We present quantum-chemical calculations for the ground and some low-lying excited states of isolated LiCa and LiSr molecules using multi-state complete active space second-order perturbation theory (MS–CASPT2). The potential energy curves (PECs) and their corresponding spectroscopic constants, obtained at the spin-free (SF) and spin-orbit (SO) levels, agree well with available experimental values. Our SO–MS-CASPT2 calculation at the atomic limit (R = 100 a.u.) with the largest basis set reproduces experimental atomic excitation energies within 3% for both LiCa and LiSr. In addition, permanent dipole moments (PDMs) and transition dipole moments (TDMs) at the SF level are also obtained. Rovibrational calculations of the ground and selected excited states, together with the spontaneous emission rates, demonstrate that the formation of ultracold LiCa and LiSr molecules in low-lying vibrational levels of the electronic ground state may be possible. I. INTRODUCTION
Since the development of laser cooling, many research groups have directed their interest towards opening new fields of physics using ultracold (<10-3K) atoms/molecules to study cold collisions, quantum degeneracy, atom optics, and precise measurements. Recently, ultracold polar molecules have attracted attention in studies of anisotropic long-range dipole-dipole interactions1 in the ultracold regime. Although the application of laser cooling to molecules is difficult, several cooling methods, such as buffer gas cooling,2 Stark or Zeeman deceleration of molecular beams,3 and production of ultracold diatomic molecules from ultracold atoms, are being employed.
In photoassociation (PA) spectroscopy,4 pairs of ultracold atoms in the electronic ground state are combined using a laser to create ultracold molecules in a rovibrational level of an electronic excited state. Through spontaneous emission, these molecules decay to highly excited rovibrational levels of either the electronic ground state or a metastable state. Subsequent laser irradiation transfers the population of this rovibrational level to the v=0 level of the electronic ground state through stimulated Raman pumping. Preliminary theoretical calculations of the ground and excited potential energy curves (PECs), permanent dipole moments (PDMs), transition dipole moments (TDMs), and spontaneous emission rates are essential in devising the type of lasers required for such experiments.
PA spectroscopy has been successfully employed for both bi-alkali (KRb, NaLi, NaCs, RbLi, RbCs, and CsLi)5 and alkaline-earth-metal atoms (Sr26 and Ca27,8). A recent theoretical investigation of SrYb9 reiterates the importance that PA experiments can play in the study of ultracold polar molecules. In our earlier study, we obtained the structure of the potential energy curves and the permanent and transition dipole moments of 6Li174Yb molecules10. Our studies support an ongoing experiment11,12 in which the simultaneous magneto-optical trapping of 174Yb and 6Li is followed by the production of 6Li174Yb molecules using PA or the Feshbach resonance in an optical lattice13. In a recent paper14, the production of 6Li174Yb molecules using Feshbach resonance was shown to be possible via a difficult experiment.
Ultracold polar molecules formed using PA or Feshbach resonance in a single quantum state are attractive systems because of the existence of PDM. PDM is a measure of dipolar interaction among molecules. Calculations for 6Li174Yb revealed a small PDM for the...