Rate Coefficients For Temperatures Research Articles

Collisional excitation of propyne (CH3CCH) by He atoms

Context. A detailed interpretation of the detected emission lines of environments where propyne (or methyl acetylene, CH3CCH) is observed requires access to its collisional rate coefficients with the most abundant species in the interstellar medium, namely, helium (He) or molecular hydrogen (H2). Aims. We present the first three-dimensional potential energy surface (3D PES) for the CH3CCH-He molecular complex. We study the dynamics of the collision and report the first set of rate coefficients for temperatures up to 100 K for the collisional excitation of the lowest 60 ortho rotational levels and 60 para rotational levels of CH3CCH by He atoms. Methods. We computed the 3D PES with the explicitly correlated coupled-cluster with a single-, double-, and perturbative triple-excitation method in conjunction with the augmented correlation-consistent triple zeta basis set (CCSD(T)-F12a/aug-cc-pVTZ). The 3D PES was then fitted to an analytical function and scattering computations of pure rotational (de-)excitation of CH3CCH by collision with He atoms were performed. State-to-state cross-sections were computed using the close coupling method for total energies up to 100 cm−1 and with the coupled states approximation at higher energies for both the ortho- and para- symmetries of CH3CCH. Results. The PES we obtained is characterised by a large anisotropy and a potential well depth of 51.04 cm−1. By thermally averaging the collisional cross-sections, we determined the quenching rate coefficients for kinetic temperatures up to 100 K. A strong and even Δj propensity rule at almost all collision energies is present for CH3CCH-He complex. To evaluate the impact of rate coefficients on the analysis of observations, we carried out non-local thermodynamic equilibrium radiative transfer computations of the excitation temperatures and we demonstrate that local thermodynamic equilibrium conditions are not typically fulfilled for the propyne molecule.

Rotational excitation of methanol in collisions with molecular hydrogen

ABSTRACT This paper presents rate coefficients for transitions between rotational levels of the A-type and E-type nuclear spin modifications of methanol induced by collisions with molecular hydrogen. These rate coefficients are required for an accurate determination of methanol abundance in the interstellar medium, where local thermodynamic equilibrium conditions generally do not apply. Time-independent close-coupling quantum scattering calculations have been employed to calculate cross-sections and rate coefficients for the (de-)excitation of methanol in collisions with para- and ortho-H2. These calculations utilized a potential energy surface (PES) for the interaction of methanol with H2 recently computed by the explicitly correlated CCSD(T)-F12a coupled-cluster method that employed a correlation-consistent aug-cc-pVTZ basis. Rate coefficients for temperatures ranging from 3 to 250 K were calculated for all transitions among the first 76 rotational levels of both A-type and E-type methanol, whose energies are less than or equal to 170 K. These rate coefficients are compared with those by Rabli and Flower who carried out coupled-state calculations using a PES computed by second-order many-body perturbation theory. Simple radiative transfer calculations using the present set of rate coefficients are also reported and compared with such calculations using the rate coefficients previously computed by Rabli and Flower.

Collision excitation of nitrous acid (HONO) by helium: isomerization effect

ABSTRACTWe generated new 3D-potential energy surfaces (3D-PESs) for the cis-HONO–He and trans-HONO–He weakly bound complexes along the intermonomer coordinates. We used the explicitly correlated Coupled Clusters with single, double, and perturbative triple excitations (CCSD(T)-F12) approach for the electronic structure computations, where the atoms were described using the aug-cc-pVTZ basis set. Then, we derived analytical forms for each PES. These PESs exhibit different shapes and present strong anisotropies. After quantum close-coupling scattering calculations for the lower rotational levels (up to ${9}_{2,7}$), and the coupled-states approximation for higher levels (up to ${22}_{1,22}$) using these PESs, we derived the collisional excitation cross sections of cis-HONO and trans-HONO by He for total energies 0.1 ≤ E ≤ 900 cm−1 and the rate coefficients for kinetic temperatures T ≤ 100 K. Our work shows that the collision data of cis-HONO and trans-HONO are different mainly because of the different 3D-PESs since the rotational energy structures of both isomers are very similar. Also, computations show that the data of the non-detected cis-HONO are as large as those of the detected trans-HONO isomer. They confirm the large values for the detected transition 52,4 → 41,3 of trans-HONO. Therefore, our work strongly suggests revisiting radiative transfer calculations to determine accurately the population of the rotational levels of these isomers. Our work should help astrophysicists for the detectability of such nitrogen oxide molecules and for the possible formation mechanisms and isomerization pathways specificities.

