Thanja Lamberts

Experimental evidence for glycolaldehyde and ethylene glycol formation by surface hydrogenation of co molecules under dense molecular cloud conditions

This study focuses on the formation of two molecules of astrobiological importance – glycolaldehyde(HC(O)CH2OH)andethyleneglycol(H2C(OH)CH2OH)–bysurfacehydrogenation of CO molecules. Our experiments aim at simulating the CO freeze-out stage in interstellar darkcloudregions,wellbeforethermalandenergeticprocessingbecomedominant.Itisshown that along with the formation of H2CO and CH3OH – two well-established products of CO hydrogenation – also molecules with more than one carbon atom form. The key step in this process is believed to be the recombination of two HCO radicals followed by the formation of a C–C bond. The experimentally established reaction pathways are implemented into a continuous-time random-walk Monte Carlo model, previously used to model the formation of CH3OH on astrochemical time-scales, to study their impact on the solid-state abundances in dense interstellar clouds of glycolaldehyde and ethylene glycol.

NO ICE HYDROGENATION: A SOLID PATHWAY TO NH2OH FORMATION IN SPACE

Icy dust grains in space act as catalytic surfaces onto which complex molecules form. These molecules are synthesized through exothermic reactions from precursor radicals and, mostly, hydrogen atom additions. Among the resulting products are species of biological relevance, such as hydroxylamine—NH2OH—a precursor molecule in the formation of amino acids. In this Letter, laboratory experiments are described that demonstrate NH2OH formation in interstellar ice analogs for astronomically relevant temperatures via successive hydrogenation reactions of solid nitric oxide (NO). Inclusion of the experimental results in an astrochemical gas–grain model proves the importance of a solid-state NO + H reaction channel as a starting point for prebiotic species in dark interstellar clouds and adds a new perspective to the way molecules of biological importance may form in space.

The kinetic monte carlo method as a way to solve the master equation for interstellar grain chemistry

The interstellar medium (ISM) is far from empty; rather it contains large molecular clouds consisting of dust and gas. It is in these clouds that new stars form—possibly with habitable planets—with molecules playing a crucial role.1,2 At the moment, over 170 different molecules have been detected: stable molecules, radicals, cations, and anions. Many molecules only consist of a few atoms and are familiar to us from a terrestrial point of view, such as water, molecular hydrogen, methanol, formaldehyde, formic acid, and dimethyl ether. Other molecules are more exotic and consist, for instance, of very unsaturated carbon chains. Gas phase chemical models show that these latter molecules can be quite easily formed through gas phase reactions.3 The saturated molecules are however only formed in small quantities through gas phase chemistry, since they require either high temperatures to be formed or three-body reactions, which are both not available in cold molecular clouds. Dust grains can act as a third body, facilitating addition reactions that lead to the formation of saturated molecules. Already in 1949 de Hulst4 realized the importance of grain surface chemistry and he suggested surface formation routes for H2 from two hydrogen atoms and the formation of H2O from O2 with hydrogen peroxide as intermediate. From various sources of information, we know that approximately 1% of the mass of the material in the ISM, and 10–10% in terms of numbers, consists of small, nanosized silicate and carbonaceous particles called dust grains. Although this seems a small portion of the total amount of matter, dust grains play an important role in the ISM, not only by acting as catalytic sites for molecule formation but also by shielding molecules from dissociating UV radiation. Depending on the physical conditions, the dust grains can be coated with a layer of “dirty” ice consisting of a mixture of different species. Their main components are water, carbon monoxide, and carbon dioxide, but also more complex molecules such as methane, methanol, formaldehyde, and ammonia may be present. Some of the ice species accrete from the gas phase, but as mentioned before, many of the simple important molecules such as H2 and H2O and also several complex organic molecules are not formed in the gas phase, but rather on the grain surfaces themselves. As a result, many modelers have begun to include grain surface processes in their models to describe chemical evolution during, for instance, star formation.1,5,6 Originally, the development of grain surface models was mostly driven from the astrochemical modeling community7−9 without much input from specialists in ice chemistry. Gas phase models were extended with a surface phase and the grain surface chemistry was treated in a way similar to that of gas phase chemistry. This was found to lead to problems in the regime where the surface abundance is low—the so-called “accretion limit”—where formation rates could be orders of magnitude off. Since then, different modeling methods have been applied to tackle this and other modeling problems. From the physicochemical point of view, refinements have been made as well. Many of the initial expressions and input parameters originated from surface science or were extensions from gas phase experiments. In surface science, mostly metal or semiconductor surfaces are studied on which absorbates are deposited within the monolayer regime, around room temperature. This is quite different from the rough grain surface on which inhomogeneous ice layers of tens of monolayers are formed at very low temperatures. Fortunately, technological developments have now enabled the experimental verification of grain surface chemistry. Many of the assumptions within the models are now put to the test by applying to experimental surface science techniques to real interstellar analogues. With this new experimental information comes the need for detailed surface models to understand the physicochemical processes behind the experimental results and to translate the experimental findings to astrochemical time scales. Different modeling techniques have been applied to grain surface astrochemistry, covering a large range of time and length scales. More molecular detail comes at the expense of more CPU time to cover the same evolution in real time. How different techniques relate to one other in this respect is indicated in Figure ​Figure1.1. Molecular dynamics simulations trace the exact location and orientation of the molecules including lattice vibrations and in some cases even the intramolecular movement, but they typically stay within the picosecond to nanosecond time frame. Rate equations on the other hand can easily handle 108 years—much longer than the lifetime of a molecular cloud—but adsorbed molecules are only treated in terms of numbers and their exact locations are not known. The present review aims to discuss the application of the kinetic Monte Carlo technique (KMC) to grain surface chemistry. This method was initially introduced into astrochemistry as a solution to the “accretion” limit, but it is now more and more applied to gain insight into the physicochemical surface processes. The method has, in principle, not a real restriction in how molecules are represented: in terms of number densities or with their exact location and orientation. The different implementations of kinetic Monte Carlo can therefore cover a huge time and length scale range. Figure 1 Overview of the different simulation methods mentioned in the present review.

