ИСТИНА

The current interest in alkali heterodiatomic molecules arises from their active utilization in the synthesis of ultracold molecular ensembles. This is most often done by laser assembly based on multi-steps optical cycles involving excited molecular states as intermediates. The search for efficient pathways requires that the highly accurate energetic and the radiative properties of the intermediate electronic states be available beforehand. Here we report accurate ab initio calculations on low-lying states of MCs (M=K,Rb) molecules. The relativistic electronic structure model employed in our study was defined by accurate two-component shape-consistent pseudopotentials of small atomic cores. The calculations were performed by the Fock space relativistic coupled cluster method (FSRCC), with the restriction of the cluster operator expansion to single and double excitations and with the assumption of the Fock space scheme (MCs)2+ → (MCs)+ → MCs. Numerical instabilities caused by the presence of intruder states were avoided by means of the energy denominator shift technique, in some cases combined with the Padé extrapolation to the zero-shift limit [1]. Excited state potential energy curves (PECs) were obtained by adding the resulting excitation energies as functions of the internuclear distance to the accurate empirical ground state PEC. Transition dipole moments (TDM) were evaluated by the finite-field (FF) technique [2], making use of approximate off-diagonal Hellmann-Feynman-like relations for FSRCC effective Hamiltonian eigenstates. All calculations were performed using the appropriately modified DIRAC17 code [3]. The deperturbation treatment of the rovibronic structure and transition probabilities for singlet-triplet complexes of the KCs and RbCs molecules has demonstrated that the present FSRCC PECs and FF TDM functions agree with their empirical counterparts to within a few percent over a wide range of internuclear distances. [1] A. Zaitsevskii, E. Eliav. Padé extrapolated effective Hamiltonians in the Fock space relativistic coupled cluster method. Int. J. Quantum Chem., 118, e25772 (2018). [2] A. V. Zaitsevskii, L. V. Skripnikov, A. V. Kudrin, et al. Electronic transition dipole moments in relativistic coupled-cluster theory: the finite-field method. Opt. Spectrosc., 124, 451 (2018). [3] L. Visscher, H. J. Aa. Jensen, R. Bast, T. Saue., et al. DIRAC, a relativistic ab initio electronic structure program, Release DIRAC17, http://www.diracprogram.org (2017).