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.

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Collisional excitation of methylene by molecular hydrogen

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stab2519 ◽

2021 ◽

Vol 508 (1) ◽

pp. 118-124

Author(s):

Paul J Dagdigian

Keyword(s):

Energy Surface ◽

Energy Levels ◽

Rate Coefficients ◽

Coupled Cluster ◽

Collisional Excitation ◽

Work Time ◽

Close Coupling ◽

Collisional Rate Coefficients ◽

Collisional Rate ◽

Cluster Level

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.

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

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa1086 ◽

2020 ◽

Vol 494 (4) ◽

pp. 5675-5681 ◽

Cited By ~ 1

Author(s):

Sanchit Chhabra ◽

T J Dhilip Kumar

Keyword(s):

Potential Energy ◽

Potential Energy Surface ◽

Ab Initio ◽

Cross Sections ◽

Energy Surface ◽

Molecular Ions ◽

Basis Set ◽

Rate Coefficients ◽

Close Coupling ◽

Collisional Rate Coefficients

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.

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Inelastic scattering of interstellar silyl cyanide (SiH3CN) by helium atoms : cross-sections and rate coefficients for A- and E-SiH3CN

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stab2451 ◽

2021 ◽

Vol 507 (4) ◽

pp. 5264-5271

Author(s):

Manel Naouai ◽

Abdelhak Jrad ◽

Ayda Badri ◽

Faouzi Najar

Keyword(s):

Inelastic Scattering ◽

Total Energy ◽

Cross Sections ◽

Three Dimensional ◽

Basis Set ◽

Rate Coefficients ◽

Close Coupling ◽

Explicitly Correlated ◽

Correlation Consistent ◽

Triple Excitation

ABSTRACT Rotational inelastic scattering of silyl cyanide (SiH3CN) molecule with helium (He) atoms is investigated. Three-dimensional potential energy surface (3D-PES) for the SiH3CN–He interacting system is carried out. The ab initio 3D-PES is computed using explicitly correlated coupled cluster approach with single, double, and perturbative triple excitation CCSD(T)-F12a connected to augmented-correlation consistent-polarized valence triple zeta Gaussian basis set. A global minimum at (R = 6.35 bohr; θ = 90○; ϕ = 60○) with a well depth of 52.99 cm−1 is pointed out. Inelastic rotational cross-sections are emphasized for the 22 first rotational levels for total energy up to 500 cm−1 via close coupling (CC) approach in the case of A-SiH3CN and for the 24 first rotational levels for total energy up to 100 cm−1 via CC and from 100 to 500 cm−1 via coupled states (CS) in the case of E-SiH3CN. Rate coefficients are derived for temperature until 80 K for both A- and E-SiH3CN–He systems. Propensity rules are obtained for |ΔJ| = 2 processes with broken parity for A-SiH3CN and for |ΔJ| = 2 processes with |ΔK| = 0 and unbroken parity for E-SiH3CN.

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Collisional excitation of H2S by molecular hydrogen

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa1091 ◽

2020 ◽

Vol 494 (4) ◽

pp. 5239-5243

Author(s):

Paul J Dagdigian

Keyword(s):

Energy Surface ◽

Rate Coefficients ◽

Cluster Method ◽

Quantum Scattering ◽

Collisional Excitation ◽

Close Coupling ◽

Accurate Knowledge ◽

Explicitly Correlated ◽

Collisional Rate Coefficients ◽

Collisional Rate

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.

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Hyperfine excitation of NS+ due to para-H2(j = 0) impact

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stz1700 ◽

2019 ◽

Vol 487 (4) ◽

pp. 5685-5691 ◽

Cited By ~ 1

Author(s):

Cheikh T Bop

Keyword(s):

Cross Sections ◽

Basis Set ◽

Rate Coefficients ◽

Cluster Method ◽

Close Coupling ◽

Nitrogenous Compounds ◽

Deep Well ◽

Explicitly Correlated ◽

Correlation Consistent ◽

Triple Excitation

ABSTRACT Sulphur bearing nitrogenous compounds have been observed in space over this last decade. Modelling their abundances has been done using rate coefficients of isoelectronic molecules. In order to satisfy the astrophysical precision required, we report the actual rate coefficients of NS+ induced by collision with the most abundant interstellar species (para-H2). Considering the 23 low-lying rotational levels of NS+, we were able to compute the (hyperfine) rate coefficients up to 100 K. These latter were carried out by averaging cross-sections over the Maxwell–Boltzmann velocity distribution. The state-to-state inelastic cross-sections were determined in the quantum mechanical close coupling approach for total energies ranging up to 1400 cm−1. These dynamic data result from a four dimensional potential energy surface (4D-PES) which was spherically averaged over the H2 orientations. The 4D-PES was calculated using the explicitly correlated coupled cluster method with simple, double, and non-iterative triple excitation (CCSD(T)–F12) connected to the augmented–correlation consistent–polarized valence triple zeta Gaussian basis set (aug–cc–pVTZ). The so-averaged PES presents a very deep well of 596.72 cm−1 at R = 5.94 a0 and θ1 = 123.20°. Discussions on the propensity rules for the (hyperfine) rate coefficients were made and they are in favour of (Δj = ΔF) Δj = 1 transitions. The results presented here may be crucially needed in order to accurately model the NS+ abundance in space. In addition, we expect that this paper will encourage investigations on the sulphur bearing nitrogenous compounds.

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