Collisional excitation of HCNH+ by He and H2: New potential energy surfaces and inelastic rate coefficients.

Protonated molecules have been increasingly detected in the interstellar medium (ISM), and usually astrochemical models fail at reproducing the abundances derived from observational spectra. Rigorous interpretation of the detected interstellar emission lines requires prior calculations of collisional rate coefficients with H2 and He, i.e., the most abundant species in the ISM. In this work, we focus on the excitation of HCNH+ induced by collision with H2 and He. Therefore, we first calculate ab initio potential energy surfaces (PESs) using the explicitly correlated and standard coupled cluster method with single, double, and non-iterative triple excitation in conjunction with the augmented-correlation consistent-polarized valence triple zeta basis set. Both the HCNH+-H2 and HCNH+-He potentials are characterized by deep global minima of 1426.60 and 271.72 cm-1, respectively, and large anisotropies. From these PESs, we derive state-to-state inelastic cross sections for the 16 low-lying rotational energy levels of HCNH+ using the quantum mechanical close-coupling approach. The differences between cross sections due to ortho- and para-H2 impacts turn out to be minor. Using a thermal average of these data, we retrieve downward rate coefficients for kinetic temperatures of up to 100K. As it could be anticipated, differences of up to two orders of magnitude exist between the rate coefficients induced by H2 and He collisions. We expect that our new collision data will help to improve the disagreement between abundances retrieved from observational spectra and astrochemical models.

The excitation of CO in CO-dominated cometary comae

ABSTRACTAn abundance of CO significantly surpassing the abundance of H2O is observed in the comae of comets at large heliocentric distances. In these environments, CO molecules can be the most abundant species and they may be therefore the dominant projectiles inducing collisional excitation of the cometary molecules. It is thus of high interest to investigate the excitation of CO by CO. This article provides a new set of CO–CO collisional rate coefficients for temperatures up to 150 K and for CO rotational levels j1 up to 10. These data are obtained from quantum scattering calculations using the coupled states approximation. They are used in a simple radiative transfer model in order to test their impact on the excitation of cometary CO. Because mutual (de-)excitations of the target and projectile are important, the CO projectile was assumed to be thermalized at the kinetic temperature. We found that the non-local thermodynamical equilibrium regime extends for CO densities in the range 103–107 cm−3. We also observed that as soon as the CO/H2O ratio is larger than 70 per cent/30 per cent, the contribution of H2O collisions can be neglected. Similarly, the excitation of CO by CO may be ignored for relatively low CO/H2O density ratios (≤30 per cent/70 per cent). Finally, when the coma is a ∼50 per cent/50 per cent mixture of CO and H2O, the contribution of both colliders is similar and has to be considered.

Reconciling spectroscopy with dynamics in global potential energy surfaces: The case of the astrophysically relevant SiC2.

