Potential energy surface and rotational energy transfers in CNNC and NCCP in collision with He and para-H2

Abstract

In the past century, our understanding of the interstellar medium (ISM) has greatly evolved. Initially thought of as empty space between stars, modern astrochemistry has revealed a complex web of physical and chemical processes in various interstellar environments. Around 300 molecules have been detected in interstellar space with the help of spectroscopy so far that are dominated by Hy and He. Because of their importance, several communities are working on experimental and theoretical astrochemistry to determine the spectroscopic properties. In ISM, the density is typically low, and collisions occur rarely, making it challenging to maintain a local thermodynamic equilibrium. Consequently, it becomes important to calculate the collisional rate with key collisional partners like Hy and He. For calculating collisional rate coefficients, the first step is to define the potential energy surface (PES) of the colliding system. The work deals with the interaction of interstellar molecules CNNC and NCCP with He and para-H,. New accurate ab initio PESs for CNNC-He, CNNC-H,, NCCPHe and NCCP-H, are generated using electronic structure calculations involving coupled-cluster singles and doubles with perturbative triples with F12 approximation (CCSD(T)-F12) and augmented correlation consistent polarized valence triple zeta (aVTZ) basis sets. To perform the dynamical calculations, the ground state PESs are expanded in terms of Legendre polynomials. The time-independent approach of quantum dynamics is employed to study the collisional systems. Inelastic cross-sections for the rotational de-excitations of all the studied systems have been computed with a close-coupling method. The cross-sections are then thermally averaged to calculate the collisional rate coefficients at interstellar temperatures. Only even Aj transitions are allowed in the case of CNNC collisions, whereas both even and odd Aj transitions are present for NCCP. NCCP-para-H, rates are determined to be 1.5-4.5 times of NCCP-He. A tremendous increase in the research on ion-molecule interactions is attributed to their involvement in the synthesis of larger molecules in interstellar clouds. These reactions become important as compared to neutral species interactions due to the long-range electrostatic interactions occurring between ion and atom. In this context, the interaction of ionic molecule COH" in collision with He is studied considering its possibility to form bound states. COH*-He has a global minimum of 836.55 cm !and He approaches from H-side of COH* in stable configuration. Inelastic and pressure broadening (PB) cross-sections are calculated and compared for COH™-He, resulting in some extra peaks in PB cross-sections. The obtained rotational rate coefficients are consistent with the previously reported data for COH*-He. Cooling molecules to ultracold temperatures has produced a new research area of ultracold chemistry due to their applications in controlled chemical reactions and ultrahigh-resolution molecular spectroscopy. In the ultracold study, rotational quenching cross-sections and rate coefficients are computed for Cy in collision with Co-*4He. The quenching cross-sections are found to obey Wigner’s threshold laws. The isotopic effect of He is analyzed by computing the scattering lengths and lifetime of quasi bound states. Quenching rate coefficients suggest that C, can be cooled with “He buffer gas.