De-excitation Rate Coefficients Research Articles

Rate coefficients for the rotational de-excitation of OCS by collision with He

Context. In typical molecular clouds, analyzing the physicochemical conditions with nonlocal thermodynamic equilibrium models requires knowledge of the collisional rate coefficients between the detected molecule and the most common colliders in the interstellar medium (ISM); for example H2, He, and H. The OCS molecule has been widely observed in the ISM. However, the collision data available for this species were calculated using a potential energy surface (PES) that shows differences with surfaces presented recently. Aims. The main goal of this work is to report a new set of rate coefficients for the collision of OCS with He based on a new PES computed at a high level of theory. Methods. We developed an analytical PES using a large set of ab initio energies calculated using the coupled cluster with single, double, and perturbative triple excitations (CCSD(T)) method at the completed basis set limit for the OCS+He complex. We used this surface in close-coupling calculations, and computed a new set of collisional rate coefficients for OCS and He. Results. We compare the de-excitation rate coefficients with previously available data. Furthermore, we observe a |Δj| = 1 propensity rule. Finally, we report a set of rate coefficients for the lower 39 rotational states of OCS, which is the largest set determined to date for this collision.

An improved artificial neural network fit of the ab initio potential energy surface points for HeH+ + H2 and its ensuing rigid rotors quantum dynamics

Artificial neural networks (ANN) have been shown for the last several years to be a versatile tool for fitting ab initio potential energy surfaces. We have demonstrated recently how a 60-neuron ANN could successfully fit a four-dimensional ab initio potential energy surface for the rigid rotor HeH+ - rigid rotor H2 system with a root-mean-squared deviation (RMSD) of 35 cm−1. We show in the present study how a (40, 40) neural network with two hidden layers could achieve a better fit with an RMSD of 5 cm−1. Through a follow-up quantum dynamical study of HeH+(j1)-H2(j2) collisions, it is shown that the two fits lead to slightly different rotational excitation and de-excitation cross sections but are comparable to each other in terms of magnitude and dependence on the relative translational energy of the collision partners. When averaged over relative translational energy, the two sets of results lead to rate coefficients that are nearly indistinguishable at higher temperatures thus demonstrating the reliability of the ANN method for fitting ab initio potential energy surfaces. On the other hand, we also find that the de-excitation rate coefficients obtained using the two different ANN fits differ significantly from each other at low temperatures. The consequences of these findings are discussed in our conclusions.

Atomic data on inelastic processes in boron–hydrogen collisions with accounting for fine structure

ABSTRACT This paper reports the results of investigations of inelastic processes in B + H and B+ + H− collisions with accounting for the fine structure. The calculations of the cross-sections and rate coefficients for excitation, de-excitation, ion-pair formation, and mutual neutralization processes (240 in total) are performed by using the quantum asymptotic method, which is modified in this paper for accounting of the fine structure, and by the multichannel approach within the framework of the Born–Oppenheimer formalism. It is found that accounting for the fine structure may change the rate coefficients significantly, and the rates calculated with accounting for the fine structure cannot be obtained by a simple redistribution of the non-relativistic rates. The processes important for non-local thermodynamic equilibrium modelling of stellar atmospheres are designated.

New de-excitation rates of the open shell dicarbon monoxide C2O(X3Σ−) in collision with He(1S)

Abstract In order to derive accurate physical conditions of the interstellar clouds (like TMC-1) from the rotational transitions of C2O, collisional rates are needed. Based on a new two-dimensional potential energy surface (2D-PES), we report a new rate coefficients calculation of C2O(X3Σ−) induced by collision with He. This PES was obtained from the explicitly correlated restricted open-shell coupled cluster with single, double and perturbative triple excitation (rccsd(t)-f12) ab initio approach associated with aug-cc-pVTZ basis sets. The C2O–He PES presents one minimum below its dissociation limit with a well depth of 42.376 cm−1. The close-coupling quantum time independent formalism was used to derive cross sections for E ≤ 370 cm−1 and N ≤ 10. De-excitation rate coefficients are obtained for thermal temperature below 80 K.