SiC2 is a fascinating molecule due to its unusual bonding and astrophysical importance. In this work, we report the first global potential energy surface (PES) for ground-state SiC2 using the combined-hyperbolic-inverse-power-representation method and accurate ab initio energies. The calibration grid data are obtained via a general dual-level protocol developed afresh herein that entails both coupled-cluster and multi-reference configuration interaction energies jointly extrapolated to the complete basis set limit. Such an approach is specially devised to recover much of the spectroscopy from the PES, while still permitting a proper fragmentation of the system to allow for reaction dynamics studies. Besides describing accurately the valence strongly bound region that includes both the cyclic global minimum and isomerization barriers, the final analytic PES form is shown to properly reproduce dissociation energies, diatomic potentials, and long-range interactions at all asymptotic channels, in addition to naturally reflect the correct permutational symmetry of the potential. Bound vibrational state calculations have been carried out, unveiling an excellent match of the available experimental data on c-SiC2(A11). To further exploit the global nature of the PES, exploratory quasi-classical trajectory calculations for the endothermic C2 + Si → SiC + C reaction are also performed, yielding thermalized rate coefficients for temperatures up to 5000K. The results hint for the prominence of this reaction in the innermost layers of the circumstellar envelopes around carbon-rich stars, hence conceivably playing therein a key contribution to the gas-phase formation of SiC, and eventually, solid SiC dust.

Formation of CO through C (2s 22p 2 3P) and O (2s 22p 4 3P) Radiative Association

The formation of CO through the radiative association of the carbon (C, 2s 22p 2 3P) and oxygen (O, 2s 22p 4 3P) atoms is investigated. The corresponding cross sections and rate coefficients for temperatures T = 10–10,000 K are calculated using the quantum-mechanical approach based on ab initio potential energy curves, permanent dipole moments, and transition dipole moments, which are obtained by the internally contracted multi-reference configuration interaction method with the Davidson correction and aug-cc-pwCV5Z-DK basis set. All dipole-allowed transitions between singlet, triplet, and quintet states converging to the C (2s 22p 2 3P) + O (2s 22p 4 3P) dissociation limit are considered. Compared to the previous results that only contain the X1Σ+ → X1Σ+, A1Π → X1Σ+, and B1Σ+ → X1Σ+ transitions, our results suggest that the a′3Σ+ → a3Π and d3Δ → a3Π transitions make significant contributions to the radiative association for T = 10–30 K. The total rate coefficient at low temperatures is estimated to be about 10−18 cm3 s−1, which shows significant deviation from the previous results, where only three transitions were considered. New rate coefficients may improve the chemical modeling of CO in the low-density region of the interstellar medium.

Rotational excitation of NS+ by H2 revisited: A new global potential energy surface and rate coefficients.

Due to the lack of specific collisional data, the abundance of NS+ in cold dense interstellar clouds was determined using collisional rate coefficients of CS as a substitute. To better understand the chemistry of sulfur in the interstellar medium, further abundance modeling using the actual NS+ collisional rate coefficients is needed. For this purpose, we have computed the first full 4D potential energy surface of the NS+-H2 van der Waals complex using the explicitly correlated coupled cluster approach with single, double, and non-iterative triple excitation in conjunction with the augmented-correlation consistent-polarized valence triple zeta basis set. The potential energy surface exhibits a global minimum of 848.24 cm-1 for a planar configuration of the complex. The long-range interaction energy, described using multipolar moments, is sensitive to the orientation of H2 up to radial distances of ∼50 a0. From this new interaction potential, we derived excitation cross sections, induced by collision with ortho- and para-H2, for the 15 low-lying rotational levels of NS+ using the quantum mechanical close-coupling approach. By thermally averaging these data, we determined downward rate coefficients for temperatures up to 50K. By comparing them with the previous NS+-H2 data, we demonstrated that reduced dimensional approaches are not suited for this system. In addition, we found that the CS collisional data underestimate our results by up to an order of magnitude. The differences clearly indicate that the abundance of NS+, in cold dense clouds retrieved from observational spectra, must be reassessed using these new collisional rate coefficients.

Rotational (de)-excitation of HMgNC in collision with He

Abstract Potential energy surface for the collision between HMgNC and He are evaluated using the high-accuracy CCSD(T)-F12 ab-initio method. This PES leads to conduct quantum dynamical scattering calculations of the rotational and hyperfine (de)-excitations rate coefficients for temperatures up to 200 K. In order to evaluate the impact of the computed data, we performed radiative transfer computations of the brightness and excitation temperatures for selected observed j = 8 to 7 transition.

