Collisional excitation of methylene by molecular hydrogen

ABSTRACT Accurate estimates of the abundance of methylene (CH2) in the interstellar medium require knowledge of both the radiative and collisional rate coefficients for the transfer of population between rotational levels. In this work, time-independent quantum close coupling calculations have been carried out to compute rate coefficients for the (de-)excitation of ortho- and para-CH2 in collisions with ortho- and para-H2. These scattering calculations have employed a recently computed, high-quality potential energy surface, based on the coupled cluster level of theory [RCCSD(T)-F12a], for the interaction of CH2 in its ground $\tilde{X} ^3B_1$ electronic state with H2. The collisional rate coefficients were obtained for all fine-structure transitions among the first 22 and 24 energy levels of ortho- and para-CH2, respectively, having energies less than 277 cm−1. These rate coefficients are compared with previous calculated values, obtained by scaling data for CH2–He. In the case of ortho-CH2, whose levels display hyperfine structure, rate coefficients for transitions between hyperfine levels were also computed, by the MJ randomization approximation. Finally, some simple radiative transfer calculations are presented.

Rotational (de)-excitation of C5 by collision with He at low temperature

An appropriate estimation of the abundance of the observed C5 radical in the interstellar medium requires accurate radiative and collisional rate coefficients.

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.

Fine and hyperfine excitation of CCN by He

ABSTRACT The relative abundance of cyanide and isocyanide molecules is a sensitive tracer of the physical conditions in the interstellar and circumstellar media. Accurate modelling of collisional and radiative processes implying these species opens the way for accurate estimation of their abundances. The present paper focuses on the computation of collisional rate coefficients for fine and hyperfine (de-)excitation of the CCN molecule (one of the C2N isomers) in collision with He, for temperatures up to 150 K. Using a time independent coupled-channel approach, scattering calculations were performed for transitions implying the lowest 56 fine structure levels and the corresponding 166 hyperfine structure levels belonging to both 2Π1/2 and 2Π3/2 spin-orbit manifolds. We provided, for the first time, realistic collisional data for the CCN radical. Propensity rules for fine and hyperfine transitions are discussed. As a first application, we evaluated the impact of these new data on the astrophysical modelling through radiative transfer calculations. We obtained the brightness and excitation temperatures of selected lines observed towards circumstellar envelopes and we found that local thermodynamic equilibrium conditions are not fulfilled for this species. As a consequence, our results indicate that the abundance of the CCN derived from the observations has to be revised since the observation modelling strongly depends on the collisional data used.

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.

Non-Maxwellian rate coefficients for electron and ion collisions in Rydberg plasmas: implications for excitation and ionization

Scattering phenomena between charged particles and highly excited Rydberg atoms are of critical importance in many processes in plasma physics and astrophysics. While a Maxwell–Boltzmann (MB) energy distribution for the charged particles is often assumed for calculations of collisional rate coefficients, in this contribution we relax this assumption and use two different energy distributions, a bimodal MB distribution and a $\unicode[STIX]{x1D705}$ -distribution. Both variants share a high-energy tails occurring with higher probability than the corresponding MB distribution. The high-energy tail may significantly affect rate coefficients for various processes. We focus the analysis to specific situations by showing the dependence of the rate coefficients on the principal quantum number of hydrogen atoms in $n$ -changing collisions with electrons in the excitation and ionization channels and in a temperature range relevant to the divertor region of a tokamak device. We finally discuss the implications for diagnostics of laboratory plasmas.

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.

The Leiden Atomic and Molecular Database (LAMDA): Current Status, Recent Updates, and Future Plans

The Leiden Atomic and Molecular Database (LAMDA) collects spectroscopic information and collisional rate coefficients for molecules, atoms, and ions of astrophysical and astrochemical interest. We describe the developments of the database since its inception in 2005, and outline our plans for the near future. Such a database is constrained both by the nature of its uses and by the availability of accurate data: we suggest ways to improve the synergies among users and suppliers of data. We summarize some recent developments in computation of collisional cross sections and rate coefficients. We consider atomic and molecular data that are needed to support astrophysics and astrochemistry with upcoming instruments that operate in the mid- and far-infrared parts of the spectrum.

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.

Relaxation of NO+ by collision with para-H2 (j = 0)

ABSTRACT The first tentative detection of the nitrosylium ion (NO+) in the interstellar medium (ISM) was reported just a few years ago. The application of non-local thermal equilibrium models requires the knowledge of the collisional rate coefficients with the most common colliders in the ISM (e.g. He, H, H2, and e). The main goals of this paper are to study the collision of the NO+ molecule with para-H2 (j = 0) and report the rate coefficients for the lower rotational states of NO+. A large set of ab initio energies was computed at the CCSD(T)/aug-cc-pV5Z level of theory. A new potential energy surface averaged over the H2 orientations was then fitted using a reproducing kernel Hilbert space procedure. The state-to-state cross-sections of NO++para-H2 (j = 0) for the first 18 rotational levels were computed using the close-coupling method. The rotational rate coefficients of this system were compared with those for NO++He, and a different propensity rule was found. Furthermore, the hyperfine rate coefficients were also calculated using the infinite-order-sudden scaling procedure.