Surface formation routes of interstellar molecules: hydrogenation reactions in simple ices
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It has been a long standing problem in astrochemistry to explain how molecules can form in a highly dilute environment such as the interstellar medium. In recent years it has become clear that not only ion/radical-molecule gas-phase reactions, but also solid state reactions on icy dust grains play an important role in the formation of new species. In order to investigate the underlying processes, laboratory based experiments are needed to simulate surface reactions induced by photon (UV) processing or particle (atom, cosmic ray, electron) bombardment of interstellar ice analogs. Here, the latest research performed on SURFace REaction SImulation DEvice (SURFRESIDE), one of the ultra-high vacuum setups in the Sackler Laboratory for Astrophysics in Leiden is reviewed. The focus is on hydrogenation, i.e., H-atom addition reactions in interstellar ice analogs for astronomically relevant temperatures. We discuss how molecules form when CO and O2 containing ices are exposed to thermal hydrogen atoms under fully controlled experimental conditions. Surface formation schemes for interstellar relevant species, such as solid methanol, water, and carbon dioxide are investigated and chemical links between molecular species in space are discussed.
KeywordsAstrochemistry Infrared: ISM ISM: atoms ISM: molecules Methods: laboratory
Our Galaxy is largely empty. By terrestrial standards the space between stars can be considered as a near-perfect vacuum: the average particle density in the solar neighborhood is roughly a factor of 1019 less than in the terrestrial atmosphere at sea level. Nevertheless, the highly diluted material present between the stars, the interstellar medium (ISM), plays a central role in the chemical evolution of our Galaxy. The ISM is the repository of ashes from previous generations of stars and it is itself the birthplace of new stars and planetary systems.
The interstellar matter consists of about 99% gas, mainly hydrogen, helium and some heavier elements (e.g., C, O, N, S), and 1% (sub)micron size silicate and carbonaceous dust grains by mass. The identification of rotational, vibrational, and electronic spectra has established the presence of a large variety of polyatomic molecules, ions and radicals in the ISM, both in the gas phase and in the solid state. In fact, over 150 different molecular species (excluding isotopomers) have been assigned. The spectra are probes of the physical conditions and chemical history of the regions in space where molecular species reside. These species include a variety of inorganic compounds (e.g., H2O, CO, CO2, NH3 and SO2), organics (e.g., CH4, H2CO, CH3OH, HCOOH, and CH3CH2OH), ions (e.g., HCO+ and C6H−) and species identified only in ice (e.g., OCN− and NH4+), as well as unsaturated hydrocarbon chains (e.g., HCnN with n as large as n = 11) (Tielens 2005). Recently, also the fullerenes C60 and C70 were unambiguously detected (Cami et al. 2010; Sellgren et al. 2010). Aromatic species such as polycyclic aromatic hydrocarbons (PAHs) are likely present in space, but not included in the count, since they have not been uniquely identified yet.
Interstellar ice feature inventory with respect to H2O ice towards dark clouds, low- and high-mass YSOs
Dark cloud (Elias 16)
L-m YSO (HH 46)
H-m YSO (W33A)
In quiescent dark clouds, interstellar grains provide a surface on which species can accrete, meet and react and to which they can donate excess energy. Grain-surface chemistry is governed by the accretion rate of gas-phase species onto the grains, the surface migration rate, which sets the reaction network, and the desorption rate. The timescale at which gas-phase species deplete-out onto grains is ∼105 years in dense cores. This time is shorter than the lifetime of dense cores, which is between 105 and 106 years. Hence, in dense regions, during the first stage of star formation virtually all species (except H2) are frozen-out onto interstellar grains. In this way, icy dust grains act both as a molecular reservoir and as a catalytic site. Subsequently, a complex grain-surface chemistry is triggered by photon/cosmic ray irradiation, thermal processing and particle bombardment (Herbst and van Dishoeck 2009).
These and other astrochemical reaction networks were based on chemical intuition and analogs from gas-phase routes. It took several decades before experimental techniques allowed laboratory astrochemists to put all these reactions to the test. The laboratory studies presented in this review have a common bottom–up approach which aims to investigate at low temperatures selected and astrochemically relevant surface reactions, starting from the hydrogenation/ deuteration of pure and binary ices. The ultimate goal of this approach is to derive fundamental and molecule specific parameters, like reaction rates and diffusion barriers. In this experimental process, several of the previously proposed reactions were proven to be efficient, whereas others were not. Also several new reaction routes were revealed. The resulting experimentally measured reaction rates and diffusion barriers can then be included in astrochemical models which simulate ice evolution under astronomical relevant timescales (105 years) and, therefore, push experimental results beyond typical laboratory timescales.
