Electronic structure, cold ion-atom elastic collision properties and possibility of laser cooling of BeCs⁺ molecular...

The BeCs⁺ system represents a possible future candidate for the realization of samples of cold or ultra-cold molecular ion species that have not yet been investigated experimentally or theoretically. With the aim of highlighting the spectroscopic and electronic structure of the cesium and beryllium cation BeCs⁺, we theoretically investigate ground and low lying excited states of ^1,3Σ⁺, ^1,3Π and ^1,3Δ symmetries below the first nine asymptotic limits dissociating into Be⁺(2s) + Cs(6s, 6p, 5d) and Be(2s², 2s2p, 2s3s, 2p²) + Cs⁺. We used a quantum chemistry approach based on a semi-empirical pseudo potential for Be²⁺ and Cs⁺ cores, core polarization potentials (CPP), large Gaussian basis sets and full configuration interaction (FCI) method for the valence electrons. Additional calculations have been performed for the ground state using CCSD(T)/CI methods with different basis sets. Adiabatic potential energy curves, spectroscopic constants, vibrational levels, and permanent and transition dipole moments are reported in this work. Furthermore, the elastic scattering properties at low energy for both ground 1¹Σ⁺ and second excited states 3¹Σ⁺, of BeCs⁺ are theoretically investigated, and isotopic effects on cold and ultra-cold energy collisions are also detected. Vibrational lifetimes of the ground state 1¹Σ⁺ are calculated taking into account both spontaneous and stimulated emissions and also the absorption induced by black body radiation at room temperature (T = 300 K). Vibrational radiative lifetimes for the first 2¹Σ⁺ and second 3¹Σ⁺ excited states are also calculated and extensively analyzed. We found that the radiative lifetimes of the lower vibrational levels of the 1¹Σ⁺ state have an order of magnitude of seconds (s), while those of 2¹Σ⁺ and 3¹Σ⁺ states have an order of nanoseconds (ns). The Franck-Condon factors are also calculated for transitions from the low lying excited 2¹Σ⁺, 3¹Σ⁺, 1¹Π states to the ground state 1¹Σ⁺. We found that the favourite vibrational transition to the 1¹Σ⁺(v = 0) ground state is obtained for 1¹Π (v''' = 0)-1¹Σ⁺(v = 0) with a diagonal structure and a large Franck-Condon factor value of 0.94. This Franck-Condon factor value is sufficiently large to make the BeCs⁺ system a favorable candidate for direct laser cooling.