Segregation effect and N2 binding energy reduction in CO-N2 system adsorbed on water ice substrates

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Surface chemistry

There are three main chemical processes on grain surfaces. The first one is known as the Eley-Rideal (E-R) mechanism. One reactant (atom/molecule) is adsorbed on the grain surface and then impact and react directly with a atom/molecule coming from the gas phase. E-R mechanism is a non-thermal surface mecha-nism because it leads to a reaction between a thermally adsorbed surface species and a reactant which has not yet been thermally accommodated to the surface. The second mechanism occurs through the surface diffusion and is known as the Langmuir-Hinshelwood (L-H) mechanism. The new molecule is formed via the reaction of two reactive partners are already adsorbed on the surface. The L-H mechanism is a thermal surface mechanism because the two reactants are thermal-ized to the surface. The last mechanism is the so called hot atoms mechanism. A gas phase species is landing on the surface with an excess of kinetic energy which implies an excess of mobility of a finite period on the surface. Thus one of the species has a temporary increased mobility, find and react with partners before reaching the thermal equilibrium. With the E-R and hot-atom mechanisms, reac-tions occur before reaching the thermal equilibrium. Thus, these mechanisms are different in regards to evaluate the activation barriers of reactions. Under typically conditions of dust-grain surface reactions, the L-H mechanism is the dominant re-active process. All three mechanisms can be followed by desorption of the reaction products, back into the gas phase.

Interstellar molecules

Molecular material in general and the complex molecules in particular can be either in the gas phase and in the solid phase (locked in the icy mantles on dust grains). They are mainly associated with molecular clouds and their dense clumps or in star forming regions. The probe of the ISM components and processes that occur in it, are derived from the molecular spectra and the chemical chain of reactions that yield to different molecular species. Molecular spectra provide some information about the physical conditions on gas and dust. In particular, the rotational and vibrational spectra give us information about the density and temperature of the gas as well as the collapse and rotation (Herbst and van Dishoeck, 2009b).
In recent years, more than 200 complex molecules have been detected in interstellar medium and circumstellar shells1 including molecules from 2 atoms to 13 atoms. The diversity of molecular components in the ISM is evidenced by the large number of known molecules.
Interstellar molecules allow us to clarify the evolution of star formation that is occurring in the cold, dense pre-stellar globules and cores (density n = 2 × 104 cm−3 and temperature T ≈ 10 K) and the abundant molecules in both gas phase and ice mantles (Herbst and van Dishoeck, 2009b).