Scattering calculation of the newly observed pentacarbon monoxide (C5O) with He atom: de-excitation rates and radiative transfer

ABSTRACT To determine the chemical composition of gases in molecular clouds, the oxygen-bearing systems CnO are needed as probe elements. The pentacarbon monoxide C5O was recently detected in TMC-1, and in order to derive accurate physical conditions from its rotational transitions, calculation of rate coefficients of C5O(1Σ+) induced by collision with He are performed for thermal temperature below 100 K. These calculations are based on a new 2D potential energy surface (2D-PES) obtained from the explicit correlated coupled cluster with single, double, and pertubative triple excitation (ccsd(t)-f12) ab initio approach associated with aug-cc-pVTZ basis sets. The C5O–He PES presents two minima below its dissociation limit with a well depths of −59.321 and −53.059 cm−1. By mean of this PES, the integral cross sections are calculated in the close-coupling quantum time independant formalism for $E\le 500 \, {\rm cm}^{-1}$ and J ≤ 20. The de-excitation rate coefficients are obtained after averaging these cross sections at low temperatures. We expect that the new collisional data will allow accurate determination of the C5O abundance in the interstellar medium, as well as the interpretation of its emission lines. These new data are crucial to understand the chemistry of carbon chain species in the interstellar gas.

State-to-state rate coefficients for HCS+ in rotationally inelastic collisions with H2 at low temperatures

ABSTRACT HCS+ ions have been detected in several regions of the interstellar medium (ISM), but an accurate determination of the chemical-physical conditions in the molecular clouds where this molecule is observed requires detailed knowledge of the collisional rate coefficients with the most common colliders in those environments. In this work, we study the dynamics of rotationally inelastic collisions of HCS+ + H2 at low temperature, and report, for the first time, a set of rate coefficients for this system. We used a recently developed potential energy surface for the HCS+–H2 van der Waals complex and computed state-to-state rotational rate coefficients for the lower rotational states of HCS+ in collision with both para- and ortho-H2, analysing the influence of the computed rate coefficients on the determination of critical densities. Additionally, the computed rate coefficients are compared with those obtained by scaling the ones from HCS+ in collision with He (an approximation that is sometimes used when data is lacking), and large differences are found. Furthermore, the approximation of using the rates for the HCO+ + H2 collision as a rough approximation for those of the HCS+ + H2 system is also evaluated. Finally, the complete set of de-excitation rate coefficients for the lowest 30 rotational states of HCS+ by collision with H2 is reported from 5 to 100 K.

Deexcitation rate coefficients of C3 by collision with H2 at low temperatures

Context. An accurate analysis of the physical-chemical conditions in the regions of the interstellar medium in which C3 is observed requires knowing the collisional rate coefficients of this molecule with He, H2, electrons, and H. Aims. The main goals of this study are to present the first potential energy surface for the C3 +H2 complex, to study the dynamics of the system, and to report a set of rate coefficients at low temperature for the lower rotational states of C3 with para- and ortho-H2. Methods. A large grid of ab initio energies was computed at the explicitly correlated coupled-cluster with single-, double-, and perturbative triple-excitation level of theory, together with the augmented correlation-consistent quadruple zeta basis set (CCSD(T)-F12a/aug-cc-pVQZ). This grid of energies was fit to an analytical function. The potential energy surface was employed in close-coupling calculations at low collisional energies. Results. We present a high-level four-dimensional potential energy surface (PES) for studying the collision of C3 with H2. The global minimum of the surface is found in the linear HH-CCC configuration. Rotational deexcitation state-to-state cross sections of C3 by collision with para- and ortho-H2 are computed. Furthermore, a reduced two-dimensional surface is developed by averaging the surface over the orientation of H2. The cross sections for the collision with para-H2 using this approximation and those from the four-dimensional PES agree excellently. Finally, a set of rotational rate coefficients for the collision of C3 with para- and ortho-H2 at low temperatures are reported.

Electronic structure calculations and quantum dynamics of rotational deexcitation of CNNC by He.

