Search

We present a theoretical analysis of optical pathways for formation of cold Ca($^1$S)Na$^+$($^1$S) molecular ions via an intermediate state. The formation schemes are based on ab initio potential energy curves and transition dipole moments calculated using effective-core-potential methods of quantum chemistry. In the proposed approach, starting from a mixture of cold trapped Ca$^+$ ions immersed into an ultracold gas of Na atoms, (NaCa)$^+$ molecular ions are photoassociated in the excited E$^{1}\Sigma^+$ electronic state and allowed to spontaneously decay either to the ground electronic state or an intermediate state from which the population is transferred to the ground state via an additional optical excitation. By analyzing all possible pathways, we find that the efficiency of a two-photon scheme, via either B$^{1}\Sigma^+$ or C$^{1}\Sigma^+$ potential, is sufficient to produce significant quantities of ground state (NaCa)$^+$ molecular ions. A single-step process results in lower formation rates that would require either a high density sample or a very intense photoassociation laser to be viable., Comment: 13 pages, 15 figures; submitted to Phys. Rev. A

We performed calculations of the optimized structure, harmonic vibrational frequencies and dissociation energies of RbOH and its anion, and investigate the interactions between Rb and OH$^-$ leading to possible associative detachment pathways. The electron affinity of RbOH was computed to be 0.2890 eV, with a bond energy of Rb+OH$^-$ half that of Rb+OH. To determine other possible charge loss pathways, the Rb+OH and Rb+OH$^-$ dissociation curves were computed using couple cluster methods along all possible collisional angles. An adiabatic curve crossing between the neutral and charged molecule was found at the inner wall of the molecular potential curve for linear geometries. Associative detachment rates were estimated using the Langevin ion capture cross-section for hydroxide. We find for $v\ge 2$ an associative detachment rate of $>2\times 10^{-9}$ cm$^3$s$^{-1}$, while for $v=0$ and 1 no appreciable rate exists. This strong dependence on vibrational level suggests the ability to control the associative detachment rate directly.

We report \textit{ab initio} calculations of the X $^2 \Sigma_{u}^+$, A $^2\Pi_u$ and B $^2 \Sigma_{g}^+$ states of the Ca$_{2}^+$ dimer. All electron CAS+MRCI calculations are performed for the X $^2 \Sigma_{u}^+$ and B $^2 \Sigma_{g}^+$ states, while valence CAS+MRCI calculations using an effective core potential are used to describe the A $^2\Pi_u$ state. A double well is found in the B $^2 \Sigma_{g}^+$ state. Spectroscopic constants, vibrational levels, transition moments and radiative lifetimes are calculated for the most abundant isotope of calcium ($^{40}$Ca). The static dipole and quadrupole polarizabilities, and the leading order van der Waals coefficients are also calculated for all three states., Comment: 6 pages, 5 figures

An accurate long-range {\em ab initio} potential energy surface has been calculated for the ground state ${}^2A'$ lithium trimer in the frozen diatom approximation using all electron RCCSD(T). The {\em ab initio} energies are corrected for basis set superposition error and extrapolated to the complete basis limit. Molecular van der Waals dispersion coefficients and three-body dispersion damping terms for the atom-diatomic dissociation limit are presented from a linear least squares fit and shown to be an essentially exact representation of the {\em ab initio} surface at large range.

We report ab initio calculations of the X $^2 \Sigma_{u}^+ $ and B $^2 \Sigma_{g}^+$ states of the Be$_{2}^+$ dimer. Full valence configuration interaction calculations were performed using the aug-cc-pVnZ basis sets and the results were extrapolated to the CBS limit. Core-core, core-valence effects are included at the CCSDT/MTsmall level of theory. Two local minima, separated by a large barrier, are found in the expected repulsive B $^2 \Sigma_{g}^+$ state. Spectroscopic constants have been calculated and good agreement is found with the recent measurements of Merritt et al. Bound vibrational levels, transition moments and lifetimes have also been calculated., Comment: 4 pages, 4 figures. Submitted to Chemical Physics Letters

The lowest doublet electronic state for the lithium trimer (2A') is calculated for use in three-body scattering calculations using the valence electron FCI method with atomic cores represented using an effective core potential. It is shown that an accurate description of core-valence correlation is necessary for accurate calculations of molecular bond lengths, frequencies and dissociation energies. Interpolation between 2A' ab initio surface data points in a sparse grid is done using the global interpolant moving least squares method with a smooth radial data cutoff function included in the fitting weights and bivariate polynomials as a basis set. The Jahn-Teller splitting of the 2E' surface into the 2A1 and 2B2 states is investigated using a combination of both CASSCF and FCI levels of theory. Additionally, preliminary calculations of the 2A'' surface are also presented using second order spin restricted open-shell Moller-Plesset perturbation theory., Comment: 7 pages, 5 figures

New experiments were proposed recently to investigate the regime of cold atomic and molecular ion-atom collision processes in a special hybrid neutral-atom--ion trap under high vacuum conditions. The collisional cooling of laser pre-cooled Ca$^+$ ions by ultracold Na atoms is being studied. Modeling this process requires knowledge of the radiative lifetime of the excited singlet A$^1\Sigma^+$ state of the (NaCa)$^+$ molecular system. We calculate the rate coefficient for radiative charge transfer using a semiclassical approach. The dipole radial matrix elements between the ground and the excited states, and the potential curves were calculated using Complete Active Space Self-Consistent field and M\"oller-Plesset second order perturbation theory (CASSCF/MP2) with an extended Gaussian basis, 6-311+G(3df). The semiclassical charge transfer rate coefficient was averaged over a thermal Maxwellian distribution. In addition we also present elastic collision cross sections and the spin-exchange cross section. The rate coefficient for charge transfer was found to be $2.3\times 10^{-16}$ cm$^3$/sec, while those for the elastic and spin-exchange cross sections were found to be several orders of magnitude higher ($1.1\times 10^{-8}$ cm$^3$/sec and $2.3\times 10^{-9}$ cm$^3$/sec, respectively). This confirms our assumption that the milli-Kelvin regime of collisional cooling of calcium ions by sodium atoms is favorable with the respect to low loss of calcium ions due to the charge transfer., Comment: 4 pages, 5 figures; v.2 - conceptual changes