Interstellar ices

Interstellar ices are formed in the cold and dense clouds where densities reach high enough (103 to 105 cm−3). At average molecular cloud densities of 104 cm−3, atoms and molecules accrete on the surface of a sub-micro sized dust grain once per day in average (Tielens and Allamandola, 1987). Low temperatures allow particles to accrete, to move on the surface and to form new molecules. Sheltered from the strong UV field, molecules are able to accumulate on the grain to form a mantle of interstellar ice.
Compositions of interstellar ice are determined by physical conditions in the am-bient gas. Hydrogen (H), oxygen (O), and nitrogen (N) may be in atomic and molecular form, whereas carbon (C) arrives on the dust grain in the form of car-bon monoxide (CO). At low temperature (typically of 10 K), atoms/molecules are able to scan on the grain surface, meet and react with other atoms/molecules. Hy-drogen and oxygen atoms may perform quantum tunneling (Manicò et al., 2001, Minissale et al., 2013, Congiu et al., 2014b, Minissale et al., 2014), the atomic oxygen, nitrogen, and carbon can do thermal hopping (Tielens and Allamandola, 1987). Simultaneously, the ice layer is a result of atoms addition reactions. Thus, the primal interstellar ices are mainly formed of water H2O with significant amount of H2CO, CO, CO2, N2, and NH3 (Tielens and Hagen, 1982).
There have been many surveys of interstellar ice to determine the ice compositions (Knez et al., 2005, Boogert et al., 2008, Pontoppidan et al., 2008, Öberg et al., 2011). Interstellar ices are dominated by water (H2O); however, it is mixed with the significant amounts of CO, CO2, CH3OH, and as well as smaller abundances of NH3, H2CO, CH4, HCOOH, NH+4, OCS and several ionic species (Van Dishoeck, 2004). Table 1.2 shows the molecular abundances are relative to the water ice (Van Broekhuizen, 2005).
Actually, these molecules freeze out onto the grain surface from the gas phase; how-ever, mantle composition does not reflect gas phase composition or abundances. For instance, CO2 has not been observed in the gas phase yet, while it is widely detected as an ice condensed onto dust grains (Boonman et al., 2000). Therefore, new molecules are formed when reactive gaseous species condense on the grain surface or when ices are energetically processed by UV radiation or cosmic rays.
Although layer ices of interstellar ices are diverse, H2O still dominates the compo-sition of interstellar ices (Boogert et al., 2008, Pontoppidan et al., 2004). In the gas phase, H2O has abundance with respect to H2 of 10−8 in the cold dense re-gions, to 10−4 in warm gas and shocked regions (van Dishoeck and Helmich, 1996, Melnick and Bergin, 2005). D’Hendecourt et al. (1985) and Hasegawa et al. (1992) studied that gas phase chemistry cannot reproduce the H2O abundance observed in ISM. However, there were laboratories that investigated and demonstrated the H2O formation; the H2O molecules was produced from the reaction of H-atoms and O-atoms initially trapped in a N2O matrix (Hiraoka et al., 1998); the reactions be-tween H atoms and O2 produced efficiently the H2O2 and H2O molecules (Miyauchi et al., 2008, Ioppolo et al., 2008). Moreover, experiments were preformed through set-up FORMOLISM (FORmation of MOLecules in the ISM, located at LERMA in the University of Cergy Pontoise) allowed to investigate the formation of water molecules by exposing the water ice substrate to D-atoms and O atoms and O2 molecules, accordingly simulated the water formation in dense interstellar clouds (Dulieu et al., 2010). Based on the experiments, they have shown that D2 does not react with O atoms or O2 molecules residing on the amorphous solid water (ASW) ice surface. The research of Oba et al. (2012) showed that quantum tunneling was responsible for the reaction OH + H2 → H2O + H and thus could happen in the ISM. Therefore, the water formation process requires atomic hydrogen or (-OH) group and molecular hydrogen. Moreover, the experimental investigation has been confirmed that hydrogenation of the molecular oxygen produces amorphous solid water (ASW) with compact (porous) structures (Oba et al., 2009, Accolla et al., 2013).
In interstellar ices, the morphology of H2O is controlled by its hydrogen-bonding character. Depending on temperature and different structures can be distin-guished. The ice growth conditions such as temperature, growth rate, and di-rectionality H2O deposition on grain surfaces affect the porosity of ASW (Berland et al., 1995, Westley et al., 1998, Stevenson et al., 1999a, Kimmel et al., 2001, Dohnálek et al., 2003). On the other hand, the porosity of ASW can be reduced by energetic processing in the ISM. Porous ASW is compacted and collapsed in structure under the bombardment of comic ray and UV photons (Palumbo, 2006, Palumbo et al., 2010, Raut et al., 2008) as well as by thermal processing (Bossa et al., 2012) or by chemical activity such as the H+H reaction (Accolla et al., 2011) Simultaneously, ice porosity of ASW also influences the efficiency of H2 formation (Roser et al., 2002), provides the effective surface areas for absorption of molecules and catalysis of chemical reactions.

