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We report the preparation of exactly one 87Rb atom and one 133Cs atom in the same optical tweezer as the essential first step towards the construction of a tweezer array of individually trapped 87Rb133Cs molecules. Through careful selection of the tweezer wavelengths, we show how to engineer species-selective trapping potentials suitable for high-fidelity preparation of Rb + Cs atom pairs. Using a wavelength of 814 nm to trap Rb and 938 nm to trap Cs, we achieve loading probabilities of 0.508(6) for Rb and 0.547(6) for Cs using standard red-detuned molasses cooling. Loading the traps sequentially yields exactly one Rb and one Cs atom in 28.4(6)% of experimental runs. Using a combination of an acousto-optic deflector and a piezo-controlled mirror to control the relative position of the tweezers, we merge the two tweezers, retaining the atom pair with a probability of. We use this capability to study hyperfine-state-dependent collisions of Rb and Cs in the combined tweezer and compare the measured two-body loss rates with coupled-channel quantum scattering calculations. [ABSTRACT FROM AUTHOR]

We investigate the potential energy surfaces and activation energies for reactions between methyl halide molecules CH3X (X = F, Cl, Br, I) and alkali-metal atoms A (A = Li, Na, K, Rb) using high-level ab initio calculations. We examine the anisotropy of each intermolecular potential energy surface (PES) and the mechanism and energetics of the only available exothermic reaction pathway, CH3X + A → CH3 + AX. The region of the transition state is explored using two-dimensional PES cuts and estimates of the activation energies are inferred. Nearly all combinations of methyl halide and alkali-metal atom have positive barrier heights, indicating that reactions at low temperatures will be slow. [ABSTRACT FROM AUTHOR]

We present elastic and inelastic spin-changing cross sections for cold and ultracold NH(X 3Σ-) + NH(X 3Σ-) collisions, obtained from full quantum scattering calculations on an accurate ab initio quintet potential-energy surface. Although we consider only collisions in zero field, we focus on the cross sections relevant for magnetic trapping experiments. It is shown that evaporative cooling of both fermionic 14NH and bosonic 15NH is likely to be successful for hyperfine states that allow s-wave collisions. The calculated cross sections are very sensitive to the details of the interaction potential, due to the presence of (quasi)bound state resonances. The remaining inaccuracy of the ab initio potential-energy surface therefore gives rise to an uncertainty in the numerical cross-section values. However, based on a sampling of the uncertainty range of the ab initio calculations, we conclude that the exact potential is likely to be such that the elastic-to-inelastic cross-section ratio is sufficiently large to achieve efficient evaporative cooling. This likelihood is only weakly dependent on the size of the channel basis set used in the scattering calculations. [ABSTRACT FROM AUTHOR]

State-of-the-art ab initio techniques have been applied to compute the potential energy surface for the lithium atom interacting with the lithium hydride molecule in the Born-Oppenheimer approximation. The interaction potential was obtained using a combination of the explicitly correlated unrestricted coupled-cluster method with single, double, and noniterative triple excitations [UCCSD(T)-F12] for the core-core and core-valence correlation and full configuration interaction for the valence-valence correlation. The potential energy surface has a global minimum 8743 cm-1 deep if the Li-H bond length is held fixed at the monomer equilibrium distance or 8825 cm-1 deep if it is allowed to vary. In order to evaluate the performance of the conventional CCSD(T) approach, calculations were carried out using correlation-consistent polarized valence X-tuple-zeta basis sets, with X ranging from 2 to 5, and a very large set of bond functions. Using simple two-point extrapolations based on the single-power laws X-2 and X-3 for the orbital basis sets, we were able to reproduce the CCSD(T)-F12 results for the characteristic points of the potential with an error of 0.49% at worst. The contribution beyond the CCSD(T)-F12 model, obtained from full configuration interaction calculations for the valence-valence correlation, was shown to be very small, and the error bars on the potential were estimated. At linear LiH-Li geometries, the ground-state potential shows an avoided crossing with an ion-pair potential. The energy difference between the ground-state and excited-state potentials at the avoided crossing is only 94 cm-1. Using both adiabatic and diabatic pictures, we analyze the interaction between the two potential energy surfaces and its possible impact on the collisional dynamics. When the Li-H bond is allowed to vary, a seam of conical intersections appears at C2v geometries. At the linear LiH-Li geometry, the conical intersection is at a Li-H distance which is only slightly larger than the monomer equilibrium distance, but for nonlinear geometries it quickly shifts to Li-H distances that are well outside the classical turning points of the ground-state potential of LiH. This suggests that the conical intersection will have little impact on the dynamics of Li-LiH collisions at ultralow temperatures. Finally, the reaction channels for the exchange and insertion reactions are also analyzed and found to be unimportant for the dynamics. [ABSTRACT FROM AUTHOR]

