Interstellar Temperatures Research Articles

Communication: Rotational excitation of HCl by H: Rigid rotor vs. reactive approaches.

We report fully quantum time-independent calculations of cross sections for the collisional excitation of HCl by H, an astrophysically relevant process. Our calculations are based on the Bian-Werner ClH2 potential energy surface and include the possibility of HCl destruction through reactive collisions. The strongest collision-induced rotational HCl transitions are those with Δj = 1, and the magnitude of the HCl-H inelastic cross sections is of the same order of magnitude as the HCl-H2 ones. Results of exact calculations, i.e., including the reactive channels, are compared to pure inelastic calculations based on the rigid rotor approximation. A very good agreement is found between the two approaches over the whole energy range 10-3000 cm(-1). At the highest collisional energies, where the reaction takes place, the rigid rotor approach slightly overestimates the cross sections, as expected. Hence, the rigid rotor approach is found to be reliable at interstellar temperatures.

Accelerated chemistry in the reaction between the hydroxyl radical and methanol at interstellar temperatures facilitated by tunnelling

Understanding the abundances of molecules in dense interstellar clouds requires knowledge of the rates of gas-phase reactions between uncharged species. However, because of the low temperatures within these clouds, reactions with an activation barrier were considered too slow to play an important role. Here we show that, despite the presence of a barrier, the rate coefficient for the reaction between the hydroxyl radical (OH) and methanol--one of the most abundant organic molecules in space--is almost two orders of magnitude larger at 63 K than previously measured at ∼200 K. We also observe the formation of the methoxy radical product, which was recently detected in space. These results are interpreted by the formation of a hydrogen-bonded complex that is sufficiently long-lived to undergo quantum-mechanical tunnelling to form products. We postulate that this tunnelling mechanism for the oxidation of organic molecules by OH is widespread in low-temperature interstellar environments.

CAN INTERSTELLAR PROPENE (CH3CHCH2) BE FORMED VIA GAS-PHASE REACTIONS?

Propene (CH3CHCH2), detected in the cold core TMC-1, is a surprisingly saturated (H-rich) species for observation in such regions. In a recently proposed gas-phase formation mechanism, interstellar propene is produced from its protonated precursor C3H+ 7 (CH3CHCH+ 3) via a dissociative recombination process. The precursor ion C3H+ 7 is itself produced via two consecutive radiative association reactions involving H2 starting from the isomer of C3H+ 3 with a linear carbon backbone (CH2CCH+). Initial calculations showed that the radiative association reactions are efficient enough to allow the production of an abundance of propene equal to that observed. However, a combination of experiments and more refined quantum chemical ab initio calculations reported here does not corroborate the initial result. Indeed, from both of these approaches, we have learned that the radiative association reactions leading to protonated propene do not occur efficiently at interstellar temperatures due to activation energy barriers. The result is that propene cannot be produced efficiently by the suggested gas-phase synthetic route. It is still difficult to say, however, that no suitable gas-phase syntheses for propene can occur in cold cores such as TMC-1.

IN-SITU PROBING OF RADIATION-INDUCED PROCESSING OF ORGANICS IN ASTROPHYSICAL ICE ANALOGS—NOVEL LASER DESORPTION LASER IONIZATION TIME-OF-FLIGHT MASS SPECTROSCOPIC STUDIES