The quantum dynamics of rotational transitions of the diisocyanogen (CNNC) molecule undergoing collision with the helium (He) atom occurring in the interstellar medium (ISM) has been studied. The rotational deexcitation cross sections are extracted by first computing an ab initio potential energy surface of CNNC-He using the coupled-cluster with single and double and perturbative triple excitations with the F12a method (CCSD(T)-F12a) employing the aug-cc-pVTZ basis set. Utilizing the multipole expansions, collisional cross sections are determined for total energies of up to 1000 cm-1 by the close coupling equations. The discussion on propensity rules suggests that the transitions have even Δj values, while odd Δj valued transitions are forbidden due to C and N nuclei spin statistics. Quasi-bound states present in the CNNC-He van der Waals complex resulted in the resonances coming from the rapid oscillations in the values of the cross sections in the region of low energy. Rotational deexcitation rate coefficients are further worked out by averaging the calculated cross sections at temperatures below 200 K. The new findings of the study will be beneficial in modeling the abundance of diisocyanogen in the ISM.

Inelastic, exchange, and reactive processes in rovibrationally excited collisions of HD with H

ABSTRACT The HD molecule is an important coolant in early universe chemistry models and a tracer of H2 in star-forming regions. Rate coefficients for collisional excitation and de-excitation of HD rotational and vibrational levels form important ingredients in astrophysical models. While collisions with He, H2, and H are the most important, available data for H + HD collisions are largely limited to temperatures less than 1000 K for the vibrational ground state, low-lying rotational levels of the v = 1 HD vibrational level, or computed without reactive contributions. Here, through explicit quantum scattering calculations, we report extensive data for rovibrational transitions in HD induced by H atoms for a range of rotational levels in v = 1 and some v = 0 levels for temperatures up to 1000 K. The significance of the computed results for astrophysical modeling is discussed.

Inelastic rate coefficients for collisions of C4H− with H2

ABSTRACT Carbon-chain anions were recently detected in the interstellar medium. These very reactive species are used as tracers of the physical and chemical conditions in a variety of astrophysical environments. However, the local thermodynamic equilibrium conditions are generally not fulfilled in these environments. Therefore, collisional as well as radiative rates are needed to accurately model the observed emission lines. We determine in this work the state-to-state rate coefficients of C4H− in collision with both ortho- and para-H2. A new ab initio 4D potential energy surface was computed using explicitly correlated coupled-cluster procedures. This surface was then employed to determine rotational excitation and de-excitation cross-sections and rate coefficients for the first 21 rotational levels (up to rotational level j1 = 20) using the close-coupling method, while the coupled-state approximation was used to extend the calculations up to j1 = 30. State-to-state rate coefficients were obtained for the temperature range 2–100 K. The differences between the ortho- and para-H2 rate coefficients are found to be small.

Vibrational excitation of N2O by an electron impact and the role of the Renner-Teller effect in the process

Cross sections and rate coefficients for vibrational excitation and de-excitation of the ${\mathrm{N}}_{2}\mathrm{O}$ molecule by a low-energy electron for transitions between the lowest vibrational levels are computed using a first-principles approach. The present theoretical approach employs the normal-mode approximation for the description of target vibrational states, the vibrational frame transformation to compute amplitudes of vibrational transitions, and the $R$-matrix method to compute ab initio electronic bound and continuum states. It was found that the nonadiabatic Renner-Teller effect, which couples partial waves of the incident electron with degenerate bending vibrations of ${\mathrm{N}}_{2}\mathrm{O}$, is responsible for the excitation of the bending mode. Theoretical results obtained agree reasonably well with available experimental data at low energies. Thermally averaged rate coefficients are computed for temperatures in the 10--10 000 K range.

New H2O–H2O collisional rate coefficients for cometary applications

ABSTRACT We re-introduce a semiclassical methodology based on theories developed for the determination of broadening coefficients. We show that this simple and extremely fast methodology provides results that are in good agreement with results obtained using the more sophisticate MQCT approach. This semiclassical methodology could be an alternative approach which allows to provide large sets of collisional data for very complex molecular systems. It saves time both on the determination of potential energy surfaces and on the collisional dynamical calculations. In addition, this paper provides more complete sets of rotational de-excitation cross-sections and rate coefficients of H2O perturbed by a thermal average of water molecules. Those data can be used in the radiative transfer modelling of cometary atmospheres.

