We present infrared absorption coefficients, cross sections, and band strengths calculated from reflection-absorption infrared spectroscopy (RAIRS) data of molecular ices deposited on a highly oriented pyrolytic graphite (HOPG) surface at 28 K. We also describe an estimated conversion factor (encompassing considerations from the metal-surface selection rule and metal-surface enhancement factor for graphite) to explain how these parameters compare to those calculated using transmission infrared methods. Further, we discuss the limitations of this method and the assumptions which must be considered when evaluating absorption parameters from RAIRS data. The two species studied here (methyl acetate and methyl propanoate) were chosen to test these methods, as they are both astrochemically relevant and have previously been reported in the literature, providing a reliable benchmark.

Carbon monoxide (CO) is a key component of the icy mantles that form on the surfaces of dust grains in the interstellar medium. In dense molecular clouds, where grain temperatures are around 10 K, CO freezes out as a nonpolar layer on top of H2O ice. This CO plays an important role in the formation of complex organic molecules (COMs) through reactions with hydrogen atoms. Interstellar grains are also exposed to photons and charged particles that can both drive chemical reactions and promote desorption of molecules, providing an important link between the solid state reservoir of molecules and the gas phase. While several studies have considered UV photon driven desorption mechanisms, the UV component of the interstellar radiation field is strongly attenuated within dense clouds, with the internal cloud field being dominated by IR photons. We have used the FELIX IR Free Electron Laser (FEL) FEL-2 to irradiate a few monolayer film of CO deposited on the top of amorphous solid water (ASW) and compared the CO desorption yields to those obtained for a pure CO film. Infrared spectroscopy, combined with mass spectrometric detection of desorbing CO molecules, reveals that excitation of vibrational modes in the underlying ASW leads to significant CO desorption. This is in contrast to direct excitation of the stretching mode of CO which results in only inefficient desorption. The desorption efficiencies we derive indicate that energy transfer within ices on interstellar grains might provide an important route to IR photon-induced desorption of volatile species, such as CO.

Ascertaining the morphology and composition of the icy mantles covering dust grains in dense, cold regions of the interstellar medium is essential to developing accurate astrochemical models, determining conditions for ice formation, constraining chemical interactions in and on icy grains and understanding how ices withstand space radiation. The widely observed infrared spectroscopic signature of H2O ice at similar to 3 mu m discriminates crystalline from amorphous structures in interstellar ices. Weaker bands seen only in laboratory ice spectra at similar to 2.7 mu m, termed 'dangling OH' (dOH), are attributed to water molecules not fully bound to neighbouring water molecules and are often considered as tracing the degree of ice compaction. We exploit the high sensitivity of JWST NIRCam to detect two dOH features at 2.703 and 2.753 mu m along multiple lines of sight probing the dense cloud Chamaeleon I, attributing these signatures to unbound dOH in cold water ice and dOH in interaction with other molecular species. These detections open a path to using the dOH features as tracers of the formation, composition, morphology and evolution of icy grains during the star and planet formation process.

Complex organic molecules (COMs) have been detected in a wide range of astrophysical environments, including the interstellar medium, comets and proto-planetary disks. The icy mantles that form on dust grains in these environments are thought to be the chemical nurseries that allow the formation of many of the COMs that have been identified. As such, the adsorption, thermal processing and desorption of COMs from dust grain surfaces are important in understanding the astrochemical networks as a whole. To study these processes, surface science techniques (temperature programmed desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS)) have been used to investigate ices of the simple esters, methyl acetate and methyl propanoate, adsorbed on a graphitic dust grain analogue surface (highly oriented pyrolytic graphite, HOPG) at 28 K. From the TPD experiments, kinetic parameters have been determined for the desorption of the esters from graphite. The data show a clear coverage dependence for the desorption energies and pre-exponential factors in the sub-monolayer regime. For methyl acetate, the desorption energies and pre-exponential factors range from 57.1 +/- 0.4 to 47.2 +/- 0.3 kJ mol-1 and 3.1 x 1019 +/- 0.2 to 1.6 x 1019 +/- 0.1 s-1 respectively. For methyl propanoate the same parameters range from 57.0 +/- 0.1 to 51.0 +/- 0.1 kJ mol-1 and 7.7 x 1019 +/- 0.1 to 4.4 x 1019 +/- 0.1 s-1. As expected, neither ester shows coverage dependent values for multilayer ices. The determined desorption energies and pre-exponential values for the multilayer ices are 43.5 +/- 0.9 kJ mol-1 and 4.2 x 1032 +/- 0.4 molecules cm-2 s-1 for methyl acetate and 45.7 +/- 0.9 kJ mol- 1 and 8.7 x 1029 +/- 0.4 molecules cm-2 s-1 for methyl propanoate. Experimental RAIRS data were also recorded, showing that the ices undergo an irreversible phase change from an amorphous to a crystalline structure when thermally processed. This study provides fundamental data for use in astrochemical models as well as the basis for a future investigation of methyl acetate and methyl propanoate adsorbed in mixed ice environments with water ice.

