Description
Title: Finite-Field Approach for Relativistic Coupled-Cluster Calculations of Isotope Shifts for the Low-Lying States of Ca II
Abstract: The first five low-lying fine-structure states of the singly charged calcium ion (Ca II) have had their isotope shift (IS) constants evaluated using a method we developed called the relativistic coupled cluster (RCC) and a code we also created. The differential values (the shifts of the spectral lines) from a previous calculation using the singles and doubles approximation of RCC theory (RCCSD method) show results for the individual states in the FF approach that deviate significantly from other theoretical results and experiments. Though calculations using the Dirac-Hartree-Fock (DHF) method show good agreement, we find a contrary trend from the FF approach using our RCCSD method. According to our findings, the partial triple excitations that are incorporated into the RCC equations through energy derivation reduce the accuracy of the perturbative approach (RCCSD(T) method), but this accuracy can be increased by incorporating triple excitations into the wave function. It is clear from the differences between the RCCSD and RCCSD(T) results that triple excitations are crucial for determining Ca II’s energies and IS constants. Last but not least, we also present ab initio values of IS’s between the S-P, S-D, and D-P transitions in the DHF, as well as RCCSD and RCCSD(T) approximations (theoretical as well as experimental).
Keywords: isotope shift; ab initio method; coupled-cluster theory
Paper Quality: SCOPUS / Web of Science Level Research Paper
Subject: Physics
Subject Category: Atomic and Molecular Physics, and Optics
Writer Experience: 20+ Years
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