Interaction of gas and grains

Gas phase

Although there is a significant fraction of neutral-neutral reactions, most of the reactions are of the ion-molecule variety and the produced species are detected via high resolution spectroscopy in the lab.
The molecular hydrogen is formed on dust grains, and then ejected to the gas phase, it can be ionised by reactions with cosmic ray to form H+2, and then reacts with its ion produced H+2 forming H+3(Herbst, 2001). The H+3 is relatively abundant because does not react with H2 in the gas phase (Geballe, 2000) and detected via infrared absorption.
The second is condensation reactions such as: C2H+2 + C2H2 → C4H+3 + H, C4H+2 + H2.
It then is the radiative association reactions such as C+ + Cn → C+n+1 + hν. As another example, the reaction of C3H+ ion is thought to associate with H2 to form a precursor to the cyclic species C3H2: C3H+ + H2 → c-C3H+3 + hν. Or ion-atom reactions followed by dissociative recombination such as: N + C3H+3 → HC3NH+ + H.
Gas phase chemistry of a variety of regions is affected by star formation. For instance, in hot cores where the temperature and densities are high, the molecular composition of gas is much more saturated than in the ambient material – molecules such as H2O, NH3, H2S, and CH3OH become much more abundant and larger molecules such as methyl formate, dimethyl ether, and ethanol are seen only in such regions. Before star formation, both gas phase and grain surface chemistry occur at low temperature and ice mantles build up on the grain surface. The ice mantles contain CH3OH, mostly produced by the hydrogenation of CO by addition H atoms landing on grains (Watanabe and Kouchi, 2002, Fuchs et al., 2009). During star formation, the surface temperature reaches 100-300 K for the evaporation of the ice mantles.
CO is the second abundant molecule after H2 in the gas phase, while the amount of N2 is uncertain because it lack a permanent dipole moment. The abundance of N2 is usually inferred from the presence of its daughter species N2H+ mainly formed via the reaction N2 + H+3 → N2H+ + H2. The freeze out of these molecules, CO and N2, on dust grains and ice mantles are different in pre-stellar cores. So, CO and N2 should have different depletion behaviour in pre-stellar cores (Pagani et al., 2012) although they have similar masses, sticking properties and desorption efficiency (Bisschop et al., 2006).
Moreover, the depletion of CO relates to the freeze out of CO toward the center and is also reflected in the abundance of many other molecules either through correlation and anti-correlation (Jørgensen et al., 2004). For instance, CO is the main destroyer of N2H+ in the gas phase, yet N2H+ is the daughter of N2 and its observational tracer, so CO freeze out is also a prerequisite to have an N2 gas phase or depletion estimate.
There are molecular species which are considered to be pre-biotic species in space such as glycoladehyde (CH2OHCHO) (Hollis et al., 2004), acetamide (CH2CONH2) (Hollis et al., 2006), amino acetonitrile (NH2CH2CN) (Belloche et al., 2008), for-mamide (NH2CHO), etc which exist in the gas phase. In particular, the presence of formamide (NH2CHO) has been observed in the gas phase in several astronom-ical environments such as pre-stellar cores and protostellar objects (Kahane et al., 2013), or massive hot molecular cores (Bisschop et al., 2007). There are hypothe-sises that suggest that the formamide formation in the gas phase via the reaction of two molecular precursors, H2CO and NH2 (Barone et al., 2015, Vasyunin and Herbst, 2013b, Vasyunin et al., 2017, Codella et al., 2017). Hence the gas phase chemistry has an important role for the synthesis of pre-biotic species which are involved in processes of the life origin.
There are many molecules observed in the ISM. Some of them, like CO can be used as a tracer of the dense media and can be synthetized in the gas phase. However, many other molecules such as H2 or hydrogenated complex organic molecules need the intervention of the solid state, which is provided by the surface of interstellar dust grains.