Icy grain mantles are the main reservoir of the volatile elements that link chemical processes in dark, interstellar clouds with the formation of planets and the composition of their atmospheres. The initial ice composition is set in the cold, dense parts of molecular clouds, before the onset of star formation. With the exquisite sensitivity of the James Webb Space Telescope, this critical stage of ice evolution is now accessible for detailed study. Here we show initial results of the Early Release Science programme Ice Age that reveal the rich composition of these dense cloud ices. Weak ice features, including 13CO2, OCN−, 13CO, OCS and complex organic molecule functional groups, are now detected along two pre-stellar lines of sight. The 12CO2 ice profile indicates modest growth of the icy grains. Column densities of the major and minor ice species indicate that ices contribute between 2% and 19% of the bulk budgets of the key C, O, N and S elements. Our results suggest that the formation of simple and complex molecules could begin early in a water-ice-rich environment. Using JWST, the molecules seen in planetary atmospheres can be traced back to their cold origins in ices formed in dense interstellar clouds, before the onset of star formation, revealing that chemical diversity and complexity is achieved early.

Using a jet spectroscopy molecular beam setup, gas phase electronic spectra of three xylene isomers (para, meta and ortho) have been collected for the neutral monomer species as well as for their clusters with one and two water molecules. Scans at a resolution of +/- 0.02 nm showed a clear 0-0 transition for each xylene isomer as well as the vibronic progression. The spectra were assigned with the help of Franck-Condon factor PGOPHER simulations from theoretical studies at the CAM-B3LYP/aug-cc-pVDZ level of theory. The vibronic spectra of the xylene.H2O and xylene.(H2O)2 clusters showed broad features between 36800-38400 cm-1 (260-272 nm) for p- and m-xylene, while the water clusters of o-xylene gave more defined bands. The separation of the vibronic bands in the clusters mirrors the progression of the neutral monomers implying that, for the S1 <- S0 transition, it is the same vibrational modes that are involved in the monomer as in the clusters with water. Both the separation and the spectral width of the bands can be explained by the calculated differences in geometries of the clusters in the ground and first electronic excited states.

Ugo Jacovella addressed all: What are the rigorous definitions and thus differences between ices, grains and dusts? Murthy S. Gudipati answered: These definitions are a bit slippery. As per my understanding, here are the definitions and differences: (a) Ice: any gas or liquid consisting of atoms and/or molecules

The infrared excitation and photodesorption of carbon monoxide (CO) and water-containing ices have been investigated using the FEL-2 free-electron laser light source at the FELIX laboratory, Radboud University, The Netherlands. CO-water mixed ices grown on a gold-coated copper substrate at 18 K were investigated. No CO photodesorption was observed, within our detection limits, following irradiation with light resonant with the C-O vibration (4.67 μm). CO photodesorption was seen as a result of irradiation with infrared light resonant with water vibrational modes at 2.9 μm and 12 μm. Changes to the structure of the water ice, which modifies the environment of the CO in the mixed ice, were also seen subsequent to irradiation at these wavelengths. No water desorption was observed at any wavelength of irradiation. Photodesorption at both wavelengths is due to a single-photon process. Photodesorption arises due to a combination of fast and slow processes of indirect resonant photodesorption (fast), and photon-induced desorption resulting from energy accumulation in the librational heat bath of the solid water (slow) and metal-substrate-mediated laser-induced thermal desorption (slow). Estimated cross-sections for the slow processes at 2.9 μm and 12 μm were found to be ∼7.5 × 10-18 cm2 and ∼4.5 × 10-19 cm2, respectively.

