ASTROCHEMISTRY: From Laboratory Studies to Astronomical Observations
855(2006); http://dx.doi.org/10.1063/1.2359534View Description Hide Description
Dissociative recombination of molecular ions with electrons is an important gas‐phase process for production of interstellar molecules. Product branching ratios in dissociative recombination have been notoriously difficult to measure in the laboratory. In recent years, the ion storage ring CRYRING in Stockholm has been used to measure branching ratios for astrophysically important molecular ions by means of a grid technique.
Astrochemistry of Magnesium Cations with Hydrogen Cyanide and Cyanoacetylene: Possible Formation of Cyclic Tetramers of Cyanoacetylene855(2006); http://dx.doi.org/10.1063/1.2359535View Description Hide Description
Laboratory measurements with the SIFT technique have shown that magnesium radical cations in a helium bath gas at 0.35 Torr and 294 K are unreactive with hydrogen cyanide but initiate a chemical sequence in cyanoacetylene that leads to the formation of cluster ions with n up to 7. Rate‐coefficient measurements for sequential addition of cyanoacetylene to Mg +• indicate an extraordinary pattern in alternating chemical reactivity while multiple‐collision induced dissociation experiments revealed an extraordinary stability for the cluster radical cation. Molecular orbital calculations with density functional theory (DFT) have provided structures and energies for the observed cations. These calculations indicate that the path of formation of appears to involve ligand‐ligand interactions mediated by Mg +• to form 2,4,6,8‐tetracyanosemibullvalene‐Mg+• or 1,2,5,6‐tetracyano‐1,3,5,7‐cyclooctatetraene‐Mg+• cations. A case is made for the formation of similar complex organomagnesium ions in the upper atmosphere of Titan where subsequent electron‐ion recombination may produce cyano derivatives of large unsaturated hydrocarbons. In contrast, circumstellar environments with their much higher content of free electrons seem less likely to give rise to such chemistry.
Neutral‐Neutral Gas‐Phase Reactions In Extraterrestrial Environments: Laboratory Investigations By Crossed Molecular Beams855(2006); http://dx.doi.org/10.1063/1.2359536View Description Hide Description
In this contribution we report experimental results obtained by the crossed molecular beam technique with mass spectrometric detection on several neutral‐neutral gas‐phase reactions of relevance in astrochemistry. More specifically, we provide experimental evidence that the reactions of electronically excited state nitrogen atoms, N(2 D), with several hydrocarbons observed in the atmosphere of Titan can generate molecular products containing a novel CN bond, thus suggesting possible routes towards the production of gas‐phase nitriles in that environment. We also report on the reaction between C(3 P) and acetylene, an important neutral‐neutral reaction in the chemistry of interstellar clouds. The branching ratio between the cyclic and linear C3H isomers has been derived for a collision energy of 3.5 kJ mol−1. Further evidence of the occurrence of the C3 formation channel is given. Finally we report on recent studies of reactions involving two open shell species, namely O+CH3 and O+C3H5. Products other than those already considered in the modeling of planetary atmospheres and interstellar medium have been identified.
855(2006); http://dx.doi.org/10.1063/1.2359537View Description Hide Description
We present new results for some astrochemically relevant reaction processes obtained using reflectron velocity map ion imaging and Megapixel ion imaging techniques developed in our laboratory recently. Using reflectron multimass velocity map ion imaging, we studied the vibrationally mediated photodissociation dynamics in the ethylene cation. We simultaneously record spatially resolved images of parent ions as well as photofragment and ions originating in dissociation from the vibronic excitations in two distinct bands, and , at roughly the same total energy. The experimental results are discussed in terms of the possible influence of the torsional excitation on competition between direct dissociation, isomerization, and radiationless transitions through conical intersections among the numerous electronic states that participate in the dissociation. In conjunction with dc slice imaging, the Megapixel ion imaging technique can improve the resolution up to ten times higher than standard video camera resolution and at very low cost. We obtained VUV nonresonant ionization and resonance enhanced multiphoton ionization (REMPI) results for the reactions of Cl with alkanes with this technique. Using REMPI detection, the product HCl (v=0, J=2) image of state‐resolved crossed beam scattering results for Cl + C2H6 was acquired at a collision energy of 6.7 ± 0.6 kcal/mol. The experimental results show strongly coupled angular and translational energy distributions revealing features of the reaction not seen in previous studies. These observations provide new insight into the dynamics of the reaction.