Solid phase sublimation

Many molecular species are formed and detected in the gas phase. However, it cannot explain the observed abundance of some complex organic molecules, therefore the grains surface should have an important role. As the chemistry in the gas phase proceeds, atoms and molecules can collide with the grain surface to accrete, diffuse, meet, and react to form more molecular complexity than it is possible in the gas phase alone. Even if gas phase species can condense onto the grain surface, the grain species can desorb back into the gas phase through the thermal desorption when the grain surface increases from 10 K to 100-300 K. At temperatures are higher than 20 K, H2 does not reside long enough on the grain surface, CO, CO2, N2, and CH4 also start to sublimate, and the other heavier elements remaining on the grain surfaces may begin to diffuse. Hence reaction networks and desorption process depends on physical parameters such as cloud densities, gas temperatures, compositions of grain surfaces or illumination, but for these processes the most important parameter is the grain temperature. In return, molecules injected from the solid phase into the gas phase will continue to chemically evolve making complex the interaction between gas and grain surface chemistry.
After hydrogenation reactions, one has to consider other important solid phase route, CO is the second most abundant molecule in the ISM (10−4-10−5 times the hydrogen abundances) (Pontoppidan et al., 2005). When CO freeze out occurs, and then CO-rich ice mantles are formed on top of the water ice. The CO reactions formed some species on grain surface such as the formation of CO2 via {CO + O} reaction (Minissale et al., 2013) or {CO + OH} (Ioppolo et al., 2011, Oba et al., 2011, Minissale et al., 2013). Most reactions are the interaction between radicals and radicals or radicals and molecules, hence they do not require or have any activation energies and can proceed at low temperatures (typically of 10 K).
Regarding the formation of COMs other than methanol on grains surface and in the gas phase, Isocyanic acid (HNCO), is the simplest molecule containing the four abundant atoms: hydrogen, nitrogen, carbon, and oxygen. Previous hypothesises assumed that HNCO was only formed in the gas phase (Iglesias, 1977), but there were studies which introduced that HNCO forms on grain surface via the thermal reaction NH + CO or hydrogenation of OCN (Garrod et al., 2008). Furthermore, the hydrogenation of HNCO was predicted to be the pathway of the formamide formation. However, Noble et al. (2015) simultaneously demonstrated that formamide is not formed from the hydrogenation of HNCO and the presence of HNCO in the gas phase can be partly due to the desorption from grains.
Therefore, grain surface chemistry is not only responsible for the formation of hydrogen (H2) but also for the hydrogenated molecules of the pre-collapse phase in particular, and almost the whole set of observed complex organic molecules (COMs). In 2006, the basic ideas of model of Garrod and Herbst (2006), radicals are trapped in the iced mantles obtained mobility and react forming COMs when the grain surface reaches the temperature ∼ 30 K. Moreover, at later stages of stars formation, other sources of energy are available for the solid phase chemistry. Jones et al. (2011) showed the formation of formamide in CO-NH3 solid state complexes via energetic electron bombardment in interstellar ice on grains at 10 K. Formamide has also been produced by hydrogenation and UV photolysis of NO in CO-rich interstellar ice analogues (Fedoseev et al., 2016).
For understanding the formation of COMs many aspects have to be considered and studied: gas phase chemistry, non-energetic solid state chemistry, energetic solid state chemistry, and finally the interplay of solid and gas phases have to be understood. The present thesis only focus on non-energetic solid state chemistry and especially hydrogenation reactions, and on some aspect of the desorption of accreted or formed species.
This chapter presents the experimental apparatus and methods used to carry out the experiments. All experiments described in this thesis have taken place at LERMA-Cergy (Laboratoire d’Etude du Rayonnement et de la Matiere en Astro-physique et Atmospheres) in the University of Cergy Pontoise thanks to a machine named VENUS (« Vers de NoUvelles Syntheses »), with means in English « Toward New Synthesis ». It is used to investigate the physical-chemical processes of atoms and molecules on cold surfaces under dust grain environments.

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Experimental apparatus

The UHV is maintained to keep the surface clean and prevent residual water molecules that are in the main chamber to be adsorbed on the surface. In fact, with the base pressure of 1 × 10−10 mbar, it takes ∼ 5000 minutes to grow 1 monolayer (ML) of water ice (Accolla, 2010). During the beam injection, especially the pressure of the hydrogen beam may rise up to few 10−10 mbar, is low enough to keep low the pollution on the sample surface. Indeed, water and others residual gases can form film of ice on the sample surface, so it may affect of the physical-chemical processes. In order to check this aspect, the residual gas analysis is made at any steps.
Moreover, there are two intermediate stages, which are called the first and the sec-ond chambers. The connection between the intermediate stages and the sources of the main chamber are made through diaphragms. It permits to create differential pumping. They have the residual pressures of 10−8 mbar for the first chamber and 10−9 mbar for the second chamber, respectively.

The sample holder and cryostat system

The sample holder is located in the center of the main chamber. It is made of a circular copper mirror coated with gold. It diameter is equal to 9 mm. The sample holder is larger than the beams which have an aperture of about 3 mm diameter. It is mounted onto the cold head of the closed cycle He cryostat (see figure 2.2). The sample temperature can be controlled in the range of 7 – 400 K by using a regulated resistive heater clamped behind the sample holder. It connects to a Lakeshore controller, that controls the temperatures by varying the current injected in the resistive heater in an automated way.
Figure 2.2 displays the place of the sample holder in the main chamber and the schematic of the cryostat with longitudinal section of the sample holder. A cryoshield is made of copper coated with nickel to protect and isolate the sample holder.
The sample holder can be translated (x, y, z) in order to get from room radiation the best alignment with both IR spectroscopy and molecular beams.