State-to-state inelastic rate coefficients of phosphine in collision with He at low to moderate temperature

ABSTRACTSeveral phosphorus-bearing molecules, such as the phosphine of interest here, have been detected in astrophysical media. With the aim of satisfying the precision required by the astrophysical community, we report the rate coefficients of PH3 in collision with helium from low to moderate temperature. To this end, we constructed the first three-dimensional potential energy surface (3D-PES) of the PH3–He van der Waals complex, which governs the nuclear motions. The 3D-PES was worked out by means of the standard coupled cluster with single, double and non-iterative triple excitation approach, in conjunction with the aug-cc-pVQZ basis set and complemented by mid-bond functions. This 3D-PES presents a well of 34.92 cm−1 at {R, θ, Φ} = {5.76 a0, 90°, 60°}. Afterwards, we incorporated this 3D-PES into time-independent close-coupling quantum dynamical computations to derive the inelastic cross-sections of rotational excitation of (ortho-) para-PH3 after collision with He up to (1000) 500 cm−1. Subsequently, we evaluated the rate coefficients for temperatures up to (100 K) 50 K populating the (41) 42 low-lying rotational levels of (ortho-) para-PH3. These data were derived by averaging the cross-sections thermally over the Maxwell–Boltzmann velocity distribution. No general propensity rules are found. We also performed a comparison with the rates for NH3–He. Differences are observed that invalidate the use of NH3 rates for deducing accurate abundances of phosphine in cold astrophysical media. Our results should be of great help in determining accurate PH3 abundances and, more generally, constraining the interstellar PH3 chemistry better.

Collisional excitation of the formyl radical (HCO) by molecular hydrogen

ABSTRACT This paper addresses the need for accurate rate coefficients for transitions between fine- and hyperfine-structure resolved rotational transitions in the formyl (HCO) radical induced by collisions with the two nuclear spin modifications of H2, the dominant molecule in the interstellar medium (ISM). These rate coefficients, as well as radiative transition rate coefficients, are required for accurate determination of the abundance of HCO in the ISM. Time-independent close-coupling quantum scattering calculations have been used to compute rate coefficients for (de-)excitation of HCO in collisions with para- and ortho-H2. These calculations utilized a potential energy surface for the interaction of HCO with H2 recently computed by the explicitly correlated RCCSD(T)-F12a coupled-cluster method. Rate coefficients for temperatures ranging from 5 to 400 K were calculated for all transitions among the fine and hyperfine levels associated with the first 22 rotational levels of HCO, whose energies are less than or equal to 144 K.

Rotational de-excitations of C3H+ (1Σ+) by collision with He: new ab initio potential energy surface and scattering calculations

ABSTRACT Molecular ions play an important role in the astrochemistry of interstellar and circumstellar media. C3H+ has been identified in the interstellar medium recently. A new potential energy surface of the C3H+–He van der Waals complex is computed using the ab initio explicitly correlated coupled cluster with the single, double and perturbative triple excitation [CCSD(T)-F12] method and the augmented correlation consistent polarized valence triple zeta (aug-cc-pVTZ) basis set. The potential presents a well of 174.6 cm−1 in linear geometry towards the H end. Calculations of pure rotational excitation cross-sections of C3H+ by He are carried out using the exact quantum mechanical close-coupling approach. Cross-sections for transitions among the rotational levels of C3H+ are computed for energies up to 600 cm−1. The cross-sections are used to obtain the collisional rate coefficients for temperatures T ≤ 100 K. Along with laboratory experiments, the results obtained in this work may be very useful for astrophysical applications to understand hydrocarbon chemistry.

