Collisional excitation of C 2 H(X 2 Σ + ) by para-H 2 ( j = 0): Fine-structure resolved transitions

New potential energy surface and rotational deexcitation cross-sections of CNNC by para-H2 (jp = 0)

The objective of this study is to enhance our understanding of the existence of molecules in interstellar space by determining the collisional rate coefficients with the most prevalent species. The study examines the impact of para-H2 collisions, specifically when it is in its ground vibrational state with a nuclear spin of para-H2, i.e., jp = 0, on causing the rotational deexcitation of the diisocyanogen (CNNC) molecule. These scattering data are obtained as a result of spherically averaging a four-dimensional potential energy surface (4DPES) over the H2 orientations. Using the CCSD(T)-F12a approach and aug-cc-pVTZ basis sets, the ab initio 4DPES for the CNNC-H2 van der Waals system is calculated. The CNNC-para-H2 4DPES attains a global minimum of 221.38 cm-1 at the CNNC and H2 center of mass distance (R) of 3.1 Å. The method of close coupling calculations is employed for the purpose of calculating the cross-sectional data of CNNC with para-H2 (jp = 0), for total energies up to 1000 cm-1. Rate coefficients are computed over the temperature range of 1 K to 100 K. Propensity suggests that even Δj transitions are strongly preferred. The rate coefficients for CNNC-H2 are determined to be 0.90-2.95 times those of CNNC-He, which implies it is not reliable to estimate the H2 rate coefficients by multiplying the rate coefficients for CNNC-He collision with a scaling factor of 1.38.

Collision excitation of c-C3H-(X1A1) by He

Accurate modeling of anionic abundances in the interstellar and circumstellar media requires calculations of collisional data with the most abundant species that are usually He atoms and H₂ molecules. In this paper, we focus on smaller cyclic molecular anion, c-C₃H^-, an astrophysical candidate, following the detection of larger C_nH^- carbon chains. From a new three-dimensional potential energy surface, the rotational (de-)excitation of the c-C₃H^-(X¹A₁) anion by collision with He is investigated. The surface is obtained in the supermolecular approach at the CCSD(T)-F12/aug-cc-pVTZ level of theory. Fully quantum close-coupling calculations of inelastic integral cross sections are performed on a grid of collisional energies large enough to ensure the convergence of the state-to-state rate coefficients for the 34 first rotational levels up to j_Ka,Kc = 7_7,0 of c-C₃H^- and temperatures ranging from 5 to 100 K. For this collisional system, rate coefficients exhibit a strong dominance in favor of 2_1,2 → l_1,1 downward transition. This transition was previously used for the detection of the cyclic parent c-C₃H. The c-C₃H^--He rate coefficients (∼10^-11 cm³ s^-1) are of the same order of magnitude as those of the detected anions C_nH^- (as C₂H^-, C₄H^-, and C₆H^-) in collision with He and one order of magnitude smaller than those with H₂. The critical densities of H₂ were also estimated, and a discussion on the validity of the local thermodynamic equilibrium conditions is carried out. This work represents the contribution to understanding and modeling abundances and chemistry of hydrocarbon radicals, C_nH, in astrophysical media.

Rotational excitation of C2H- anion in collision with H2

The discovery of anions in the interstellar medium has shown that they are very reactive species. This gave them great importance in the modeling of the chemical and astrophysical evolution of the interstellar medium. The detection of the first anion C6H- followed by the other anions C4H-, C8H- and CN- in the interstellar medium has encouraged research on other detectable anions. The C2H- anion was observed for the first time in the circumstellar envelope of IRC+10216 and in TMC-1. In these cold and low-density regions, precise modeling of the chemical and physical conditions of the observed emission lines requires knowledge of the radiative and collisional excitation rates. We present here the first new two-dimensional Potential Energy Surface (PES) for C2H-H2 interaction. Rotational excitation of the anion by collision with para-H2(j H2 = 0) is investigated. The PES is obtained in the super-molecular approach based on a single and double excitation coupled cluster method with perturbative contributions from triple excitations (CCSD(T)). In all our calculations, all atoms were described using the augmented correlation-consistent triple zeta (aug-cc-pVTZ) basis sets and bond f unctions. Fully-quantum close-coupling calculations of inelastic integral cross sections are done on a grid of collision energies large enough to ensure converged state-to-state rate coefficients for the 16 first rotational levels of C2H- and for temperatures ranging from 5 to 120 K. For this collisional system, rate coefficients exhibit a strong propensity in favor of even Δj transitions.

Interaction of C2H with molecular hydrogen: Ab initio potential energy surface and scattering calculations

The potential energy surface (PES) describing the interaction of the ethynyl (C₂H) radical in its ground X̃²Σ⁺ electronic state with molecular hydrogen has been computed through restricted coupled cluster calculations including single, double, and (perturbative) triple excitations [RCCSD(T)], with the assumption of fixed molecular geometries. The computed points were fit to an analytical form suitable for time-independent quantum scattering calculations of rotationally inelastic cross sections and rate constants. A representative set of energy dependent state-to-state cross sections is presented and discussed. The PES and cross sections for collisions of H₂(j = 0) are compared with a previous study [F. Najar et al., Chem. Phys. Lett. 614, 251 (2014)] of collisions of C₂H with H₂ treated as a spherical collision partner. Good agreement is found between the two sets of calculations when the H₂ molecule in the present calculation is spherically averaged.