Initial state-selected total reaction probabilities and integral reaction cross sections (ICSs) of, C(
3
P) + OH (X
2
Π) → CO(a
3
Π) + H (
2
S), reaction on its second excited electronic state (1
4
...A″) are calculated with the aid of a time-dependent wave packet propagation method within the coupled states approximation. Partial wave contributions for the total angular momentum quantum number, J=0-48, were necessary to obtain converged ICSs for OH (v=0, j=0) upto a collision energy of ∼0.25 eV. In case of rotationally and vibrationally excited OH, many more partial wave contributions had to be included. Dense oscillatory structures are found in total reaction probabilities due to the formation of quasibound collision complexes inside the wells (of depth ∼2.25 eV and ∼1.85 eV) present on the potential energy surface. While reagent vibrational excitation promotes the reaction, no such general trend is found with rotational excitation. The results of the present study are compared with the literature data. Mechanistic details of the C + OH reaction occurring on its ground, first and second excited electronic states are compared and discussed.
Context. Dark cloud chemical models usually predict large amounts of O2, often above observational limits. Aims. We investigate the reason for this discrepancy from a theoretical point of view, ...inspired by the studies of Jenkins and Whittet on oxygen depletion. Methods. We use the gas-grain code Nautilus with an up-to-date gas-phase network to study the sensitivity of the molecular oxygen abundance to the oxygen elemental abundance. We use the rate coefficient for the reaction O + OH at 10 K recommended by the KIDA (KInetic Database for Astrochemistry) experts. Results. The updates of rate coefficients and branching ratios of the reactions of our gas-phase chemical network, especially N + CN and H\hbox{$_3^+$}+3 + O, have changed the model sensitivity to the oxygen elemental abundance. In addition, the gas-phase abundances calculated with our gas-grain model are less sensitive to the elemental C/O ratio than those computed with a pure gas-phase model. The grain surface chemistry plays the role of a buffer absorbing most of the extra carbon. Finally, to reproduce the low abundance of molecular oxygen observed in dark clouds at all times, we need an oxygen elemental abundance smaller than 1.6 × 10-4. Conclusions. The chemistry of molecular oxygen in dense clouds is quite sensitive to model parameters that are not necessarily well constrained. That O2 abundance may be sensitive to nitrogen chemistry is an indication of the complexity of interstellar chemistry.
The dynamics of the D+ + H2 → HD + H+ reaction on a recent ab initio potential energy surface (Velilla, L.; Lepetit, B.; Aguado, A.; Beswick, J. A.; Paniagua, M. J. Chem. Phys. 2008, 129, 084307) has ...been investigated by means of a time-independent quantum mechanical approach. Cross-sections and rate coefficients are calculated, respectively, for collision energies below 0.1 eV and temperatures up to 100 K for astrophysical application. An excellent accord is found for collision energy above 5 meV, while a disagreement between theory and experiment is observed below this energy. We show that the rate coefficients reveal a slightly temperature-dependent behavior in the upper part of the temperature range considered here. This is in agreement with the experimental data above 80 K, which give a temperature independent value. However, a significant decrease is found at temperatures below 20 K. This decrease can be related to quantum effects and the decay back to the reactant channel, which are not considered by simple statistical approaches, such as the Langevin model. Our results have been fitted to appropriate analytical expressions in order to be used in astrochemical and cosmological models.
We report extensive, accurate fully quantum, time-independent calculations of cross sections at low collision energies, and rate coefficients at low temperatures for the H⁺ + H₂(v = 0, j) → H⁺ + H₂(v ...= 0, j') reaction. Different transitions are considered, especially the ortho-para conversion (j = 1 → j' = 0) which is of key importance in astrophysics. This conversion process appears to be very efficient and dominant at low temperature, with a rate coefficient of 4.15 × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹ at 10 K. The quantum mechanical results are also compared with statistical quantum predictions and the reaction is found to be statistical in the low temperature regime (T < 100 K).