Collisional excitation of H2S by molecular hydrogen

ABSTRACT Accurate estimates of the abundance of H2S, and inferences about the unmeasured H2 density, require accurate knowledge of radiative and collisional rate coefficients. Time-independent close-coupling quantum scattering calculations have been employed to compute rate coefficients for (de-)excitation of para- and ortho-H2S in collisions with para- and ortho-H2. These calculations utilized a potential energy surface for the interaction of H2S with H2 recently computed by the explicitly correlated CCSD(T)-F12a coupled-cluster method. Rate coefficients for temperatures ranging from 5 to 500 K were calculated for all transitions among the first 19 rotational levels of H2S, whose energies are less than or equal to 405 K. These rate coefficients are compared with previous estimates of these quantities.

Collisional energy transfer in the HeH+-H reactive system.

The HeH+ molecule is the first to be formed in the Universe. Its recent detection, in the interstellar medium, has increased the interest in the study of the physical and chemical properties of this ion. Here, we report exact quantum time-independent calculations of the collisional cross sections and rate coefficients for the rotational excitation of HeH+ by H. Reactive and exchange channels are taken into account in the scattering calculations. Cross sections are computed for energies of up to 10 000 cm-1, enabling the computation of rate coefficients for temperatures of up to 500 K. The strongest collision-induced rotational HeH+ transitions are those with Δj = 1. Previous results obtained using approximate treatment are compared to the new ones, and significant differences are found. The new rate coefficients are also compared to those for electron-impact rotational excitation, and we found that collisions with H dominate the excitation of HeH+ in media where the electron fraction is less than 10-4. In the light of those results, we recommend the use of the new HeH+-H collisional data in order to accurately model HeH+ excitation in both the interstellar media and early Universe.

The excitation of NH2 in the interstellar medium

ABSTRACT Accurate estimation of the abundance of the NH2 radical in the interstellar medium requires accurate radiative and collisional rate coefficients. The calculation of hyperfine-resolved rate coefficients for the collisional (de-)excitation of NH2 by both ortho- and para-H2 is presented in this work. Hyperfine-resolved rate coefficients are calculated from pure rotational close-coupling rate coefficients using the Mj randomizing approximation. Rate coefficients for temperatures ranging from 5 to 150 K were computed for all hyperfine transitions among the first 15 rotational energy levels of both ortho- and para-NH2 in collisions with ortho- and para-H2. The new data were then employed in radiative transfer calculations to simulate the excitation of NH2 in typical star-forming regions such as W31C, where NH2 is seen in emission. We compared the excitation and brightness temperatures for different NH2 transitions obtained using the new and the previously available collisional data. It is found that the new rate coefficients increase the simulated line intensities by a factor ∼10–30. As a consequence, NH2 abundance derived from the observations will be significantly reduced by the use of the present rate coefficients.

Hyperfine excitation of CH and OH radicals by He

Context. Because of their high reactivity, the CH and OH radicals are of particular interest in astrochemistry. Modeling of CH and OH molecules requires the calculation of accurate radiative Einstein coefficients and rate coefficients for (de)excitation by collisions with the most abundant species such as H2 and He. Aims. The present paper focuses on the calculation of inelastic rate coefficients among the lowest OH/CH hyperfine levels in their ground vibrational state induced by collisions with He atoms. Methods. Calculations of hyperfine (de)excitation of CH/OH by He were performed using the close-coupling and recoupling methods from the most recent ab initio potential energy surfaces. Results. Cross sections for transitions among the 60 and 56 lowest hyperfine levels of CH and OH, respectively, were calculated for collision energies up to 2500 cm−1. These cross-sections were used to calculate the rate coefficients for temperatures between 10 and 300 K. A propensity rule for transitions with ΔF = Δj was observed. Conclusions. The new rate coefficients will help significantly in interpreting the CH/OH spectroscopic data observed with current and future telescopes, and help to accurately describe the OH masers and the hyperfine anomalies in CH emission spectra.