This paper describes the application of a relatively simple, but computationally tractable, “particle-on-a-sphere” (POS) model for quantum-mechanical calculation of large-amplitude, H atom dynamics in polyatomic hydrides (XHn), based on radially relaxed, two-dimensional angular motion of H atoms on the surface of a sphere. This work focuses on systems with many degrees of freedom, i.e., XH4 (eight dimensional) and XH5 (ten dimensional), with corresponding molecular analogs of CH4 and CH5+ and is applicable to rovibrationally excited states with J>=0. A pairwise-additive potential fit for CH5+, which yields remarkable agreement with geometries, energies, and barrier heights on the full-dimensional surface of Brown et al. [J. Chem. Phys. 121, 4105 (2004)] is presented. Comparisons with experimental data and diffusion quantum Monte Carlo (DMC) methods test convergence for the POS model and provide insight into multidimensional quantum rovibrational dynamics. In particular, POS energy-level patterns for a series of scaled CH5+ potentials indicate an absence of strong tunneling behavior, consistent with the highly delocalized wave functions, large zero-point energies, and small interconversion barriers noted in previous DMC studies of Brown et al. [ABSTRACT FROM AUTHOR]

A potential energy surface for the lowest quartet electronic state (4A′) of lithium trimer is developed and used to study spin-polarized Li+Li2 collisions at ultralow kinetic energies. The potential energy surface allows barrierless atom exchange reactions. Elastic and inelastic cross sections are calculated for collisions involving a variety of rovibrational states of Li2. Inelastic collisions are responsible for trap loss in molecule production experiments. Isotope effects and the sensitivity of the results to details of the potential energy surface are investigated. It is found that for vibrationally excited states, the cross sections are only quite weakly dependent on details of the potential energy surface. [ABSTRACT FROM AUTHOR]

The ground-state energies and HF vibrational frequency shifts of ArnHF clusters have been calculated on the nonadditive potential-energy surfaces (PESs) for n=2-7 and on the pairwise-additive PESs for the clusters with n=1-12, using the diffusion Monte Carlo (DMC) method. For n>3, the calculations have been performed for the lowest-energy isomer and several higher-lying isomers which are the closest in energy. They provide information about the isomer dependence of the HF redshift, and enable direct comparison with the experimental data recently obtained in helium nanodroplets. The agreement between theory and experiment is excellent, in particular, for the nonadditive DMC redshifts. The relative, incremental redshifts are reproduced accurately even at the lower level of theory, i.e., the DMC and quantum five-dimensional (rigid Arn) calculations on the pairwise-additive PESs. The nonadditive interactions make a significant contribution to the frequency shift, on the order of 10%–12%, and have to be included in the PESs in order for the theory to yield accurate magnitude of the HF redshift. The energy gaps between the DMC ground states of the cluster isomers are very different from the energy separation of their respective minima on the PES, due to the considerable variations in the intermolecular zero-point energy of different ArnHF isomers. [ABSTRACT FROM AUTHOR]