The beamline system

The separated four beamlines system allows us to deposit different molecular species at the same time on the same surface. They are called the right, cen-tral, top, and bottom beams (see figure 2.3). We used alignment lasers to find the best position for each beam. A gas expansion zone and a nozzle are located in a chamber pumped by a turbo molecular pump. For each beam, the gases are regulated by an automated flow regulator by Bronkhorst High-tech. The incoming flux is around 0.1 sccm1 which correspond to 0.1 cm3 per minute of gas at atmo-spheric pressure.
Figure 2.3: Photo of the beamlines system for VENUS before moving to the new laboratory in March 2018. The beamline system component consists of the right, central, top and bottom beams. Central beam is behind the right beam on the photo.
and then the second chamber through the tiny diaphragms, pumped down by the turbo molecular pumps. Finally, when the gas arrives in the main chamber, it does not change the pressure in the main chamber too much where the residual pressure is of about 10−10 mbar. But the flux in the beam at the sample surface is around 2 × 1012 mol/cm2/s (8 minutes for a monolayer (ML)).

Quadrupole Mass Spectrometer (QMS)

The QMS is able to detect and measure the composition and the abundance of the residual gas in the main chamber or analysing the atomic/molecular jet coming from the beamlines. It can also check the molecules desorbing from the sample holder during a Thermally Programmed Desorption (TPD) experiment.
The QMS is mounted on the bottom of the main chamber. The QMS can be translated vertically or rotated. In a low position, the QMS can analyze the com-position of present residual species in the main chamber. At the higher position, it is in front of the beamline and the sample holder. It can therefore characterise species coming from the beamlines or measure the desorption rate from the sample holder. We put the QMS in the low position during gas deposition or IR recording.
Figure 2.4 shows the QMS is in the low position (left panel) and the high position (right panel) where the QMS is positioned in front of the sample holder.
Figure 2.4: Photo of QMS is at the low position (left panel) and the schematic of QMS at the high position (right panel).
A QMS consists of an ionizer (bombardment by electron from a hot filament), an ion accelerator, and a mass filter consisting of four parallel metal rods. Residual molecules or desorbing molecules from the sample surface enter the quadrupole probe, they are ionized via electron bombardment by passing near to a heated tungsten filament and they are subsequently accelerated toward the four metallic rods. These four parallel rods represent the ion mass filter to select the species according to their mass charge ratio (m/z); In fact, a voltage combination of a direct and a radio frequency component is applied between adjacent and opposite rods. Varying the direct and the radio frequency component, the QMS is capable of scanning all ions up to a chosen mass to charge ratio technically fixed.
The ion detector is a Channeltron (an electron multiplier). The current output generated in the Channeltron, is converted into a digital signal. The digitalized signal is then controlled by a software provided by HIDEN. It allows not only to monitor and record the acquired information, but also to adjust the electronic setting of the QMS and the dwelling times between two measurements. Moreover, it is possible to record simultaneously the sample temperature measured by the Lakeshore controller during TPD experiments. In our experiments, we use a 30 eV kinetic energy for the electrons which is limitative somehow the cracking of the molecules.

The infrared spectroscopy

VENUS is equipped with a VERTEX 70v Fourier transform Infrared Spectrometer (FTIR) which used to monitor adsorbed/formed species in situ. Figure 2.5 shows the photos of external and internal FTIR. It consists of the mid-infrared (MIR) source. It is the global thermal source made of a silicon carbide rod heated up to 1000oC-1500oC, and emitting a polychromatic infrared radiations from 2.2 to 14.3 µm wavelength. The light passes through an aperture (typically of 1.5 mm). For studies mid-infrared is ranged from 4500 – 750 cm−1 for vibrational identification for species. The interferometer is composed of a beamsplitter (KBr) that is used for splitting the light into two parts, a fixed mirror, and a moving mirror (see figure 2.5). The moving mirror is the basic linear scanner with different optical path length. The beamsplitter recombines the beams coming from two of the mirrors. The resulting beam passes through an exit port and focuses on the sample holder of the UHV chamber. External to the UHV chamber, the reflected infrared beam from the sample holder is directed on the gold-plated mirrors which are mounted in a differentially pumped housing adjacent the UHV chamber and combined with an external pump. The reflected infrared beam is subsequently collected and focused onto the liquid nitrogen cooled mercury cadmium telluride (MCT) detector (see figure 2.5 in the right panel). MCT detector collects the raw data, and are subsequently sent to the computer. Afterwards, OPUS software assembles all these data and turn the interferometer (the raw data, light absorption for each mirror position) into the typical IR spectrum through a Fourier transform. Each spectrum is obtained with a resolution of 4 cm−1 prior or subsequently to either deposition or TPD.