Quantum mechanical study of the high-temperature H+ + HD → D+ + H2 reaction for the primordial universe chemistry

ABSTRACT We use the time-independent quantum-mechanical formulation of reactive collisions in order to investigate the state-to-state H+ + HD → D+ + H2 chemical reaction. We compute cross-sections for collision energies up to 1.8 eV and rate coefficients for temperatures up to 10 000 K. We consider HD in the lowest vibrational level v = 0 and rotational levels j = 0–6, and H2 in vibrational levels v′ = 0–3 and rotational levels j′ = 0–9. For temperatures below 4000 K, the rate coefficients strongly vary with the initial rotational level j, depending on whether the reaction is endothermic (j ≤ 2) or exothermic (j ≥ 3). The reaction is also found less and less probable as the final vibrational quantum number v′ increases. Our results illustrate the importance of studying state-to-state reactions, in the context of the chemistry of the primordial universe.

Hyperfine excitation of SH+ in collisions with para- and ortho-H2

ABSTRACT Time-independent close-coupling quantum scattering calculations are employed to compute hyperfine-resolved rate coefficients for (de-)excitation of SH+ in collisions with para- and ortho-H2. These calculations utilized a potential energy surface for the interaction of SH+(X3Σ−) with H2 recently computed by the explicitly correlated RCCSD(T)-F12a coupled-cluster method. Rate coefficients for temperatures ranging from 10 to 500 K were calculated for all transitions among the first 37 hyperfine energy levels of SH+, with rotational angular momenta n ≤ 6, in collisions with para- and ortho-H2. As a first application of these data, the rate coefficients were employed in simple radiative transfer calculations to simulate the excitation of SH+ in typical molecular clouds.

Collisional excitation of NH(3Σ-) by Ar: A new ab initio 3D potential energy surface and scattering calculations.

Collisional excitation of light hydrides is important to fully understand the complex chemical and physical processes of atmospheric and astrophysical environments. Here, we focus on the NH(X3Σ-)-Ar van der Waals system. First, we have calculated a new three-dimensional Potential Energy Surface (PES), which explicitly includes the NH bond vibration. We have carried out the ab initio calculations of the PES employing the open-shell single- and double-excitation couple cluster method with noniterative perturbational treatment of the triple excitations. To achieve a better accuracy, we have first obtained the energies using the augmented correlation-consistent aug-cc-pVXZ (X = T, Q, 5) basis sets and then we have extrapolated the final values to the complete basis set limit. We have also studied the collisional excitation of NH(X3Σ-)-Ar at the close-coupling level, employing our new PES. We calculated collisional excitation cross sections of the fine-structure levels of NH by Ar for energies up to 3000 cm-1. After thermal average of the cross sections, we have then obtained the rate coefficients for temperatures up to 350 K. The propensity rules between the fine-structure levels are in good agreement with those of similar collisional systems, even though they are not as strong and pronounced as for lighter systems, such as NH-He. The final theoretical values are also compared with the few available experimental data.

Rotational excitation of sodium hydroxide molecule (NaOH) by helium (He) at low temperature

This paper reports the rotational excitation of the alkali sodium hydroxide molecule () by collision with helium atom. We present a new 2D potential energy surface (PES) for NaOH–He van der Waals complex. The PES was computed at the coupled cluster level of theory with single, double and perturbative triple excitation (CCSD(T)) using the aug-cc-pVQZ basis sets for the four atoms with bound functions. A global minimum of −100.15 cm−1 is found at linear geometry He–NaOH and a local minimum of −21.94 cm−1 is located at linear geometry He–HONa. The scattering calculations were performed, via close coupling approach, for a grid of total energies up to 1500 cm−1 to ensure converged state-to-state rate coefficients for kinetic temperatures ranging from 5 K up to 300 K among the first 13 rotational levels. Propensity rules that favor Δj = ±1 transitions are found. These new rates have great potential for NaOH detection in the interstellar medium.