The evolution of star-forming regions and their thermal balance are strongly influenced by their chemical composition, that, in turn, is determined by the physico-chemical processes that govern the transition between the gas phase and the solid state, specifically icy dust grains (e.g., particles adsorption and desorption). Gas-grain and grain-gas transitions as well as formation and sublimation of interstellar ices are thus essential elements of understanding astrophysical observations of cold environments (e.g., pre-stellar cores) where unexpected amounts of a large variety of chemical species have been observed in the gas phase. Adsorbed atoms and molecules also undergo chemical reactions which are not efficient in the gas phase. Therefore, the parameterization of the physical properties of atoms and molecules interacting with dust grain particles is clearly a key aspect to interpret astronomical observations and to build realistic and predictive astrochemical models. In this consensus evaluation, we focus on parameters controlling the thermal desorption of ices and how these determine pathways towards molecular complexity and define the location of snowlines, which ultimately influence the planet formation process. We review different crucial aspects of desorption parameters both from a theoretical and experimental point of view. We critically assess the desorption parameters commonly used in the astrochemical community for astrophysical relevant species and provide tables with recommended values. In addition, we show that a non-trivial determination of the pre-exponential factor nu using the Transition State Theory can affect the binding energy value. Finally, we conclude this work by discussing the limitations of theoretical and experimental approaches currently used to determine the desorption properties with suggestions for future improvements.

Monocyclic aromatic hydrocarbons such as benzene, toluene and xylene are thought to play an important role as precursors to the formation of polycyclic aromatic hydrocarbons (PAHs) and their methylated counterparts in a range of astrophysical environments. Benzene has been detected in two carbon rich objects and models have predicted that it could also be present in the interstellar medium (ISM). It has hence been speculated that small aromatic molecules are present in molecular clouds in the ISM, although they have not been detected to date. If they are present in the ISM, they are likely to exist in water-ice dominated icy mantles on the surface of dust grains. We present a laboratory study of benzene, toluene and two xylene isomers (ortho- and para-xylene) in the presence of water ice on a carbonaceous model dust grain surface (highly oriented pyrolytic graphite, HOPG). Temperature programmed desorption (TPD) shows how the desorption of the molecules is affected by the presence of water ice. The importance of these data for astrophysical situations is demonstrated by the use of TPD-derived kinetic parameters to generate a simple model of desorption in dense molecular clouds on an astrophysical timescale. Since benzene, toluene and xylene have not been detected in water-dominated icy mantles to date, desorption has been simulated in a range of different water-containing environments to show the different behaviour expected depending on ice composition. The simulations demonstrate how future observations of aromatic molecules in dense molecular clouds at known temperatures could reveal which environments the molecules are in. Data from these experiments are also used to predict the behaviour of other, larger, aromatic molecules such as PAHs. Reflection absorption infrared spectroscopy (RAIRS) is also used to record the infrared spectra of the small molecules in different water ice configurations. These spectra can be used to aid identification of these icy aromatics in future observations, such as those that will be possible with the James Webb Space Telescope (JWST). In all cases, spectra of mixed ices consisting of the aromatic molecule and amorphous water ice show evidence of interactions between the water ice and the aromatic species.

Surface science methodologies, such as reflection-absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD), are ideally suited to studying the interaction of molecules with model astrophysical surfaces. Here we describe the use of RAIRS and TPD to investigate the adsorption, interactions and thermal processing of acetonitrile and water containing model ices grown under astrophysical conditions on a graphitic dust grain analogue surface. Experiments show that acetonitrile physisorbs on the graphitic surface at all exposures. At the lowest coverages, repulsions between the molecules lead to a decreasing desorption energy with increasing coverage. Analysis of TPD data gives monolayer desorption energies ranging from 28.8–39.2 kJ mol−1 and an average multilayer desorption energy of 43.8 kJ mol−1. When acetonitrile is adsorbed in the presence of water ice, the desorption energy of monolayer acetonitrile shows evidence of desorption with a wide range of energies. An estimate of the desorption energy of acetonitrile from crystalline ice (CI) shows that it is increased to ~37 kJ mol−1 at the lowest exposures of acetonitrile. Amorphous water ice also traps acetonitrile on the graphite surface past its natural desorption temperature, leading to volcano and co-desorption. RAIRS data show that the C≡N vibration shifts, indicative of an interaction between the acetonitrile and the water ice surface.