855(2006); http://dx.doi.org/10.1063/1.2359538View Description Hide Description
Crossed molecular beams experiments of dicarbon, , and tricarbon molecules, , with unsaturated hydrocarbons acetylene (C2H2), ethylene (C2H4), methylacetylene (CH3CCH), and allene (H2CCCH2) were carried out at 27 collision energies between 11 and 128 kJmol−1 utilizing a universal crossed beams machine. The data suggest that all reactions of C2 and C3 are indirect and involve cyclic intermediates formed by an initial addition of dicarbon or tricarbon molecule to the π bond of the hydrocarbon; these cyclic intermediates isomerize to acyclic structures. The latter were found to decompose via atomic/molecular hydrogen loss to C4H [HCCCC], i‐C4H3 [H2CCCCH], C5H[HCCCCC], C5H2[H2CCCCC], n‐C5H3 [HCCCCCH2], i‐C5H3[HCCCHCCH], and C6H3 [H2CCCCCCH]. All dicarbon reactions are barrier‐less, whereas pronounced entrance barriers between 40 and 80 kJmol−1 are present in the entrance channel of the tricarbon reactions. The identification of the dicarbon versus atomic hydrogen and tricarbon versus atomic/molecular hydrogen exchange pathways suggest the possibility of these reactions to form hydrogen‐deficient, often resonantly‐stabilized free hydrocarbon radicals in combustion flames and in extraterrestrial environments via reactions of small carbon clusters with unsaturated hydrocarbons.
Isotopic Segregation of Molecular Hydrogen on Water Ice Surface at Low Temperature: Importance for Interstellar Grain Chemistry855(2006); http://dx.doi.org/10.1063/1.2359539View Description Hide Description
A first step, before studying the formation of molecules on surfaces at low temperature by recombination of atoms, is a good understanding of the adsorption/desorption mechanisms of the molecules themselves and a correct interpretation of the results of TPD (Thermally Programmed Desorption) experiments. Experimental studies (performed on our newly built experimental setup “FORMOLISM”) of adsorption and desorption of molecular hydrogen and deuterium on an amorphous porous solid water (ASW) ice surface between 10 and 35 K reveal a very efficient isotopic segregation process. The slight difference in the maximum desorption temperature between the two isotopes leads to a preferential coverage of the sites binding more tightly of the ice surface by D2, independent of the gas deposition process history. A statistical model, which takes into account thermodynamic aspects of adsorption sites as well as isotopic competition, is proposed to understand the enhancement of deuterium fractionation. Model results are in very good agreement with the experimental ones. This mechanism could play a key role in chemistry at the surface of interstellar dust grains. It could in particular explain the isotopic enrichment observed in some dark clouds. The next step will be to measure the rovibrational energy of the molecular hydrogen formed on such surfaces using a REMPI‐TOF detection which allows for selective quantum state identification.
855(2006); http://dx.doi.org/10.1063/1.2359540View Description Hide Description
Water and methanol have been shown to form a stable clathrate hydrate under conditions of relevance to the interstellar medium. Observational evidence indicates that methanol may be present in ices in proximity to protostars at local concentrations well above its overall solid state abundance, suggesting that segregation processes occur in such ices. In previous research we have demonstrated the influence of water ice morphology on the desorption behaviour of other species adsorbed on, or mixed in water ice films. Using CO desorption as a probe, in combination with infrared spectroscopy, we present here the results of an extension of this experimental program, in which the influence of varying methanol concentrations on the morphology of water ice have been studied.
855(2006); http://dx.doi.org/10.1063/1.2359541View Description Hide Description
We performed an experiment in which pure solid CO was simultaneously exposed to cold H and D atoms at 10 K, and observed efficient formation of H2CO, CH3OH and deuterated isotopologues HDCO, D2CO, CH2DOH, CHD2OH, and CD3OH. The D/H ratios obtained for formaldehyde and methanol are in good agreement with previously reported observations. We found that the H‐D substitution reaction in methanol is a key process in the formation of deuterated methanol, especially multi‐deuterated isotopologues CHD2OH and CD3OH.
855(2006); http://dx.doi.org/10.1063/1.2359542View Description Hide Description
Interstellar dust particles (IDPs) constitute most of the solid matter in the universe. Large quantities of IDPs are also present in the Solar System and fall on Earth. IDPs are also of interest as they can catalyze astrochemical reactions and prebiotic synthesis, and their organic contents are believed to have contributed to the origins of life. Their chemical composition is similar to carbonaceous chondrite comets, asteroids and meteorites. The IDPs are microporous web‐like aggregates of 10–100 nm phyllosilicate particles with morphologies similar to particles produced by the Laser Vaporization Controlled Condensation (LVCC) method. IDPs are available only as microscopic samples, and simulated IDPs are needed to study their chemical and catalytic effects. To produce such simulated IDPs, we formed nanoparticles from carbonaceous chondrite meteorites by LVCC processing. The compositions, morphologies, particle size distribution, FTIR spectra, and catalytic properties of the meteorite‐based nanoparticles were investigated and compared with the original meteorite materials and reference minerals.