Ices are monitored by means of reflection-absorption infrared spectroscopy (RAIRS) using a Fourier transform infrared (FTIR) spectrometer, which covers the range between 4,000 and 700 cm−1 (2.5−14 μm). A spectral resolution between 1 and 4 cm−1 is used and several scans are co-added. In Fuchs et al. (2009), Ioppolo et al. (2008, 2010, 2011a) and Romanzin et al. (2011), the ice is first deposited and then hydrogenated/deuterated. In this case, RAIR difference spectra with respect to the initial deposited ice are acquired during H/D exposure. In Cuppen et al. (2010) the procedure is different and molecules are co-deposited with H atoms. RAIR difference spectra are acquired with respect to the bare substrate during co-deposition. In all cases, newly formed solid species are monitored by RAIRS using unique IR spectral signatures. Spectra are recorded at different stages during hydrogenation, providing time resolved information about the destruction (i.e., use-up) of the precursor ice (the deposited ice layer) and the formation of new molecules that are identified through their spectral fingerprints. The intensity of a spectrum can be translated into a column density using a modified Lambert-Beer equation (Bennett et al. 2004). At the end of the H-atom addition a temperature programmed desorption (TPD) experiment can be performed to constrain the spectroscopic results. Surface hydrogenation reactions of simple ices, like pure CO, O2, O3, and CO:O2 mixtures are investigated for a full range of different laboratory conditions including H/D-atom fluxes, ice temperatures, ice thicknesses, ice structures, and mixture ratios. This makes it possible to unravel the physics and chemistry of molecule formation and where applicable to examine in more detail the astronomical implications.
3 Bottom–up versus top–down approach
In the past the chemistry of inter- and circumstellar ice analogs has been studied using a top–down scenario: ice mixtures of astronomical constituents with more or less realistic mixing ratios were chemically triggered through UV/cosmic ray irradiation. The resulting residue was shown to consist of more complex organic compounds (e.g., Hagen et al. 1979; Allamandola et al. 1988; Gerakines et al. 1995; Hudson and Moore 2000; Strazzulla and Palumbo 2001; Mennella et al. 2004, 2006; Bennett and Kaiser 2007; Palumbo et al. 2008). The experimental results using this approach have been compared to interstellar ices generally in a more qualitative than quantitative way. More recently, a bottom–up approach has become experimentally possible, through the use of UHV setups in which individual reactions of simpler ices (i.e., not the cumulative outcome of a chemical network in an ice mixture) can be studied in situ and in real time under fully controlled laboratory conditions. This approach makes it possible to derive fundamental and molecule specific parameters, like reaction rates and diffusion barriers, which can then be included in astrochemical models to simulate the ice evolution under much longer timescales (105 years) than accessible in the laboratory (<1 day).
The work presented in the next section follows a bottom–up approach and summarizes a representative sample of relevant experiments (e.g., Watanabe and Kouchi 2002; Watanabe et al. 2004, 2006; Fuchs et al. 2009; Miyauchi et al. 2008; Ioppolo et al. 2008, 2010, 2011a, b; Matar et al. 2008; Oba et al. 2009, 2010; Cuppen et al. 2010; Mokrane et al. 2009; Romanzin et al. 2011; Öberg et al. 2009). These experiments prove that species like H2CO, CH3OH and H2O can be formed at low temperatures by simple hydrogenation (i.e., without the need for thermal, UV or cosmic ray processing) and provide the basic molecular data to simulate their formation on astronomical timescales (e.g., Cuppen et al. 2009), even though the ice as a whole is not representative for a realistic astronomical ice.
4.1 Surface formation of methanol
4.2 Surface formation of water
Tielens and Hagen (1982) proposed that interstellar water forms on grain surfaces through three reaction channels: hydrogenation of atomic, molecular oxygen and ozone. Using a Monte Carlo approach, Cuppen and Herbst (2007) and Cazaux et al. (2010) showed that the contribution of the different formation channels strongly depends on the local environment in interstellar clouds. They concluded that the atomic oxygen channel is the main route in translucent and diffuse clouds, while the molecular oxygen channel, together with the ozone route, is more efficient in dense cold molecular clouds.
