Potential energy surfaces and quantum dynamics of cold and ultracold collisions of H+, H, and He with C3 and NCCN

Abstract

Interstellar chemistry is primarily organic in nature. Around three-fourths of the nearly 200 molecules detected to date in the interstellar medium (ISM) contain at least one carbon atom. Carbon chain and small hydrocarbon molecules play an important role in the astrochemistry of the interstellar and circumstellar medium, leading to the formation of large organic molecules. Because of their importance, several communities are working on experimental and theoretical astrochemistry to determine the spectroscopic properties and collision rates. Observation and study of interstellar molecular clouds requires the knowledge of molecular data to derive the physical conditions of these media. These regions are generally not at the local thermodynamic equilibrium. Modeling of molecular emission spectra of these species from interstellar clouds requires the calculation of rate coe cients for excitations by collisions with H+, H and He. For calculating collisional rate coe cients, the rst step is to de ne the potential energy surface (PES) of the colliding system. The present thesis focuses on the quantum dynamics of interstellar molecules, C3 and NCCN, beginning from the construction of the PES of the electronic states, to dynamical studies of collisions occurring on these systems. New accurate PESs for the C3-H+, C3-H, C3H+-He, NCCN-He and NCCN-H+ species are generated with quantum chemistry methods. For C3-H+ and C3-H species, ground state and low-lying excited states are computed using the multireference con- guration interaction method including Davidson's correction and augmented correlation consistent polarized valence quadruple zeta basis sets. Nonadiabatic e ects leading to avoided crossings are observed between the ground and excited states po tential energy curves in C3 collision with H+, and H. Demkov coupling is observed between the ground and rst excited states while Landau-Zener coupling is observed between the rst and second excited states. For other collision systems, the ground state surfaces are computed using coupled-cluster methods. To perform the dynamical calculations, the ground state PESs are tted in terms of Legendre polynomials. The time-independent quantum dynamics is employed to study the collisional systems. Inelastic cross-sections for the rotational excitations of all the studied systems have been computed with a close-coupling method. The cross-sections are then used to calculate the collisional rate coe cients at cold temperatures, and collisional propensity rules are discussed. Cooling molecules to ultracold temperatures are important due to their applications in controlled chemical reactions and ultrahigh-resolution molecular spectroscopy. In the ultracold study, rotational quenching cross-sections and rate coe cients are computed for ultracold rotational transitions of NCCN induced by collision with 3;4He. The inelastic quenching cross-sections are found to obey Wigner's threshold laws. The isotopic e ect of He is analyzed by computing the scattering lengths and lifetime of quasi bound states. The present results will bene t future experimental studies of these or similar systems to cool and trap neutral molecules.