On the formation of small molecules by radiative association

Abstract

This thesis is devoted to the theoretical investigation of radiative association, which is one of the processes contributing to formation of molecules in the interstellar medium. The formation of the CN, SiN, SiP and CO molecules through radiative association of the corresponding atoms in their ground electronic states is studied by the means of classical and quantum dynamical calculations. In all cases the Born–Oppenheimer approximation is employed. The corresponding rate coefficients are calculated and the Breit–Wigner theory is used to properly account for the resonance contributions. Some common features of the radiative association process for the considered systems are discovered. For example, a drop in magnitude of the cross sections at high energy and, consequently, the high-temperature rate coefficients is observed. Also, is is shown that in the absence of a potential barrier the semi-classical formalism provides a good estimate for the rate coefficients. A pronounced isotope effect is discovered for the formation of CO by radiative association of C(3P) and O(3P) atoms. It is shown that the presence of a potential barrier on the A1Π electronic state of carbon monoxide leads to different formation channels for the 12CO and 13CO isotopologues at low temperatures. The role of spin-orbit and rotational couplings in radiative association of C(3P) and N(4S) atoms is investigated. Couplings among doublet electronic states of the CN radical are considered, giving rise to a 6-state model of the process. Comparison of the energy-dependent rate coefficients calculated with and without spin-orbit and rotational couplings shows that these have a strong effect on the resonance structure and low-energy baseline of the rate coefficient.