Rotational relaxation of HCO+ and DCO+ by collision with H2

ABSTRACT The HCO+ and DCO+ molecules are commonly used as tracers in the interstellar medium. Therefore, accurate rotational rate coefficients of these systems with He and H2 are crucial in non-local thermal equilibrium models. We determine in this work the rotational de-excitation rate coefficients of HCO+ in collision with both para- and ortho-H2, and also analyse the isotopic effects by studying the case of DCO+. A new four-dimensional potential energy surface from ab initio calculations was developed for the HCO+–H2 system, and adapted to the DCO+–H2 case. These surfaces are then employed in close-coupling calculations to determine the rotational de-excitation cross-sections and rate coefficients for the lower rotational states of HCO+ and DCO+. The new rate coefficients for HCO+ + para-H2 were compared with the available data, and a set of rate coefficients for HCO+ + ortho-H2 is also reported. The difference between the collision rates with ortho- and para-H2 is found to be small. These calculations confirm that the use of the rate coefficients for HCO+ + para-H2 for estimating those for HCO+ + ortho-H2 as well as for DCO+ + para-H2 is a good approximation.

Rotational relaxation of H2S by collision with He

Context. The H2S molecule has been detected in several regions of the interstellar medium (ISM). The use of non-LTE models requires knowledge of accurate collisional rate coefficients of the molecules detected with the most common collider in the ISM. Aims. The main goal of this work is to study the collision of H2S with He. Methods. A grid of ab initio energies was computed at the coupled cluster level of theory including single, double, and perturbative triple excitations (CCSD(T)) and using the augmented correlation consistent polarized quadruple zeta (aug-cc-pVQZ) basis set supplemented by a set of mid-bond functions. These energies were fitted to an analytical function, which was employed to study the dynamics of the system. Close coupling calculations were performed to study the collision of H2S with He. Results. The rate coefficients determined from the close coupling calculation were compared with those of the collision with H2O+He, and large differences were found. Finally, the rate coefficients for the lower rotational de-excitation of H2S by collision with He are reported.

First quantum study of the rotational excitation of HCN by para-H2O: Convergence of quantum results, influence of the potential energy surface, and approximate rate coefficients of interest for cometary atmospheres.

The rotational excitation of HCN by H2O, the main perturber in cometary atmospheres, is investigated using quantum methodologies. We provide approximate rotational de-excitation rate coefficients among the first levels of HCN perturbed by thermalized para-water in the temperature range T=5K to T=150K. Because of the novelty of the system for quantum rotational excitation, the current study includes a detailed appreciation of the parameters involved in the convergence of the cross-sections and of rate coefficients calculations. A compromise on the convergence of the rate coefficients with respect to the rotational basis set is taken because of the computing time cost. Moreover, because of the cost also involved in calculating the 5D potential energy surfaces necessary for the current dynamical calculations, several potential energy surfaces of increased quality are tested and it is shown that, within the current approximations on the collisional calculations, average quality potential energy surfaces are sufficient for cometary applications.

Low-energy collisions between electrons and hydrogen fluoride cations

We have studied slow electron collisions with hydrogen fluoride cations in the framework of multi-channel quantum defect theory. Assuming energy-independent interactions, the short-range reaction matrix has been evaluated in the second order of the Born expansion of the Lippmann–Schwinger equation. The isotopic effects on the cross sections and thermal rate coefficients for dissociative recombination, vibrational excitation and de-excitation have been investigated.

