cold temperatures (10-20 K) to form molecular ices. Understanding the adsorption and desorption of these ices is crucially important in understanding the processes that lead to star and planet formation, and may even help to lead to an understanding of the origin of life itself. High sensitivity surface science techniques, including temperature programmed desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS), are being increasingly used to investigate the interactions between dust grains and interstellar ices. This perspective provides an overview of the current level of understanding of the adsorption and desorption of astrophysically relevant molecules from a range of dust grain analogue surfaces. Whilst the focus of this review is on interstellar ices, the results discussed are equally valid to discussions of cometary and planetary ices

Photodesorption and photochemical dynamics on roughened silver: Sulphur dioxide and carbonyl sulphide

The photodesorption of SO2, both monolayer and multilayer, from roughened silver has been investigated. The photodesorption cross-section is enhanced by a factor of 2.6 +/- 0.4 compared to the Ag(111) surface. The peak in the enhancement occurs at 340 nm. Time-of-flight mass spectrometry shows that the same mechanism operates on both the rough and Ag(111) surfaces. Coverage dependent experiments show that the enhancement is a first layer phenomenon, and coverage of greater than one monolayer effectively suppresses photodesorption. These data point to a surface plasmon resonance enhanced substrate mediated (hot electron attachment) photodesorption mechanism. (c) 2005 Elsevier B.V. All rights reserved

Surface science investigations of the role of CO2 in astrophysical ices

We have recorded reflection–absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD) data for a range of CO2-bearing model astrophysical ices adsorbed on a graphitic dust grain analogue surface. Data have been recorded for pure CO2, for CO2 adsorbed on top of amorphous solid water, for mixed CO2:H2O ices and for CO2 adsorbed on top of a mixed CH3OH:H2O ice. For the TPD data, kinetic parameters for desorption have been determined, and the trapping behaviour of the CO2 in the H2O (CH3OH) ice has been determined. Data of these types are important as they can be used to model desorption in a range of astrophysical environments. RAIR spectra have also shown the interaction of the CO2 with H2O and CH3OH and can be used to compare with astronomical observations, allowing the accurate assignment of spectra