855(2006); http://dx.doi.org/10.1063/1.2359543View Description Hide Description
Simulation experiments for the formation of CH3OH, HNC/HCN, and NH3 in solid‐phase reactions were performed. CH3OH and H2CO were formed as major products from the 100 eV electron‐irradiated mixed CH4/H2O solid at 10 K. There found to be two pathways for the formation of methanol with about equal importance, i.e., the recombination reaction: CH3 + OH → CH3OH, and the insertion reaction: CH2+H2O → CH3OH. One CH3OH molecule was formed per 60 electron irradiation with the electron energy of 100 eV. By using mixed H2O/CD4 ice, it was confirmed that formaldehyde was formed by the insertion reaction, C + H2O →H2CO.
A mixed gas of N2/HCN (50/1) at a few Torr was activated by a dc discharge and was deposited on the gold‐plated copper substrate at 10, 15, and 20 K. During the deposition of plasma‐activated sample gas, D atoms produced by the dc discharge of D2 were simultaneously sprayed over the solid film. The association reactions of CN with D at 10 K were found to generate DNC and DCN with the intensity ratio DNC/DCN of about 3 in the infrared absorption spectra. This high ratio is in line with the high abundance ratios of HNC/HCN observed in the dark clouds. The formation of DNC and DCN became negligible at 20 K, due to the decrease of the sticking probability of D atoms on the solid surface. Ammonia was not detected as a reaction product from reaction of D with N atoms trapped in the N2 matrix.
Investigating the Formation of Intermediates in the Reactions of Carbon Dioxide (CO2) with Suprathermal Oxygen and Nitrogen Atoms855(2006); http://dx.doi.org/10.1063/1.2359544View Description Hide Description
Chemical reactions involving carbon dioxide are prominent in a variety of environments and, therefore, important for modeling reaction pathways and quantifying molecular abundances. In atmospheres and in outer solar system ices, for example, radiation induced degradation of abundant chemical species like ozone, oxygen, carbon dioxide, or molecular nitrogen can liberate high energy oxygen or nitrogen atoms that may react with carbon dioxide. This work presents a study of these reactions where in the carbon dioxide — oxygen atom reaction, two carbon trioxide isomers (C2v and D3h symmetry) were found to form. In the carbon dioxide — nitrogen atom system, the bent OCNO radical was formed. Rate constants have been derived for these reaction pathways and the dynamics of the reactions are investigated.
855(2006); http://dx.doi.org/10.1063/1.2359545View Description Hide Description
The reaction of cold H and D atoms with solid formaldehyde (D2CO and H2CO) deposited on amorphous H2O ice at 15 K was studied experimentally by in‐situ FTIR spectroscopy. New routes of formation of deuterated formaldehyde (HDCO and D2CO) were identified. In the reaction of D atoms with H2CO, H‐D substitution was observed, H2CO → HDCO → D2CO, and for the reaction of H atoms with D2CO, the analogous D‐H substitution was observed, D2CO → HDCO → H2CO. In the reaction of D atoms with H2CO, the rate of H‐D substitution was greater than that of formation of deuterated methanol‐d 2–4 by addition of 2D atoms to formaldehyde‐d 0–2. However, in the reaction of H atoms with D2CO, the rate of D‐H substitution was comparable to that of addition of 2H atoms to D2CO.
855(2006); http://dx.doi.org/10.1063/1.2359546View Description Hide Description
Recent results from the Raymond and Beverly Sackler Laboratory for Astrophysics on spectroscopy and processing of interstellar ice analogues are summarized. This includes thermal desorption studies of pure, layered and mixed CO, N2 and O2 ices, and infrared spectroscopy and heating of CO‐CO2, CO‐H2O, CO‐HCOOH, CO‐CH4 and CO‐CH3OH layered and mixed ices. Laboratory data of CO‐surface adsorbates show good agreement with the unidentified 2175 cm−1 interstellar feature. Complementary ab initio quantum chemical calculations and molecular dynamics simulations have been performed to provide insight into the gas‐grain interactions and interstellar ice processing. This includes the first molecular dynamics study of the photodissociation of water ice and the corresponding photodesorption efficiencies. The relevance of these data in the analysis of astronomical data is emphasized throughout.