Understanding the evolution of organic molecules in ice grains in the interstellar medium (ISM) under cosmic rays, stellar radiation, and local electrons and ions is critical to our understanding of the connection between ISM and solar systems. Our study is aimed at reaching this goal of looking directly into radiation-induced processing in these ice grains. We developed a two-color laser-desorption laser-ionization time-of-flight mass spectroscopic method (2C-MALDI-TOF), similar to matrix-assisted laser desorption and ionization time-of-flight (MALDI-TOF) mass spectroscopy. Results presented here with polycyclic aromatic hydrocarbon (PAH) probe molecules embedded in water-ice at 5 K show for the first time that hydrogenation and oxygenation are the primary chemical reactions that occur in astrophysical ice analogs when subjected to Lyα radiation. We found that hydrogenation can occur over several unsaturated bonds and the product distribution corresponds to their stabilities. Multiple hydrogenation efficiency is found to be higher at higher temperatures (100 K) compared to 5 K—close to the interstellar ice temperatures. Hydroxylation is shown to have similar efficiencies at 5 K or 100 K, indicating that addition of O atoms or OH radicals to pre-ionized PAHs is a barrierless process. These studies—the first glimpses into interstellar ice chemistry through analog studies—show that once accreted onto ice grains PAHs lose their PAH spectroscopic signatures through radiation chemistry, which could be one of the reason for the lack of PAH detection in interstellar ice grains, particularly the outer regions of cold, dense clouds or the upper molecular layers of protoplanetary disks.

Spatial Distributions and Interstellar Reaction Processes

Methyl formate presents a challenge for the conventional chemical mechanisms assumed to guide interstellar organic chemistry. Previous studies of potential formation pathways for methyl formate in interstellar clouds ruled out gas-phase chemistry as a major production route, and more recent chemical kinetics models indicate that it may form efficiently from radical-radical chemistry on ice surfaces. Yet, recent chemical imaging studies of methyl formate and molecules potentially related to its formation suggest that it may form through previously unexplored gas-phase chemistry. Motivated by these findings, two new gas-phase ion-molecule formation routes are proposed and characterized using electronic structure theory with conformational specificity. The proposed reactions, acid-catalyzed Fisher esterification and methyl cation transfer, both produce the less stable trans-conformational isomer of protonated methyl formate in relatively high abundance under the kinetically controlled conditions relevant to interstellar chemistry. Gas-phase neutral methyl formate can be produced from its protonated counterpart through either a dissociative electron recombination reaction or a proton transfer reaction to a molecule with larger proton affinity. Retention (or partial retention) of the conformation in these neutralization reactions would yield trans-methyl formate in an abundance that exceeds predictions under thermodynamic equilibrium at typical interstellar temperatures of ≤100 K. For this reason, this conformer may prove to be an excellent probe of gas-phase chemistry in interstellar clouds. Motivated by new theoretical predictions, the rotational spectrum of trans-methyl formate has been measured for the first time in the laboratory, and seven lines have now been detected in the interstellar medium using the publicly available PRIMOS survey from the NRAO Green Bank Telescope.

Storage ring measurements of the dissociative recombination rate of rotationally cold H3+

We present dissociative recombination measurements, using the CRYRING ion storage ring, of H3+ ions produced in a supersonic expansion discharge source. Before and after the CRYRING measurements, the ion source was characterized in Berkeley using infrared cavity ringdown spectroscopy, and was found to exhibit a typical rotational temperature of ∼30 K. Our measurement of the dissociative recombination cross section using this ion source revealed resonances that had not been observed clearly in previous experiments that used rotationally hot ion sources. Based on the present measurements, we infer a thermal dissociative recombination rate coefficient for ions at interstellar temperatures of ∼ 2.6 × 10−7 cm3s−1. Our results are in general agreement with theoretical calculations of the dissociative recombination cross section by Kokoouline and Greene. We will review the enigma of the abundance of H3+ in the diffuse interstellar medium, and discuss the impact of these experiments, especially in the context of the recent observation of H3+ towards ζ Persei.

