Ab initio study of laser cooling of AlF+ and AlCl+ molecular ions

Abstract

A theoretical investigation on the suitability of laser cooling AlF+ and AlCl+ molecular ions is performed by employing ab initio calculations. Four low-lying electronic states are determined using the multi-reference configuration interaction method plus Davidson correction. The calculated spectroscopic constants are in good agreement with the available theoretical and experimental data. The Franck–Condon factors (FCFs) and radiative lifetimes are verified by calculating the potential energy curves (PECs) and transition dipole moments for the C 2Π (v′) → X 2Σ+ (v), C 2Π (v′) → A 2Π (v) and C 2Π (v′) → B 2Σ+ (v) transitions. Our calculation indicates that the C 2Π (v′) → X 2Σ+ (v) transition is provided with sufficiently diagonally distributed FCFs and the radiative lifetimes for the C 2Π (v′) vibrational state are short enough for rapid laser cooling for both AlF+ and AlCl+ molecular ions. However, our calculation also indicates that the AlF+ radical is not suitable for laser cooling due to the predissociation of the C 2Π state. The calculation and analysis of the electronic states demonstrate the possibility of AlCl+ for laser cooling, and give more insight into the behavior of these four low-lying electronic states for further experimental and theoretical study. Its laser-cooling scheme requires one main pumping laser (λ00 = 268 nm) and three repumping lasers (λ10 = 272 nm, λ21 = 273 nm and λ31 = 278 nm). Moreover, the low mass, short radiative lifetime and small vibrational branching loss ratios to the intervening states make the AlCl+ radical suitable for laser cooling in theory.