Ab initio calculations of diatomic constants and ro-vibrational parameters for the ground state of singly charged aluminium monohalides

Abstract: We report electronic, vibrational, and rotational spectroscopic parameters for the ground state, X$^2\Sigma^{+}$, of singly charged aluminium monohalides, employing single-reference coupled-cluster theory with single and double excitations (CCSD) together with the relativistic basis sets. Higher order correlation effects coming from triple excitations are treated using perturbative CCSD(T) approach. Most of the molecular ions in the AlX$^+$ series, particularly barring the first two, have been studied here for the first time for their ground state electronic and vibrational structure. The vibrational parameters have been calculated by solving the vibrational Schr\"odinger equation utilizing potential energy curves and permanent dipole moment curves. Further, spontaneous and black-body radiation induced lifetimes have also been computed using relative energy separation and the transition dipole moments between the vibrational levels. The lifetimes of the lowest ro-vibrational states are found to be 10.63 s, 40.39 s, 23.13 s, 31.26 s, 13.43 s, and 8.08 s for the AlF$^+$, AlCl$^+$, AlBr$^+$, AlI$^+$, AlAt$^+$, and AlTs$^+$ ions, respectively. Furthermore, the rotational parameters such as Einstein coefficients and Franck-Condon factors for the lowest six vibrational states are also computed and reported in this work.