No Barrier for the Gas-Phase C2H + NH3 Reaction

The absolute rate constant of the reaction C2H + NH3 → products has been experimentally determined over the temperature range 295−765 K at a pressure of 10 Torr (He) and over the pressure range 5−30 Torr at 295 K. The concentration of C2H radicals was monitored in real time using the CH chemiluminescence method following their production by pulsed laser photolysis of C2H2 at 193 nm in a slow-flow reaction vessel. The pressure-independent rate constant is large and exhibits a pronounced negative temperature dependence: k1(T) = (1.2 ± 0.2) × 10-11 exp[(370 ± 40) K/T] cm3 s-1, both unusual for reaction of a radical with a saturated molecule. This may be rationalized by strong dipole−dipole and dipole − quadrupole interactions, as have been put forward for the isoelectronic CN + NH3 reaction. The C2H + NH3 reaction is somewhat faster than CN + NH3, even though ab initio calculations predict the CCH dipole moment to be smaller than that for CN. However the opposite sign of the dipole moment of C2H implies an initial HCC- - -HNH2 alignment, favorable for subsequent H-abstraction. The (extrapolated) high rate constant both at combustion temperatures and at interstellar temperatures should make this reaction very important in those environments.

A quantitative analysis of OCN-formation in interstellar ice analogs

The 4.62 μm absorption band, observed along the line-of-sight towards various young stellar objects, is generally used as a qualitative indicator for energetic processing of interstellar ice mantles. This interpretation is based on the excellent fit with OCN - , which is readily formed by ultraviolet (UV) or ion-irradiation of ices containing H 2 O, CO and NH 3 . However, the assignment requires both qualitative and quantitative agreement in terms of the efficiency of formation as well as the formation of additional products. Here, we present the first quantitative results on the efficiency of laboratory formation of OCN - from ices composed of different combinations of H 2 O, CO, CH 3 OH, HNCO and NH 3 by UV- and thermally-mediated solid state chemistry. Our results show large implications for the use of the 4.62 μm feature as a diagnostic for energetic ice-processing. UV-mediated formation of OCN - from H 2 O/CO/NH 3 ice matrices falls short in reproducing the highest observed interstellar abundances. In this case, at most 2.7% OCN - is formed with respect to H 2 O under conditions that no longer apply to a molecular cloud environment. On the other hand, photoprocessing and in particular thermal processing of solid HNCO in the presence of NH 3 are very efficient OCN - formation mechanisms, converting 60%-85% and ∼100%, respectively of the original HNCO. We propose that OCN - is most likely formed thermally from HNCO given the ease and efficiency of this mechanism. Upper limits on solid HNCO and the inferred interstellar ice temperatures are in agreement with this scenario.

On the deuteration of C$\mathsf{_{3}}$H$\mathsf{_{2}}$ in dense interstellar clouds via the reaction C$\mathsf{_{3}}$H$\mathsf{_{3}^{+}}$ + HD $\longrightarrow$ C$\mathsf{_{3}}$H$\mathsf{_{2}}$D++ H$\mathsf{_{2}}$

Although deuterium isotope fractionation in cold, dark interstellar clouds is reasonably well understood via gas-phase chemistry, there are some discrepancies between observation and theory. For example, the observed abundance ratio between the deuterated species C 3 HD and the cyclic molecule C 3 H 2 is significantly higher than most theoretical values unless the exchange reaction C 3 H$_{3}^{+}$ + HD $\longrightarrow$ C 3 H 2 D + + H 2 is efficient. In this paper, we report quantum chemical and dynamical calculations on this reaction, which show it to possess a large activation energy barrier and to be very slow at all normal interstellar temperatures.

Deuterium enrichment of polycyclic aromatic hydrocarbons by photochemically induced exchange with deuterium-rich cosmic ices.

The polycyclic aromatic hydrocarbon (PAH) coronene (C24H12) frozen in D2O ice in a ratio of less than 1 part in 500 rapidly exchanges its hydrogen atoms with the deuterium in the ice at interstellar temperatures and pressures when exposed to ultraviolet radiation. Exchange occurs via three different chemical processes: D atom addition, D atom exchange at oxidized edge sites, and D atom exchange at aromatic edge sites. Observed exchange rates for coronene (C24H12)-D2O and d12-coronene (C24D12)-H2O isotopic substitution experiments show that PAHs in interstellar ices could easily attain the D/H levels observed in meteorites. These results may have important consequences for the abundance of deuterium observed in aromatic materials in the interstellar medium and in meteorites. These exchange mechanisms produce deuteration in characteristic molecular locations on the PAHs that may distinguish them from previously postulated processes for D enrichment of PAHs.

