Electronic properties of francium diatomic compounds and prospects for cold molecule formation

Abstract

In this work, we investigate the possibility of creating cold Fr2, 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 Rb2, Cs2 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 Rb2 and Cs2 no double-well excited potential should be present in Fr2, probably preventing an efficient formation of cold dimers via the photoassociation of cold francium atoms.