AB INITIO STUDIES OF ELECTRONIC STRUCTURE AND PROPERTIES OF DIATOMIC MOLECULES CONTAINING ALKALINE-EARTH ATOMS

Abstract

With recent advancements in experimental techniques, the study of diatomic molecules at cold and ultracold temperatures has attracted considerable recognition in various areas of research such as controlled chemical reactions, quantum computing, quantum phase transitions, fundamental interactions, fundamental physical constants, etc. In this context, the theoretical knowledge of numerous electronic and vibrational properties of diatomic molecules plays an important role in the design and development of various such experiments. In this thesis, we have investigated singly charged alkaline - earth lithides (AELi+): BeLi+, MgLi+ and CaLi+, which have recently been considered for the examination of elastic - and inelastic processes occurring during the ion - atom collisions and for predicting the feasibility of formation of these ions via photoassociation processes [1–4]. We have performed all electron calculations for the potential energy curves (PECs), spectroscopic constants: equilibrium bond lengths (Re), dissociation energies (De), harmonic frequencies (ωe), anharmonic frequencies (ωexe) and rotational constants (Be and αe), and the molecular properties: dipole moments (μ0), quadrupole moments ( zz), components of dipole polarizabilities (αk, α⊥, ¯α, γ) and dipole polarizabilities at the asymptotic limit (α100) for the ground state of these molecular ions i ii using higher - order correlation methods and large optimized basis sets. The results of quadrupole moments and dipole polarizabilities are reported for the first time in our work, to the best of our knowledge. The errors due to the size of basis sets and correlation methods are also estimated for all diatomic constants and molecular properties of ground electronic states. Further, the effect of diffuse functions on molecular properties has also been investigated. It has been found that the inclusion of relativistic effects alters the electronic energies for the ground states of the molecular ions considered in this work noticeably. However, the difference between the results of diatomic constants calculated with the relativistic and the non - relativistic methods is small and lies within the estimated error bars at the non - relativistic level of theory. In addition, by using the PECs and permanent dipole moment (PDM) curves of ground electronic states, we have calculated the vibrational parameters: energies, wavefunctions, rotational constants and transition dipole moments (TDMs) between the vibrational levels. The lifetimes of vibrational states are then computed using relative vibrational energy spacings and vibrational TDMs. The lifetimes of ro - vibrational ground states are found to be 4.67 s for BeLi+, 2.81 s for MgLi+, and 3.19 s for CaLi+. The lifetime of highest vibrational level of the ground electronic state of MgLi+ (CaLi+) is calculated to be 19.3 s (2.38 s) which is much larger than (comparable to) the lowest vibrational level. We have also investigated the low- lying excited states of these ions for their PECs, diatomic constants and PDM curves. Further, the transition energies (Te) and the TDMs for the transitions from the ground to the excited electronic states are reported. We have made detailed comparison of our calculations for electronic and vibrational properties of AELi+ with the results available in the literature and emphasized on the high accuracy attained in our work. Since very few calculations, particularly, for the excited states are reported in the literature, our all - electron calculations computed with large active space could stand as benchmarks for the theoretical studies on these iii molecular ions in future. Another sub - species of diatomic molecules, viz., the heavier members of alkaline - earth monofluorides (AEMFs), BaF and RaF, are the potential candidates for probing the nuclear anapole moment that is of immense interest in contemporary physics [5–7]. In this connection, we have studied the molecular properties such as PDMs, components of dipole polarizability (DP), parity (P) and time - reversal (T ) - odd interaction constants: Wd and Ws, and hyperfine structure (HFS) constants for the ground state of AEMFs, and also the static DPs of alkaline - earth and fluorine atoms. The PDM of RaF, molecular polarizabilities for CaF and RaF, HFS constants of BeF and Ws for BeF, MgF, CaF and SrF molecules are computed and reported for the first time. Further, the effect of augmentation of the basis sets on the valence properties; and the contribution of triple excitations and also the contributions of inner - core electrons on the HFS constants of lower members of this series are investigated. The calculated results using frozen core approximation show good agreement with the existing data, wherever available. We have observed, however, that the accurate calculations of the HFS constants require the inclusion of core electrons as well as triple excitations at the correlation level. We believe that the results presented as part of this thesis using sophisticated many - body theories and sufficiently large optimized basis sets would serve as benchmarks for similar calculations in the future. They would also be beneficial for the experimental spectroscopists who might consider working on these molecular systems in future.