The rate of the reaction between C2H and C2H2 at interstellar temperatures.

The reaction between the radical C2H and the stable hydrocarbon C2H2 is one of the simplest neutral-neutral hydrocarbon reactions in chemical models of dense interstellar clouds and carbon-rich circumstellar shells. Although known to be rapid at temperatures > or = 300 K, the reaction has yet to be studied at lower temperatures. We present here ab initio calculations of the potential surface for this reaction and dynamical calculations to determine its rate at low temperature. Despite a small potential barrier in the exit channel, the calculated rate is large, showing that this reaction and, most probably, more complex analogs contribute to the formation of complex organic molecules in low-temperature sources.

The rate of the reaction between CN and C2H2 at interstellar temperatures.

The rate coefficient for the important interstellar reaction between CN and C2H2 has been calculated as a function of temperature between 10 and 300 K. The potential surface for this reaction has been determined through ab initio quantum chemical techniques; the potential exhibits no barrier in the entrance channel but does show a small exit channel barrier, which lies below the energy of reactants. Phase-space calculations for the reaction dynamics, which take the exit channel barrier into account, show the same unusual temperature dependence as determined by experiment, in which the rate coefficient at first increases as the temperature is reduced below room temperature and then starts to decrease as the temperature drops below 50-100 K. The agreement between theory and experiment provides strong confirmation that the reaction occurs appreciably at cool interstellar temperatures.

Theoretical analysis of the rate constants for the interstellar reaction N+OH→NO+H

AbstractThe title reaction, a key elementary process involved in the chemistry of molecular clouds, has been theoretically studied over the 5–600 K temperature range. Rate constants calculations have been carried out using the full version of the statistical adiabatic channel model in conjunction with a potential energy surface that has been derived from recent ab initio quantum chemical data. By using various switching functions, the influence of the attenuation of the bound‐complex bending frequency upon NOH bond elongation on the temperature dependence of the reaction was investigated. The rate constants exhibit a slightly positive temperature dependence with a calculated rate constant value at 300 K in very good agreement with the measured value. A comparison with the available experimental data between 250 and 515 K suggests that recrossing trajectories might occur with increasing importance as the temperature increases. However, the nonstatistical recrossing effects are expected to be of minor importance at interstellar temperatures such that the rate constants over the 5–200 K temperature range are given by k = 8.41 × 10−12 T+0.30 cm3 molecule−1 s−1. The rate constant calculated at 10 K is consistent with that derived in the astrochemical modeling of the L134N dark cloud. Rate constants for individual quantum states are also presented. © 1995 John Wiley & Sons, Inc.

Infrared spectra of crystalline phase ices condensed on silicate smokes at T less than 20 K

Infrared spectra of H2O, CH3OH, and NH3 condensed at T less than 20 K on amorphous silicate smokes reveal that predominantly crystalline phase ice forms directly on deposit. Spectra of these molecules condensed on aluminum substrates at T less than 20 K indicate that amorphous phase ice forms. On aluminum, crystalline phase H2O and CH3OH are formed by annealing amorphous deposits to 155 K and 130 K, respectively (or by direct deposit at these temperatures); crystalline NH3 is formed by direct deposit at 88 K. Silicate smokes are deposited onto aluminum substrates by evaporation of SiO solid or by combustion of SiH4 with O2 in flowing H2 followed by vapor phase nucleation and growth. Silicate smokes which are oxygen-deficient may contain active surface sites which facilitate the amorphous-to-crystalline phase transition during condensation. Detailed experiments to understand the mechanism are currently in progress. The assumption that amorphous phase ice forms routinely on grains at T less than 80 K is often used in models describing the volatile content of comets or in interpretations of interstellar cloud temperatures. This assumption needs to be reexamined in view of these results.