Experimental methods

We have described the QMS operation for the detection of atoms/molecules com-ing from the sample surface. We have shown that molecules were detected after their ionization. Because of the electron impact, dissociation of species could occur in addition to ionization. The fragments distribution of species which result from the dissociation and the ionization of molecules is called cracking pattern. The probabilities of ionization or dissociation of species depend on molecular geometry, energy of ionizing electron, and angle impact between molecules and ionizing elec-tron of the QMS. Hence we obtain different peaks/signals of the same molecules through the QMS.

Table of contents :

1 Introduction 
1.1 Star evolution
1.1.1 Interstellar gas
1.1.2 Interstellar dust
1.1.3 Surface chemistry
1.1.4 Interstellar molecules
1.1.5 Interstellar ices
1.2 Interaction of gas and grains
1.2.1 Gas phase
1.2.2 Solid phase sublimation
2 Experimental apparatus and Methods 
2.1 Experimental apparatus
2.1.1 The main chamber
2.1.2 The sample holder and cryostat system
2.1.3 The beamline system
2.1.4 Quadrupole Mass Spectrometer (QMS)
2.1.5 The infrared spectroscopy
2.2 Experimental methods
2.2.1 Mass spectroscopy Cracking pattern Thermal Programmed Desorption (TPD)
2.2.2 Water ice substrates on the sample holder
2.2.3 Calibration of the beamline system Optimization of the injection flow Determination of the geometrical area of the beam deposition zone on the sample holder Determination of the beam overlap Dissociation of H2 and D2 beam
3 Segregation effect and N2 binding energy reduction in CO-N2 system adsorbed on water ice substrates
3.1 Introduction
3.2 Experimental protocol
3.3 Experimental results
3.3.1 Pure species
3.3.2 Mixed species
3.4 Analysis and discussion
3.5 Conclusions
4 Experimental study of the chemical network of the hydrogenation of NO on interstellar dust grains 
4.1 Introduction
4.2 Experimental setup
4.3 Experimental results
4.3.1 Completeness of the reactions of the {NO + H} system before the TPD
4.3.2 Temperature dependency
4.3.3 The {NO + D} reactive system at various temperatures
4.4 Catalytic role of water
4.5 The possibility of back reaction NH2OH+H −! H2NO+H2
4.6 Experimental conclusions
4.7 Astrophysical implications
5 Study of the penetration of oxygen and deuterium atoms into porous water ice 
5.1 Introduction
5.2 Experimental methods
5.2.1 Experimental set-up
5.2.2 Water ice characterization
5.3 Experimental results
5.3.1 Oxygenation of NO ices
5.3.2 Deuteration of NO ices
5.4 Model and Discussion
5.5 Astrophysical implications and conclusions
6 Efficient formation route of the pre-biotic molecule formamide on interstellar dust grains 
6.1 Introduction
6.2 Experimental methods
6.3 Experimental results
6.4 Astrophysical implications
7 Experimental study of the hydrogenation of Acetonitrile and Methyl Isocyanide on Interstellar dust grains 
7.1 Introduction
7.2 Experimental conditions
7.3 Experimental results
7.3.1 (Non) Reaction between Acetonitrile (CH3CN) and H atoms at 10 K
7.3.2 Reactivity between Methyl Isocyanide CH3NC and H atoms at 10 K
7.3.3 CH3NC and its hydrogenation in presence of H2O. Desorption of CH3NC mixed with H2O Reaction of CH3NC and H atoms in the presence of H2O
7.3.4 Reactivity of CH3NC and D atoms on the golden surface at 10 K
7.3.5 Reactivity of CH3NC and H atoms at various temperatures
7.4 Analysis and Discussion
7.4.1 Activation barrier and quantum tunneling
7.4.2 The catalytic role of H2O for chemical reactions
7.4.3 Orientation of CH3NC at different surface temperatures
7.4.4 Astrophysical implications
7.4.5 New estimation of the cracking patterns and of the binding energy of CH3NCH2
7.5 Summary and conclusion
8 Conclusions and perspectives
8.1 Remarks and Astrophysical implications
8.2 Perspectives


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