The third water formation channel (the hydrogenation of solid O3) was tested by Mokrane et al. (2009) and more recently by Romanzin et al. (2011). Since this channel is connected to the O2 channel after the first reaction step, special care was taken in Romanzin et al. (2011) to deposit a pure O3 ice by keeping the substrate temperature between the O2 and O3 desorption temperature during deposition. If such a temperature is also kept during H-atom addition, the O2 molecules formed upon O3 hydrogenation will desorb from the surface of the ice. In this way the reaction of OH to form water via H or H2 addition can be probed. The hydrogenation of O3 is found to behave more similar to CO hydrogenation in the sense that only the top few monolayers of O3 are hydrogenated. Moreover, the reaction OH + H2 may be more efficient than the reaction OH + H: reaction OH + H2 likely proceeds through tunneling, while reaction OH + H needs to dissipate 5.3 eV of excess energy with just one final product, which could be difficult.
These experimental results complete the reaction scheme on water formation initially proposed (Tielens and Hagen 1982). The general conclusion that the three channels (O/O2/O3 + H) are strongly linked, is of importance for astrochemical models focusing on water formation under interstellar conditions (Wakelam et al. 2010).
4.3 Surface formation of CO2
5 Extending the laboratory data to ISM conditions
The conditions in the experiments discussed in the previous sections do not fully reproduce the conditions in space. Although temperature, pressure, thickness and substrate, can be approached quite accurately, the experimental particle flux can never be close to the flux in the ISM due to timescale issues. Typically, fluences are reached within a few hours in the laboratory that are similar to interstellar fluences after a million years.
For multiple step processes such as surface reactions, which are the result of a sequence of diffusion and reaction events, the product yield does not necessarily scale with the fluence. The separate steps need to be disentangled and characterized by rates. The ultimate goal of the bottom–up experiments applied here is, therefore, to obtain physico-chemical parameters such as reaction rates that can be included in astrochemical models to simulate the chemical evolution of different astrophysical objects under a range of different physical conditions.
One way of doing this is by first simulating the experimental conditions using these input parameters as fitting parameters. The same simulation routine can then be applied to simulate the chemical evolution under interstellar conditions. Using continuous-time, random-walk (CTRW-) Monte Carlo simulations this has be done for methanol formation. First, the experimental conditions have been simulated using the reaction rates of H + CO and H + H2CO as fitting parameters (Fuchs et al. 2009). The Monte Carlo simulation were found in good agreement with the experimental data especially at low temperatures, where the laboratory results are less affected by experimental limits like the presence of H2 molecules in the H-atom beam or the lower sticking probability of the thermal H atoms (300 K) at higher substrate temperatures. The resulting values were then applied to simulate formaldehyde and methanol formation in cold dense cores (Cuppen et al. 2009).
The advantage of using CTRW-Monte Carlo simulations is that species can be followed on the surface. Layering is, therefore, automatically taken into account. This is important as only the top layers are affected and the atoms do not penetrate deeply into the solid. The simulations of the experiments showed that the production rate of formaldehyde decreases and that the penetration depth into the ice increases with temperature.
Simulations for different interstellar parameters, including density and temperature, have been performed in Cuppen et al. (2009). Formaldehyde and methanol were found to form efficiently in cold dense cores or the cold outer envelopes of young stellar objects. Again layering plays an important role, since the grain mantle is found to have a layered structure with CH3OH on top at the end of the freeze-up time. The species CO and H2CO are found to exist predominantly in the lower layers of ice mantles where they are not available for hydrogenation at late times. This finding is in contrast with previous gas-grain models, which do not take into account the layering of the ice. Observational solid H2CO/ CH3OH and CO/CH3OH abundance ratios in the outer envelopes of an assortment of young stellar objects agree reasonably well with our model results, which also suggests that the large range in CH3OH/H2O observed abundance ratios is due to variation in the evolutionary stages of the selected YSOs.
In this overview the surface formation of H2CO, CH3OH, H2O, and CO2 at low temperatures is experimentally shown using a bottom–up approach, in which individual surface reactions are experimentally investigated starting from the hydrogenation of simple and binary ices (i.e., pure CO, O2, O3 ices and mixed CO:O2 ices). These studies prove that molecules like methanol, water and carbon dioxide can be formed in the solid state in the ISM without the need for energetic processing such as thermal, UV/cosmic ray processing (i.e., through H-atom addition). Here, several of the formation routes proposed in the past by astrochemical models based on gas-phase data were proven to be efficient, whereas others were not. Also several new reaction routes were revealed. The experimental outcome can be used in astrochemical models with the intent to simulate the formation of new species in the solid phase on astronomical timescales (e.g., Cuppen et al. 2009), extending the laboratory results beyond experimental constraints.
The research leading to these results has received funding from NOVA, the Netherlands Research School for Astronomy and the European Community’s Seventh Framework Programme (FP7/2007–2013) under grant agreement no. 238258.
This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
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