SURFRESIDE2: An ultrahigh vacuum system for the investigation of surface reaction routes of interstellar interest

A new ultrahigh vacuum experiment is described to study atom and radical addition reactions in interstellar ice analogues for astronomically relevant temperatures. The new setup - SURFace REaction SImulation DEvice (SURFRESIDE(2)) - allows a systematic investigation of solid state pathways resulting in the formation of molecules of astrophysical interest. The implementation of a double beam line makes it possible to expose deposited ice molecules to different atoms and/or radicals sequentially or at the same time. Special efforts are made to perform experiments under fully controlled laboratory conditions, including precise atom flux determinations, in order to characterize reaction channels quantitatively. In this way, we can compare and combine different surface reaction channels with the aim to unravel the solid state processes at play in space. Results are constrained in situ by means of a Fourier transform infrared spectrometer and a quadrupole mass spectrometer using reflection absorption infrared spectroscopy and temperature programmed desorption, respectively. The performance of the new setup is demonstrated on the example of carbon dioxide formation by comparing the efficiency through two different solid state channels (CO + OH → CO2 + H and CO + O → CO2) for which different addition products are needed. The potential of SURFRESIDE(2) to study complex molecule formation, including nitrogen containing (prebiotic) compounds, is discussed.

Efficient surface formation route of interstellar hydroxylamine through NO hydrogenation. II. The multilayer regime in interstellar relevant ices

Hydroxylamine (NH(2)OH) is one of the potential precursors of complex pre-biotic species in space. Here, we present a detailed experimental study of hydroxylamine formation through nitric oxide (NO) surface hydrogenation for astronomically relevant conditions. The aim of this work is to investigate hydroxylamine formation efficiencies in polar (water-rich) and non-polar (carbon monoxide-rich) interstellar ice analogues. A complex reaction network involving both final (N(2)O, NH(2)OH) and intermediate (HNO, NH(2)O·, etc.) products is discussed. The main conclusion is that hydroxyl-amine formation takes place via a fast and barrierless mechanism and it is found to be even more abundantly formed in a water-rich environment at lower temperatures. In parallel, we experimentally verify the non-formation of hydroxylamine upon UV photolysis of NO ice at cryogenic temperatures as well as the non-detection of NC- and NCO-bond bearing species after UV processing of NO in carbon monoxide-rich ices. Our results are implemented into an astrochemical reaction model, which shows that NH(2)OH is abundant in the solid phase under dark molecular cloud conditions. Once NH(2)OH desorbs from the ice grains, it becomes available to form more complex species (e.g., glycine and β-alanine) in gas phase reaction schemes.