A general, full-dimensional computational method for the accurate calculation of rotationally and vibrationally excited states of tetra-atomic molecules is further developed. The resulting computer program may be run in serial and parallel modes and is particularly appropriate for molecules executing wide-amplitude motions and isomerizations. An application to the isomerizing acetylene/vinylidene system is presented. Large-scale calculations using a coordinate system based on orthogonal satellite vectors have been performed in six dimensions and vibrational term values and wave functions for acetylene and vinylidene states up to ≈23 000 cm-1 above the potential minimum have been determined. This has permitted the characterization of acetylene and vinylidene states at and above the isomerization barrier. These calculations employ more extensive vibrational basis sets and hence consider a much higher density of states than in any variational calculations reported hitherto for this system. Comparison of the calculated density of states with that determined empirically suggests that our calculations are the most realistic achieved for this system to date. Indeed more states have been converged than in any previous study of this system. Calculations on lower lying excited states of acetylene based on HC–CH diatom-diatom coordinates give nearly identical results to those based on orthogonal satellite vectors. Comparisons are also made with calculations based on HH–CC diatom-diatom coordinates. [ABSTRACT FROM AUTHOR]

Semiempirical potential energy surfaces for the lowest three electronic states of the open-shell complex Br–HF are constructed, based on existing empirical potentials for Kr–HF and Kr–Ne and coupled-cluster electronic structure calculations for Br–Ne. Coupled cluster calculations are also described for He–F, Ne–F and Ar–F. Electrostatic interactions that arise from the quadrupole of the Br atom and the permanent multipoles of HF are also included in the Br–HF surfaces. The well depth of the lowest adiabatic surface is found to be 670 cm-1 at a linear equilibrium geometry. The results of helicity decoupled and full close-coupling calculations of the bound states of the complex are also described. The ground state, with total angular momentum projection quantum number |P|=3/2, is found 435 cm-1 below dissociation to Br ([sup 2]P[sub 3/2])+HF (j=0). The lowest-frequency intermolecular bending and stretching vibrations are predicted around 145 and 211 cm-1, respectively. Parity splittings are found to be extremely small for bound states with projection quantum number |P|=3/2. The relevance of the results to recently recorded spectra of Br–HF is discussed. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A general, six-dimensional computational method for the accurate calculation of rotationally and vibrationally excited states of tetra-atomic molecules is developed. The resulting program is particularly appropriate for molecules executing wide-amplitude motions and isomerizations. An application to the Ar[sub 2]HF van der Waals trimer is presented in which the HF intramolecular stretching coordinate is separated out adiabatically and is not treated explicitly. Vibrational term values up to about 100 cm[sup -1] with absolute convergence to better than 0.1 cm[sup -1] are reported. These calculations employ more extensive vibrational basis sets and hence consider a much higher density of states than hitherto. States that sample Ar-Ar-HF linear configurations and approach Ar-HF-Ar linear configurations are characterized for the first time. Results for total angular momentum J = 0 and 1 provide the first accurate calculations of rotational constants for this system. The rotational constants for the HF bending states of Ar[sub 2]HF in the ground and first vibrationally excited states of the HF monomer are in good agreement with experiment, confirming the accuracy of the potential used in this work. [ABSTRACT FROM AUTHOR]

The equilibrium and low-lying isomeric structures of Ar[sub n]OH (X²Π) clusters for n = 1 to 15 are investigated by simulated annealing calculations. Potential energy surfaces are obtained by a pairwise-additive approach, taking into account the open-shell nature of OH X²Π and including spin-orbit coupling. It is found that the spin-orbit coupling suppresses the Jahn-Teller effect, and many of the clusters have high-symmetry structures (C[sub nv] with n > 2) which would be forbidden in the absence of spin-orbit coupling. The structures are generally similar to those previously found for the closed-shell systems Ar[sub n]HF and Ar[sub n]HCl, but different from those for the open-shell systems Ar[sub n]NO and Ar[sub n]CH. This is because Ar-OH (X²Π), like Ar-HF and Ar-HCl but unlike Ar-NO and Ar-CH, has a near-linear equilibrium structure. Ar[sub n]OH clusters for n up to 6 have all Ar atoms in a single shell around OH. In the clusters with n = 7 to 9, OH is under a pentagonal pyramid formed by six Ar atoms, while the others bind to its exterior, away from OH. For n = 10 to 12, the minimum-energy structures have OH inside an Ar[sub n] cage, which is essentially icosahedral for n = 12 but has vacancies for n = 10 and 11. For n > 12, the extra Ar atoms begin to form a second solvation shell. The global minimum of Ar[sub n],OH may be constructed from the minimum-energy structure of Ar[sub n + 1] by replacing one Ar atom with OH. [ABSTRACT FROM AUTHOR]