Radiative and collisional association of mesitylene ion with parent neutral at 196 K

The dimerization of mesitylene (1,3,5-trimethylbenzene) ion was measured at 196 and 300 K in the ICR ion trap. Radiative association at 196 K had a rate constant 7×10 −12 cm 3 molecule −1 s −1. Analysis gave a collisional efficiency of 0.006, an IR photon emission rate of 29 s −1 and a complex redissociation rate of 5×10 3 s −1. The three-body association rate was 2.9×10 −22 cm 6 molecule −2 s −1 at 196 K and 9×10 −25 at 300 K. Mesitylene ion radiative association kinetics at 196 K should be a good analog for benzene ion association at interstellar cloud temperatures around 50 K.

Rate of the reaction: Formula at interstellar temperatures

Presentation des resultats des calculs des constantes de vitesse pour la reaction OH+CO→CO 2 +H

Study of He–H2CO collisions at interstellar temperatures using the L2 R-matrix method

Total cross sections for collisions of He(1S) atoms with H2CO molecules at interstellar temperatures (18–120 K) have been calculated using the L2 R-matrix method developed previously. Using 13 slightly nonorthogonal radial basis functions, excellent agreement has generally been obtained with an earlier close-coupled study. However, in two crucial regions (32.7 and 47.7 K) where strong resonances had been reported, we find that the cross sections are smooth. The accuracy of the present calculations were checked at intervals using the de Vogelaere method with parameters set very tightly. Strong resonances in the 20.2 K region were however found and characterized as being of Feshbach (compound state) type, with the He atom lying in the potential well near the O atom of formaldehyde. Similar resonances in the 127 K region are also predicted. Consequently the state-to-state rate constants, upon which the collision pump mechanism for the cooling of the k doublets of formaldehyde depends, now need to be recomputed using the new values for the cross sections. This series of studies shows that the use of slightly nonorthogonal radial basis functions—and which as a result possess arbitrary derivatives at the R-matrix boundary—is the key to the reliable application of this very stable and versatile method to molecular collision problems.

Ion-Molecule Reaction Studies below 80 K by the CRESU Technique

The basic principles of the CRESU technique (Cinétique de Réactions en Ecoulement Supersonique Uniforme) are presented. This technique allows ion-molecule reaction rate coefficients under true thermal conditions at interstellar temperatures. Various behaviors of both third-body association and binary reactions with temperature have been observed, including ion-polar molecule reactions whose rate coefficients sharply increase at very low temperatures.

On the Mechanism of H2 Formation in the Interstellar Medium

The problem of the formation of molecular hydrogen in interstellar clouds is revisited. the role played by cosmic ray bombardment under certain circumstances is considered mainly in the light of the low formation rate of H2 on grains due to the reduced mobility of adsorbed H atoms on their amorphous surfaces at interstellar temperatures.

A reanalysis of the HCO+/HOC+ abundance ratio in dense interstellar clouds.

New theoretical and experimental results have prompted a reinvestigation of the HCO+/HOC+ abundance ratio in dense interstellar clouds. These results pertain principally but not exclusively to the reaction between HOC+ and H2, which was previously calculated by DeFrees, McLean, and Herbst to possess a large activation energy barrier. New calculations, reported here, indicate that this activation energy barrier is quite small and may well be zero. In addition, experimental results at higher energy and temperature indicate strongly that the reaction proceeds efficiently at interstellar temperatures. If HOC+ does indeed react efficiently with H2 in interstellar clouds, the calculated HCO+/HOC+ abundance ratio rises to substantially greater value under standard dense cloud conditions than in deduced via the tentative observation of HOC+ in Sgr B2.