A reflection-absorption optical (RAO) spectrometer, operating across the ultra-violet/visible (UV/visible) wavelength region, has been developed that allows simultaneous measurements of optical properties and thickness of thin solid films at cryogenic temperatures in ultrahigh vacuum. The RAO spectrometer enables such measurements to be made after ice deposition, as opposed to most current approaches which make measurements during deposition. This allows changes in the optical properties and in the thickness of the film to be determined subsequent to thermal, photon or charged particle processing. This is not possible with current techniques. A data analysis method is presented that allows the wavelength dependent n and k values for ices to be extracted from the reflection-absorption spectra. The validity of this analysis method is shown using model data from the literature. New data are presented for the reflection UV/visible spectra of amorphous and crystalline single component ices of benzene, methyl formate and water adsorbed on a graphite surface. These data show that, for benzene and methyl formate, the crystalline ice has a larger refractive index than amorphous ice, reflecting changes in the electronic environment occurring in the ice during crystallisation. For water, the refractive index does not vary with ice phase.

Temperature-programmed desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS) have been used to investigate the interactions between ethyl formate and water and ethyl formate and ethanol in model astrophysical ices adsorbed on a graphitic model grain surface. Experiments show that the ethyl formate forms hydrogen bonds to both water and ethanol via the oxygen lone pairs. This leads to the observation of shifts in the vibrational wavenumber of the C=O and C-O-C modes of ethyl formate, which can potentially be used to identify the environment of this complex organic molecule in astronomical observations. TPD data show that the interaction of ethyl formate with water is stronger than that with ethanol with an additional species being observed in the TPD spectrum corresponding to the desorption of ethyl formate directly bonded to the water ice surface. The desorption energy of ethyl formate adsorbed on water ice was found to be 48.5 kJ mol(-1), compared to 43.2 kJ mol(-1) for pure ethyl formate monolayers. Ethyl formate also traps in water ice and undergoes volcano desorption at the water amorphous to crystalline phase transition temperature. In contrast to the water, ethanol has very little effect on the desorption of ethyl formate with the two species behaving independently even in a codeposited ice.

We present a novel UV/visible reflection-absorption spectrometer for determining the refractive index, n, and thicknesses, d, of ice films. Knowledge of the refractive index of these films is of particular relevance to the astrochemical community, where they can be used to model radiative transfer and spectra of various regions of space. In order to make these models more accurate, values of n need to be recorded under astronomically relevant conditions, that is, under ultra-high vacuum (UHV) and cryogenic cooling. Several design considerations were taken into account to allow UHV compatibility combined with ease of use. The key design feature is a stainless steel rhombus coupled to an external linear drive (z-shift) allowing a variable reflection geometry to be achieved, which is necessary for our analysis. Test data for amorphous benzene ice are presented as a proof of concept, the film thickness, d, was found to vary linearly with surface exposure, and a value for n of 1.43 ± 0.07 was determined.

The first comparative study of the surface behavior of four small aromatic molecules, benzene, toluene, p-xylene, and o-xylene, adsorbed on graphite at temperatures ≤30 K, is presented. Intermolecular interactions are shown to be important in determining the growth of the molecules on the graphite surface at low (monolayer) exposures. Repulsive intermolecular interactions dominate the behavior of benzene and toluene. By contrast, stronger interactions with the graphite surface are observed for the xylene isomers, with islanding observed for o-xylene. Multilayer desorption temperatures and energies increase with the size of the molecule, ranging from 45.5 to 59.5 kJ mol−1 for benzene and p-xylene, respectively. Reflection absorption infrared spectroscopy gives insight into the effects of thermal processing on the ordering of the molecules. Multilayer benzene, p-xylene, and o-xylene form crystalline structures following annealing of the ice. However, we do not observe an ordered structure for toluene in this study. The ordering of p-xylene shows a complex relationship dependent on both the annealing temperature and exposure.

Strong interactions between 2-propanol and water ice cause marked changes in the crystallisation kinetics and desorption of water.

Here we describe novel RAIRS, TPD and LID studies of CO, NO and coadsorbed CO and NO on Pd.