Ab Initio Investigation of the Electronic Structure and Spectroscopy of LiTe and LiTe+ Including Spin–Orbit Coupling: Prospects for Laser Cooling

Abstract The electronic structure and spectroscopic properties of neutral LiTe and ionic LiTe+ molecules are investigated using high-level ab initio calculations based on the CASSCF/MRCI + Q method, employing the aug-cc-pV5Z basis set for Li and an effective core potential for Te. Potential energy curves, spectroscopic constants, permanent and transition dipole moments, and vibrational energy levels are determined for the ground and low-lying excited states Σ±, Π, and Δ, correlated with the three lowest dissociation limits. To the best of our knowledge, this work represents the first comprehensive characterization of the low-lying electronic states of both LiTe and LiTe+. The effect of spin–orbit coupling is subsequently examined, resulting in 41 Ω states for LiTe+ through the splitting of the corresponding Λ–S states. The radiative lifetimes of vibrational levels in the lowest electronic states are determined from spontaneous emission and blackbody radiation transition rates. Franck–Condon factors are also calculated for the LiTe 12Π → 12Σ+ and 12Π → 22Π transitions and the LiTe+ 13Σ– → 13Π transition. Highly diagonal Franck–Condon matrices are obtained, with f00= 0.983, f11= 0.958, and f22= 0.938 for LiTe+, while the corresponding spin–orbit coupled transitions in LiTe yield similarly large values of 0.988, 0.964, and 0.940. These highly diagonal Franck–Condon matrices, together with the calculated radiative properties, identify both LiTe and LiTe+ as promising candidates for laser cooling and trapping. The present results provide reliable spectroscopic benchmarks and theoretical guidance for future experimental investigations and the development of laser-cooling schemes involving heavy polar diatomic molecules.