Optical cycling in charged complexes with Ra-N bonds

The extension of laser cooling and trapping techniques to polyatomic molecular ions would have advanced scientific applications such as search of physics outside of the Standard Model, ultracold chemistry etc. We apply the Fock space relativistic coupled cluster method to study low-lying electronic states of molecular ions with Ra--N bonds, namely RaNCH$^+$, RaNH$^+_3$ and RaNCCH$^+_3$. Prospects of laser cooling of these species are estimated, and the peculiarities of unpaired-electron distributions are analyzed from the point of view of the molecular electronic structure. RaNH$^+_3$ and RaNCCH$^+_3$ are the first symmetric top molecular ions expected to be suitable for direct laser cooling

Compound-tunable embedding potential method to model local electronic excitations on ff-element ions in solids: Pilot relativistic coupled cluster study of Ce and Th impurities in yttrium orthophosphate, YPO4_4

A method to simulate local properties and processes in crystals with impurities via constructing cluster models within the frame of the compound-tunable embedding potential (CTEP) and highly-accurate {\it ab initio} relativistic molecular-type electronic structure calculations is developed and applied to the Ce and Th-doped yttrium orthophosphate crystals, YPO$_4$, having xenotime structure. Two embedded cluster models are considered, the "minimal" one, YO$_8$@CTEP$_{\rm min}$, consisting of the central Y$^{3+}$ cation and its first coordination sphere of eight O$^{2-}$ anions (i.~e.\ with broken P--O bonds), and its extended counterpart, Y(PO$_4$)$_6$@CTEP$_{\rm ext}$, implying the full treatment of all atoms of the PO$_4^{3-}$ anions nearest to the central Y$^{3+}$ cation. CTEP$_{\rm min,ext}$ denote here the corresponding cluster environment described within the CTEP method. The relativistic Fock-space coupled cluster (FS RCC) theory is applied to the minimal cluster model to study electronic excitations localized on Ce$^{3+}$ and Th$^{3+}$ impurity ions. Calculated transition energies for the cerium-doped xenotime are in a good agreement with the available experimental data (mean absolute deviation of ca.0.3 eV for $4f{\to}5d$ type transitions). For the thorium-doped crystal the picture of electronic states is predicted to be quite complicated, the ground state is expected to be of the $6d$ character. The uncertainty for the excitation energies of thorium-doped xenotime is estimated to be within 0.35 eV. Radiative lifetimes of excited states are calculated at the FS RCC level for both doped crystals. The calculated lifetime of the lowest $5d$ state of Ce$^{3+}$ differs from the experimentally measured one by no more than twice

Diagrammatic formulation of the second-order many-body multipartitioning perturbation theory

The second-order multireference perturbation theory employing multiple partitioning of the many-electron Hamiltonian into a zero-order part and a perturbation is formulated in terms of many-body diagrams. The essential difference from the standard diagrammatic technique of Hose and Kaldor concerns the rules of evaluation of energy denominators which take into account the dependence of the Hamiltonian partitioning on the bra and ket determinantal vectors of a given matrix element, as well as the presence of several two-particle terms in zero-order operators. The novel formulation naturally gives rise to a sum-over-orbital procedure of correlation calculations on molecular electronic states, particularly efficient in treating the problems with large number of correlated electrons and extensive one-electron bases

On the finite-field transition dipole moment calculations by effective Hamiltonian methods

A simple finite-field scheme of calculations on electronic transition dipole moments in molecules by effective Hamiltonian methods is presented and discussed. The reliability of underlying approximations is analyzed by means of the quasidegenerate perturbation theory and corroborated by the results of pilot numerical applications

Ab initio quasi-relativistic calculations on angular momentum and magnetic couplings of molecular electronic states.

We formulate an ab initio method of quasirelativistic calculations on angular momentum and magnetic transition matrix elements between adiabatic electronic states of molecules. Our approach is based on the construction of a state-selective effective Hamiltonian and transition density matrices by means of the multireference many-body perturbation theory. Pilot applications to the evaluation of B0+u→B″1u predissociation matrix elements in I2 and interactions in the B0+unot, vert, similarB1u complex of Te2 are reported