Ladjimi Hela, Zrafi Wissem, Farjallah Mohamed, Bejaoui Mohamed, Berriche Hamid
Laboratory of Interfaces and Advanced Materials, Faculty of Science, University of Monastir, 5019 Monastir, Tunisia.
Department of Mathematics and Natural Sciences, School of Arts and Sciences, American University of Ras Al Khaimah, RAK, P.O. Box 10021, United Arab Emirates.
Phys Chem Chem Phys. 2022 Aug 10;24(31):18511-18522. doi: 10.1039/d2cp00808d.
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 Σ, Π and Δ symmetries below the first nine asymptotic limits dissociating into Be(2s) + Cs(6s, 6p, 5d) and Be(2s, 2s2p, 2s3s, 2p) + Cs. We used a quantum chemistry approach based on a semi-empirical pseudo potential for Be 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 1Σ and second excited states 3Σ, of BeCs are theoretically investigated, and isotopic effects on cold and ultra-cold energy collisions are also detected. Vibrational lifetimes of the ground state 1Σ are calculated taking into account both spontaneous and stimulated emissions and also the absorption induced by black body radiation at room temperature ( = 300 K). Vibrational radiative lifetimes for the first 2Σ and second 3Σ excited states are also calculated and extensively analyzed. We found that the radiative lifetimes of the lower vibrational levels of the 1Σ state have an order of magnitude of seconds (s), while those of 2Σ and 3Σ states have an order of nanoseconds (ns). The Franck-Condon factors are also calculated for transitions from the low lying excited 2Σ, 3Σ, 1Π states to the ground state 1Σ. We found that the favourite vibrational transition to the 1Σ( = 0) ground state is obtained for 1Π (''' = 0)-1Σ( = 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.
玻色-爱因斯坦凝聚体（BeCs）系统代表了一种未来可能的候选方案，用于实现尚未进行实验或理论研究的冷或超冷分子离子物种样本。为了突出铯和铍阳离子BeCs的光谱和电子结构，我们从理论上研究了低于前九个渐近极限的Σ、Π和Δ对称性的基态和低激发态，这些极限会解离为Be(2s) + Cs(6s, 6p, 5d)以及Be(2s, 2s2p, 2s3s, 2p) + Cs。我们采用了一种量子化学方法，该方法基于对Be和Cs核心的半经验赝势、核心极化势（CPP）、大高斯基组以及对价电子的全组态相互作用（FCI）方法。使用不同基组的CCSD(T)/CI方法对基态进行了额外的计算。本文报道了绝热势能曲线、光谱常数、振动能级以及永久和跃迁偶极矩。此外，从理论上研究了BeCs基态1Σ和第二激发态3Σ在低能下的弹性散射特性，并检测了同位素对冷和超冷能量碰撞的影响。考虑到自发和受激发射以及室温（ = 300 K）下黑体辐射引起的吸收，计算了基态1Σ的振动寿命。还计算并广泛分析了第一激发态2Σ和第二激发态3Σ的振动辐射寿命。我们发现，1Σ态较低振动能级的辐射寿命量级为秒（s），而2Σ和3Σ态的辐射寿命量级为纳秒（ns）。还计算了从低激发态2Σ、3Σ、1Π到基态1Σ跃迁的弗兰克-康登因子。我们发现，对于具有对角结构且弗兰克-康登因子值为0.94的1Π（''' = 0）-1Σ（ = 0）跃迁，是到1Σ( = 0)基态最有利的振动跃迁。这个弗兰克-康登因子值足够大，使得BeCs系统成为直接激光冷却的有利候选者。