This paper presents a theoretical study of the bound states of the open-shell OH radical in its ground electronic state (X[sup 2]H) interacting with n Ar atoms, for n from 4 to 12. After freezing the geometry of the Ar[sub n] cage or subunit at the equilibrium structure (preceding paper), we carry out nonadiabatic five-dimensional quantum dynamics calculations on two coupled potential energy surfaces, using an extension of the method previously applied to closed-shell Ar[sub n]HF clusters [J. Chem. Phys. 103, 1829 (1995)]. The method is based on a discrete variable representation (DVR) for the translational motion of OH relative to Ar[sub n], combined with a finite basis representation of the OH hindered rotation and electronic structure, including spin-orbit effects. The pattern of OH hindered rotor levels in clusters is similar to that in Ar-OH itself, though extended over three to four times the energy range for n = 4 to 9. Ar[sub 12]OH has a nearly spherical shell of Ar atoms around the OH, so the anisotropic splitting is very small. For n = 10 and 11, the anisotropy may be viewed as arising from holes in an otherwise spherical shell, and the resulting patterns of hindered rotor levels are inverted versions of those for Ar2OH and Ar-OH. [ABSTRACT FROM AUTHOR]

The near-dissociation microwave rovibronic spectra of HeN[sup +] [Carrington et al., Chem. Phys. Lett. 262, 598 (1996)] are used to obtain coupled potential energy curves for the six electronic states correlating with He+N[sup +] ³P[sub 0], ³P[sub 1], and ³P[sub 2]. High-quality ab initio calculations are carried out, using a spin-restricted open-shell coupled-cluster method with an augmented correlation-consistent quintuple-zeta basis set (aug-cc-pV5Z). Fully coupled calculations of bound and quasibound states are performed, including all six electronic states, and suggest two possible assignments of the observed transitions. The potentials are then morphed (scaled) to reproduce the experimental frequencies. One of the two assignments, designated SH1, is preferred because it gives a more satisfactory explanation of the observed hyperfine splittings. The corresponding morphed potential has well depths of 1954 cm[sup -1] and 192 cm[sup -1] for the spin-free ³Σ[sup -] and ³Π curves, respectively. [ABSTRACT FROM AUTHOR]

The full interaction potential between Ne(¹S) and Ne[sup +](²P) is determined by least-squares fitting of potential parameters to spectroscopic data, principally from the near-dissociation microwave spectra of the Ne[sup +][sub 2] complex. The potential obtained in this way incorporates the potential curves for all six electronic states correlating with Ne(¹S)+Ne[sup +](²P) and the couplings between them. Coupled-channel calculations on the potential take account of breakdown of the Born—Oppenheimer approximation and provide an accurate description of the microwave rovibronic spectrum involving levels within ∼10 cm[sup -1] of the first dissociation limit. The Ne[sup +][sub 2] ions are both vibrationally and rotationally hot: the spectrum involves levels up to at least J = 25/2 and there is evidence for transitions involving levels near the second dissociation limit. The long-range levels involved have up to 12 Å, compared with an equilibrium bond length of 1.756 Å for the ground electronic state. The long-range parameters of the interaction can be extracted from the fit and are compared with recent theoretical values. [ABSTRACT FROM AUTHOR]