855(2006); http://dx.doi.org/10.1063/1.2359547View Description Hide Description
Using cold atomic hydrogen and deuterium beams, hydrogenation and deuteration of solid CO on the surface of H2O ice were investigated in the surface‐temperature range 8–20 K. In hydrogenation experiments, formaldehyde and methanol were produced efficiently by successive hydrogenation of CO below 20 K in conditions similar to those in molecular clouds. In deuteration experiments, we found, for the first time, that D2CO and CD3OD are formed from CO, with the reaction proceeding similarly to hydrogenation although at a much slower rate. From the attenuation curve of the parent CO at 15 K, the relative reaction rate of CO‐deuteration to ‐hydrogenation was determined to be about 0.1. This isotope effect is considered to be due to tunneling reaction.
855(2006); http://dx.doi.org/10.1063/1.2359548View Description Hide Description
Astrochemistry is the study of chemical processes under those conditions prevalent in astronomical environments. Due to the temperatures found in many astrophysical environments much of this chemistry is heterogeneous chemistry occurring within icy mantles found on planetary surfaces, within comets and on dust grains in the interstellar medium. The physical properties and chemical reactivity of such icy surfaces depends upon its morphology. Thus it is necessary to explore how the morphology of astrochemical ices is influenced by their local environment (e.g. temperature and pressure) and the mechanisms by which they are processed. In this paper we briefly review the results of a series of experiments to explore the morphology of a variety of molecular ices using VUV spectroscopy. Several spectral features are reported that may be used to identify the morphology of astrochemical ices using remote sensing techniques.
855(2006); http://dx.doi.org/10.1063/1.2359549View Description Hide Description
Metal hydride molecules are important in astronomy: they are found in sunspots, cool stellar atmospheres and in substellar objects called brown dwarfs. Molecular opacities needed to simulate the spectral energy distributions emitted by these objects. The astronomical observation of metal hydrides is reviewed and the calculation of opacities is discussed for CrH, FeH and TiH.
855(2006); http://dx.doi.org/10.1063/1.2359550View Description Hide Description
Cyanoallene (H2CCCHCN) has been observed with the 100‐m Effelsberg Telescope towards the dark Taurus molecular cloud (TMC‐1) in four different rotational transitions (JK−K+ = 40,4−30,3, 50,5−40,4, 60,6−50,5 and 70,7−60,6) at the frequency range from 20 to 35 GHz. The existence of its isomer methylcyanoacetylene was confirmed with its K = 0 and K = 1 components of the 5 K −4 K transition. This result was predicted from laboratory measurements. Our attempt of searching for both isomers towards Orion‐KL, however, remains unsuccessful. Possible explanations are briefly discussed.
855(2006); http://dx.doi.org/10.1063/1.2359551View Description Hide Description
Recent millimeter‐wave and infrared spectroscopic observations have found evidence of rapid synthesis of complex organic molecules in the late stages of stellar evolution. The chemical synthesis begins with the formation of acetylene, the first building block of benzene, in carbon stars. In the following proto‐planetary nebula stage, emission features corresponding to stretching and bending modes of aliphatic compounds are detected. When these objects evolve to become planetary nebulae, aromatic C‐H and C‐C stretching and bending modes become strong. These results show that complex carbonaceous compounds can be produced in a circumstellar environment over a period of only a few thousand years.
Isotopic analysis of meteorites and interplanetary dust collected in the upper atmosphere have revealed the presence of pre‐solar grains similar to those formed in evolved stars. This provides a direct link between star dust and the solar system and raises the possibility that the early solar system was chemically enriched by stellar ejecta.
Carbon Isotope Ratios In Circumstellar Envelopes: Constraints For Nucleosynthesis And Galactic Chemical Evolution855(2006); http://dx.doi.org/10.1063/1.2359552View Description Hide Description
Observations of the 1→0 transition of the 12C and 13C isotopomers of CO and CN at 1.2 and 3 mm have been conducted towards various circumstellar envelopes, using the Kitt Peak 12m antenna and the Submillimeter Telescope (SMT) of the Arizona Radio Observatory (ARO). These observations have been used to establish 12C/13C ratios in these objects. CN is particularly useful in this regard because its spectra exhibit hyperfine structure from which accurate opacities can be evaluated. Observations include a variety of hydrogen‐deficient stars, supergiants, Li‐rich, M, S, and C stars. Measurements towards Asymptotic Giant Branch (AGB) stars (e.g. IRC+10216, CRL618, CRL2688, and IRC+40540), suggest preliminary 12C/13C isotope ratios in the range of 20–76, while in supergiants, 12C/13C ∼ 3–10. Theory predicts that carbon‐12 is primarily formed in massive stars that evolve into Type II supernovae, while carbon‐13 is produced from the CNO cycle occurring in intermediate mass AGB stars. Ratios determined for these objects with multiple molecular species help constrain nucleosynthesis models and provide insight into the 12C/13C Galactic gradient measured in molecular clouds, as well as local variations of this ratio found in the interstellar medium.