Inelastic processes in low-energy iron-hydrogen collisions

The simplified model approach is applied to low-energy collisions of iron atoms and cations with hydrogen atoms and anions.Inelastic collisional processes for all transitions involving 97 low-lying covalent Fe + H states and two ionic Fe+ + H- molecular states are treated, and rate coefficients for excitation, de-excitation, ion-pair formation and neutralization processes are calculated within the Σ+6Π6,Δ6,Σ-4,Π4,Δ4 and 4Φ molecular symmetries.Rate coefficients with highest values correspond to the mutual neutralization processes Fe+(3d64s 6D) + H-→ Fe(3d6(3G)4s4p(3P°) w5G°, v5F°, y5H°) + H(1s2S) and Fe+(3d74F) + H-→ Fe(3d7(2G) 4p w3F°, y3H°, v3G°) + H(1s 2S). The highest rate coefficients for excitation and de-excitation correspond to the Fe(3d6(3G)4s4p(3P°) v5F°) + H(1s2S)→ Fe(3d6(3G)4s4p(3P°) y5H°) + H(1s 2S) process within the sextet molecular symmetries and the Fe(3d7(2D2)4p v3F°) + H(1s2S) → Fe(3d7(2G)4p v3G°) + H(1s 2S) process within the quartet molecular symmetries.

Monitoring the removal of excited particles in He/Ar/H2low temperature afterglow plasma at 80–300 K

Stationary afterglow (SA) experiments with cavity ring down absorption spectrometer (cw-CRDS) have been used to study recombination of H 3 + ions with electrons. To characterize the plasma during the afterglow we monitored the time evolution of density of He 2 excited dimers ( a 3 Ʃ u + ) in plasmas in pure helium and in helium with small admixture of Ar and H 2 . By monitoring the plasma decay and its dependence on [H 2 ] and [Ar] in the afterglow in pure He and in He/Ar/H 2 mixture we estimated the rate of plasma thermalization in the temperature range of 80–300 K. The inferred deexcitation rate coefficients for reaction of helium metastable atoms with H 2 were (0.9 ± 0.3) × 10 −10 cm 3 s −1 , (1.9 ± 0.2) × 10 −10 cm 3 s −1 and (2.7 ± 0.2) × 10 −10 cm 3 s −1 at 80 K, 140 K and 300 K, respectively. The effective recombination rate coefficients for H 3 + were evaluated from the decay of the electron number density. We propose the lower estimate for the saturation of the effective recombination rate coefficient in H 3 + and D 3 + dominated plasma.

Ultracold rotational deexcitation of CO (1Σ+) collision with proton

Ultracold rotational deexcitation cross sections and rate coefficients have been computed for CO collision with proton using close-coupling method on the two-dimensional ground state potential energy surface computed at the MRCI/cc-pVTZ level of theory. State-to-state rate coefficients have been obtained for temperature range from 10−5K to 200K. The system displays higher propensity with j′=0 cross section magnitude closer to j′=1 state, and j′=2 cross section closer to j′=3 state, from initial j=4 state. In ultracold region, the magnitude of cross section increases with the decrease in collision energy following Wigner’s threshold law.

QUANTUM CALCULATION OF INELASTIC CO COLLISIONS WITH H. III. RATE COEFFICIENTS FOR RO-VIBRATIONAL TRANSITIONS

We present calculated rate coefficients for ro-vibrational transitions of CO in collisions with H atoms for a gas temperature range of 10 K $\leq T \leq$ 3000 K, based on the recent three-dimensional ab initio H-CO interaction potential of Song et al(2013). Rate coefficients for ro-vibrational $v=1,j=0-30 \rightarrow v'=0, j'$ transitions were obtained from scattering cross sections previously computed with the close-coupling method by Song et al(2015). Combining these with the rate coefficients for vibrational $v=1-5 \rightarrow v' < v$ quenching obtained with the infinite-order sudden approximation, we propose a new extrapolation scheme that yields the rate coefficients for ro-vibrational $v=2-5,j=0-30 \rightarrow v',j'$ de-excitation. Cross sections and rate coefficients for ro-vibrational $v=2, j=0-30 \rightarrow v'=1,j'$ transitions calculated with the close-coupling method confirm the effectiveness of this extrapolation scheme. Our calculated and extrapolated rates are very different from those that have been adopted in the modeling of many astrophysical environments. The current work provides the most comprehensive and accurate set of ro-vibrational de-excitation rate coefficients for the astrophysical modeling of the H-CO collision system. Application of the previously available and new data sets in astrophysical slab models shows that the line fluxes typically change by 20-70% in high temperature environments (800 K) with an H/H$_2$ ratio of 1; larger changes occur for lower temperatures.