Low-temperature surface formation of NH3 and HNCO: hydrogenation of nitrogen atoms in CO-rich interstellar ice analogues

Solid-state astrochemical reaction pathways have the potential to link the formation of small nitrogen-bearing species, like NH_3 and HNCO, and prebiotic molecules, specifically amino acids. To date, the chemical origin of such small nitrogen-containing species is still not well understood, despite the fact that ammonia is an abundant constituent of interstellar ices towards young stellar objects and quiescent molecular clouds. This is mainly because of the lack of dedicated laboratory studies. The aim of this work is to experimentally investigate the formation routes of NH_3 and HNCO through non-energetic surface reactions in interstellar ice analogues under fully controlled laboratory conditions and at astrochemically relevant temperatures. This study focuses on the formation of NH_3 and HNCO in CO-rich (non-polar) interstellar ices that simulate the CO freeze-out stage in dark interstellar cloud regions, well before thermal and energetic processing start to become relevant. We demonstrate and discuss the surface formation of solid HNCO through the interaction of CO molecules with NH radicals – one of the intermediates in the formation of solid NH_3 upon sequential hydrogenation of N atoms. The importance of HNCO for astrobiology is discussed.

The formation of ice mantles on interstellar grains revisited--the effect of exothermicity.

Modelling of grain surface chemistry generally deals with the simulation of rare events. Usually deterministic methods or statistical approaches such as the kinetic Monte Carlo technique are applied for these simulations. All assume that the surface processes are memoryless, the Markov chain assumption, and usually also that their rates are time independent. In this paper we investigate surface reactions for which these assumptions are not valid, and discuss what the effect is on the formation of water on interstellar grains. We will particularly focus on the formation of two OH radicals by the reaction H + HO2. Two reaction products are formed in this exothermic reaction and the resulting momentum gained causes them to move away from each other. What makes this reaction special is that the two products can undergo a follow-up reaction to form H2O2. Experimentally, OH has been observed, which means that the follow-up reaction does not proceed with 100% efficiency, even though the two OH radicals are formed in each others vicinity in the same reaction. This can be explained by a combined effect of the directionality of the OH radical movement together with energy dissipation. Both effects are constrained by comparison with experiments, and the resulting parametrised mechanism is applied to simulations of the formation of water ice under interstellar conditions.

Quantum tunneling during interstellar surface-catalyzed formation of water: the reaction H + H2O2 → H2O + OH

Quantification of surface reaction rate constants of the reaction H + H2O2 → H2O + OH at low temperatures with the use of instanton theory.

Relevance of the H2 + O reaction pathway for the surface formation of interstellar water - Combined experimental and modeling study

The formation of interstellar water has been commonly accepted to occur on the surfaces of icy dust grains in dark molecular clouds at low temperatures (10-20 K), involving hydrogenation reactions of oxygen allotropes. As a result of the large abundances of molecular hydrogen and atomic oxygen in these regions, the reaction H2 + O has been proposed to contribute significantly to the formation of water as well. However, gas phase experiments and calculations, as well as solid-phase experimental work contradict this hypothesis. Here, we use precisely executed temperature programmed desorption (TPD) experiments in an ultra-high vacuum setup combined with kinetic Monte Carlo simulations to establish an upper limit of the water production starting from H2 and O. These reactants are brought together in a matrix of CO2 in a series of (control) experiments at different temperatures and with different isotopological compositions. The amount of water detected with the quadrupole mass spectrometer upon TPD is found to originate mainly from contamination in the chamber itself. However, if water is produced in small quantities on the surface through H2 + O, this can only be explained by a combined classical and tunneled reaction mechanism. An absolutely conservative upper limit for the reaction rate is derived with a microscopic kinetic Monte Carlo model that converts the upper limit into a maximal possible reaction rate. Incorporating this rate into simulations run for astrochemically relevant parameters, shows that the upper limit to the contribution of the reaction H2 + O in OH, and hence water formation, is 11% in dense interstellar clouds. Our combined experimental and theoretical results indicate however, that this contribution is likely to be much lower.

Thermal H/D exchange in polar ice – deuteron scrambling in space

We have investigated the thermally induced proton/deuteron exchange in mixed amorphous H