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The Journal of Physical Chemistry A

Non-Relativistic Electronic-Structure Computation of Neutral and Cationic Systems [Fr2, Fr–AEM+ (AEM= Ca, Sr, Ba)]

Soulef Jellali, Héla Habli

Published Jun 7, 2022• 7 citations
Publisher

Abstract

The experimental field of ultracold ion-atom mixtures including an alkali-metal atom and an alkaline-earth-metal ion as well as of homonuclear alkali dimers has paved the way for creating and manipulating the ultracold molecules. The present paper is focused on a study of molecules such us francium dimer and a comparative spectroscopic investigation of the cationic systems Fr–(Ca +, Sr +, Ba + ). We adopt a computational scheme without spin–orbit coupling reposed on the full configuration interaction and semi-empirical pseudo-potential theory of the atomic cores Fr +, Ca 2+, Sr 2+, and Ba 2+ with extended and optimized basis sets. We have determined the adiabatic potentials with their relative spectroscopic constants, the electric dipole moments and the vibrational levels spacings for the 1,3 Σ u,g + and 1,3 Σ + electronic states for Fr 2 and Fr–AEM +, respectively, correlated toward {Fr(7s) + Fr(7s, 7p, 6d, 8s, 8p)}, {Ca(4s 2, 4s4p, 4s3d), Sr(5s 2, 5s5p, 5s4d), Ba(6s 2, 6s6p, 6s5d) + Fr + }, and {Ca + (4s, 3d), Sr + (5s, 4d), Ba + (6s, 5d) + Fr(7s, 7p)}. The accuracy and reliability of the current results are discussed by comparing with theoretical data available in the literature. The occurrence of some avoided crossings between the neighboring electronic states is leading to a charge or excitation transfer for atom–ion collisions in the diverse charge or excited states. The Σ + –Σ + transitions are determined in order to evaluate the future radiative lifetimes of vibrational states serving the direct laser cooling.

DOI
10.1021/acs.jpca.1c10801

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Abstract Alkaline-earth and alkali-metal mixtures have an electronic structure that is perfect for laser cooling. This makes them highly attractive for trapping and laser cooling experiments, allowing the formation of cold molecules. For this object, potential-energy curves and relevant spectroscopic parameters of the low-lying electronic excited states of 1,3Σ+, 1,3Π, and 1,3Δ symmetries of molecular-ion systems composed of alkaline-earth-ion and Francium alkali-metal-atom: ALKE-Fr+ (ALKE = Be, Mg, Ca and Sr), are determined using advanced theoretical technique in quantum chemistry, including a non-empirical pseudopotential, core-valence correlation, large Gaussian basis sets and Full Configuration Interaction (FCI). In order to obtain a more accurate understanding of the electronic structure of these systems, we also determined transition and permanent dipole moments and vibrational properties. Thereafter, the spontaneous and the black-body stimulated transition rates were determined and were employed to calculate lifetimes for all vibrational states of the ground electronic states 11Σ+ of molecular-ions under consideration. For the first and the second excited states, radiative lifetimes were investigated via the Franck–Condon approximation including bound-bound and bound-free transitions. High diagonal structure and large Franck Condon Factor (FCF) values f 00 = 0.987, f 11 = 0.959 and f 22 = 0.919 were obtained for the 11Π (v′ = 0, 1, 2)→ 11Σ+ (v = 0, 1, 2) transition making the BeFr+ system a good candidate for laser cooling. Furthermore, the current results could be used to investigate elastic scattering properties in cold-ion-atom collisions for the first excited states and may help the experimentalists for possible formation, spectroscopy, and photoassociation of cold ion-atom mixtures.
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Abstract Experiments with cold ion–atom mixtures have recently opened the way for the production and application of ultracold molecular ions. Here, in a comparative study, we theoretically investigate ground and several excited electronic states and prospects for the formation of molecular ions composed of a calcium ion and an alkali-metal atom: CaAlk+ (Alk = Li, Na, K, Rb, Cs). We use a quantum chemistry approach based on non-empirical pseudopotentials, operatorial core-valence correlation, large Gaussian basis sets, and full configuration interaction method for valence electrons. Adiabatic potential energy curves, spectroscopic constants, and transition and permanent electric dipole moments are determined and analyzed for the ground and excited electronic states. We examine the prospects for ion-neutral reactive processes and the production of molecular ions via spontaneous radiative association and laser-induced photoassociation. After that, spontaneous and stimulated blackbody radiation transition rates are calculated and used to obtain radiative lifetimes of vibrational states of the ground and first-excited electronic states. The present results pave the way for the formation and spectroscopy of calcium–alkali-metal-atom molecular ions in modern experiments with cold ion–atom mixtures.
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The accurate knowledge of electronic properties is important for creating and manufacturing ultracold molecules. We report here the ab initio quantum chemistry calculations on the properties of alkali-metal-ytterbium AM-Yb (AM = Li, Na, K, Rb, Cs) and alkaline-earth-metal-ytterbium AEM-Yb (AEM = Be, Mg, Ca, Sr, Ba) molecules for their electronic ground state. The potential energy curves (PECs) and permanent dipole moments (PDMs) are calculated on the basis of the multireference configuration interaction (MRCI) level of theory, where the core-valence correlations and scalar relativistic effects are included. The related spectroscopic constants are also determined. The results demonstrate that the dissociation energies and PDMs of AEM-Yb are smaller than those of AM-Yb molecules, and an interesting trend of the dissociation energy has been observed. This work provides favorable information for the experimental study of forming ultracold molecules via photoassociation technique.
In this work, we investigate the possibility of creating cold Fr 2 , RbFr and CsFr molecules through the photoassociation of cold atoms. Potential curves, permanent and transition dipole moments for the francium dimer and for the RbFr and RbCs molecules are determined for the first time. The francium atom is modelled as one valence electron moving in the field of the Fr + core, which is described by a new pseudopotential with averaged relativistic effects, and including effective core-polarization potential. The molecular calculations are performed for both the ionic species Fr + 2 , RbFr + , CsFr + and the corresponding neutrals, through the CIPSI quantum chemistry package where we used new extended Gaussian basis sets for Rb, Cs and Fr atoms. As no experimental data are available, we discuss our results by comparison with the Rb 2 , Cs 2 and RbCs systems. The dipole moment of CsFr reveals an electron transfer yielding a Cs + Fr − arrangement, while in all other mixed alkali pairs the electron is transferred towards the lighter species. Finally, the perturbative treatment of the spin–orbit coupling at large distances predicts that in contrast with Rb 2 and Cs 2 no double-well excited potential should be present in Fr 2 , probably preventing an efficient formation of cold dimers via the photoassociation of cold francium atoms.