The gas-phase chemistry of carbon chains in dark cloud chemical models

We review the reactions between carbon chain molecules and radicals, namely Cn, CnH, CnH2, C2n+1O, CnN, HC2n+1N, with C, N and O atoms. Rate constants and branching ratios for these processes have been re-evaluated using experimental and theoretical literature data. In total 8 new species have been introduced, 41 new reactions have been proposed and 122 rate coefficients from kida.uva.2011 (Wakelam et al. 2012) have been modified. We test the effect of the new rate constants and branching ratios on the predictions of gas-grain chemical models for dark cloud conditions using two different C/O elemental ratios. We show that the new rate constants produce large differences in the predicted abundances of carbon chains since the formation of long chains is less effective. The general agreement between the model predictions and observed abundances in the dark cloud TMC-1 (CP) is improved by the new network and we find that C/O ratios of 0.7 and 0.95 both produce a similar agreement for different times. The general agreement for L134N (N) is not significantly changed. The current work specifically highlights the importance of O + CnH and N + CnH reactions. As there are very few experimental or theoretical data for the rate constants of these reactions we highlight the need for experimental studies of the O + CnH and N + CnH reactions, particularly at low temperature.Comment: Accepted for publication in MNRA

Quantum Behavior of Spin-Orbit Inelastic Scattering of C-Atoms by D2 at Low Energy

Fine-structure populations and collision–induced energy transfer in atoms are of interest for many fields, from combustion to astrophysics. In particular, neutral carbon atoms are known to play a role in interstellar media, either as probes of physical conditions (ground state 3Pj spin-orbit populations), or as cooling agent (collisional excitation followed by radiative decay). This work aims at investigating the spin-orbit excitation of atomic carbon in its ground electronic state due to collisions with molecular deuterium, an isotopic variant of H2, the most abundant molecule in the interstellar medium. Spin-orbit excitations of C(3Pj) by H2 or D2 are governed by non-adiabatic and spin-orbit couplings, which make the theoretical treatment challenging, since the Born-Oppenheimer approximation no longer holds. Inelastic collisional cross-sections were determined for the C(3P0) + D2 → C(3Pj) + D2 (with j = 1 and 2) excitation process. Experimental data were acquired in a crossed beam experiment at low collision energies, down to the excitation thresholds (at 16.42 and 43.41 cm−1, respectively). C-atoms were produced mainly in their ground spin-orbit state, 3P0, by dissociation of CO in a dielectric discharge through an Even-Lavie pulsed valve. The C-atom beam was crossed with a D2 beam from a second valve. The state-to-state cross-sections were derived from the C(3Pj) (j = 1 or 2) signal measured as a function of the beam crossing angle, i.e., as a function of the collision energy. The results show different quantum behaviors for excitation to C(3P1) or C(3P2) when C(3P0) collides with ortho-D2 or normal-D2. These experimental results are analyzed and discussed in the light of highly accurate quantum calculations. A good agreement between experimental and theoretical results is found. The present data are compared with those obtained for the C-He and C-H2 collisional systems to get new insights into the dynamics of collision induced spin-orbit excitation/relaxation of atomic carbon

Kinetics and Dynamics of the S(^1D_2) + H_2 \to SH + H Reaction at Very Low Temperatures and Collision Energies

We report combined studies on the prototypical S(^1D_2) + H2 insertion reaction. Kinetics and crossed-beam experiments are performed in experimental conditions approaching the cold energy regime, yielding absolute rate coefficients down to 5.8 K and relative integral cross sections to collision energies as low as 0.68 meV. They are supported by quantum calculations on a potential energy surface treating long range interactions accurately. All results are consistent and the excitation function behavior is explained in terms of the cumulative contribution of various partial waves

Review of OCS gas-phase reactions in dark cloud chemical models

The association reaction S + CO {\to} OCS + hnu has been identified as being particularly important for the prediction of gas-phase OCS abundances by chemical models of dark clouds. We performed detailed ab-initio calculations for this process in addition to undertaking an extensive review of the neutral-neutral reactions involving this species which might be important in such environments. The rate constant for this association reaction was estimated to be several orders of magnitude smaller than the one present in current astrochemical databases. The new rate for this reaction and the introduction of other processes, notably OH + CS {\to} OCS + H and C + OCS {\to} CO + CS, dramatically changes the OCS gas-phase abundance predicted by chemical models for dark clouds. The disagreement with observations in TMC-1 (CP) and L134N (N), suggests that OCS may be formed on grain surfaces as is the case for methanol. The observation of solid OCS on interstellar ices supports this hypothesis.Comment: Accepted for publication in MNRA

Tunneling in the reaction of acetone with OH

Based on recent detailed quantum mechanical computations of the mechanism of the title reaction (Phys. Chem. Chem. Phys., 2003, 5, 333) and (J. Chem. Phys., 2003, 119, 10 600), this paper presents kinetics analysis of the overall rate constant and its temperature dependence, for which ample experimental data are available for comparison. The analysis confirms that the principal channel is the formation of acetonyl radical + H2O, while the channel leading to acetic acid is of negligible importance. It is shown that the unusual temperature dependence of the overall rate constant, as observed experimentally, is well accounted for by standard RRKM treatment that includes tunneling. This treatment is applied at the microcanonical level, with chemically activated distribution of entrance species, i.e. using a stationary rather than a thermal distribution that incorporates collisional energy transfer and competition between the redissociation and exit channel. A similar procedure is applied to the isotopic reaction acetone-d6 + OH with equally satisfying results, so that the experimental temperature dependence of the KIE (kinetic isotope effect) is perfectly reproduced. This very good agreement between calculation and experiment is obtained without any fitting to experimental values and without any adjustment of the parameters of calculation

Correction to “Probing Nonadiabatic Effects in Low-Energy C( 3 P j ) + H 2 Collisions”

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