Abstract
We present a detailed theoretical study of the rotational excitation of CH+ due to reactive and non-reactive collisions involving C+(2P), H2, CH+, H and free electrons. Specifically, the ...formation of CH+ proceeds through the reaction between C+(2P) and H$_2(\nu _{\rm H_2} = 1, 2)$, while the collisional (de)excitation and destruction of CH+ is due to collisions with hydrogen atoms and free electrons. State-to-state and initial-state-specific rate coefficients are computed in the kinetic temperature range 10–3000 K for the inelastic, exchange, abstraction and dissociative recombination processes using accurate potential energy surfaces and the best scattering methods. Good agreement, within a factor of 2, is found between the experimental and theoretical thermal rate coefficients, except for the reaction of CH+ with H atoms at kinetic temperatures below 50 K. The full set of collisional and chemical data are then implemented in a radiative transfer model. Our non-LTE calculations confirm that the formation pumping due to vibrationally excited H2 has a substantial effect on the excitation of CH+ in photon-dominated regions. In addition, we are able to reproduce, within error bars, the far-infrared observations of CH+ towards the Orion Bar and the planetary nebula NGC 7027. Our results further suggest that the population of $\nu _{\rm H_2} = 2$ might be significant in the photon-dominated region of NGC 7027.
Quantum state-selected dynamics of C((3)P) + OH (X(2)Π) → CO(a(3)Π) + H ((2)S) reaction on its first excited electronic potential energy surface (1(2)A(")) is examined here using a time-dependent ...wave packet propagation approach. All partial wave contributions for the total angular momentum, J = 0-95, are included to obtain the converged cross sections and initial state-selected rate constants in the temperature range of 10-500 K. The reaction probability, as a function of collision energy, exhibits dense oscillatory structures owing to the formation of resonances during collision. These resonance structures also persist in reaction cross sections. The effect of reagent rotational and vibrational excitation on the dynamical attributes is examined and discussed. Reagent rotational excitation decreases the reactivity whereas, vibrational excitation of the reagent has minor effects on the reactivity. The results presented here are in good accord with those obtained using the time-independent quantum mechanical and quasi-classical trajectory methods.
Initial state-selected dynamics of the S(3P) + OH (X2Π) → SO (X3Σ–) + H (2S) reaction on its electronic ground potential energy surface (X̃2A″) is investigated here by a time-dependent wave packet ...propagation (TDWP) approach. Total reaction probabilities for the three-body rotational angular momentum up to J = 138 are calculated to obtain converged integral reaction cross sections and state-specific rate constants employing the centrifugal sudden (CS) approximation. The convergence of the latter quantities is checked by varying all parameters used in the numerical calculations. The cross section and rate constant results are compared with those available in the literature, calculated with the aid of the quasi-classical trajectory method on the same potential energy surface. Reaction probabilities obtained with the TDWP approach exhibit dense oscillatory structures, implying formation of a metastable quasi-bound complex during the collision process. The effect of rotational and vibrational excitations of reagent OH on the dynamical attributes is also examined. While the rotational excitation of reagent OH decreases the reactivity, its vibrational excitation enhances the same.
Total and state-to-state probabilities have been determined for the N + NO → N2 + O reaction for collision energies up to 0.6 eV using a time-independent quantum mechanical method. The probabilities ...as a function of collision energy show broad oscillations, in strong contrast with previous theoretical results obtained by means of a time-dependent wave packet method that show a dense resonance structure. The rate constant has been calculated in the J-shifting approach for temperatures between 10 and 400 K. It is in good agreement with previous theoretical results obtained only at 100 K and above 200 K and experiments in a wide temperature range.
We have studied the quantum dynamics of the N + OH → NO + H reaction for collision energies up to 0.7 eV. The hyperspherical method has been used in a time-independent formalism. State-to-state ...reaction probabilities for a total angular momentum J = 0 have been computed. The results show a high reactivity below 0.45 eV and a very small one above this collision energy. Rotational and vibrational product distributions are presented for three collision energies (0.05, 0.1, and 0.5 eV). The vibrational distributions are found to be noninverted at 0.1 eV and inverted peaking at other energies. Rotational distributions are rather hot even if some low rotational states are strongly populated. These features are consistent with both direct and indirect reaction mechanisms.
Accurate three-dimensional quantum-mechanical scattering calculations using a time-indepedent hyperspherical method have been performed for the C(3 P) + OH(X2Π) → CO(a3Π) + H(2 S) reaction on the ...second excited potential energy surface of 14A″ symmetry. State-to-state reaction probabilities at a total angular momentum J = 0 have been computed in a wide range of collision energies. Many pronounced resonances have been found, espcially at low energy. The product vibrational distributions are noninverted. The present results therefore suggest that the title reaction proceeds via a long-lived intermediate complex. An approximate quantum-mechanical rate constant has also been calculated, and large differences are observed with the quasi-classical trajectory prediction.
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