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arXiv:1302.2746 · Physical Review A

van der Waals coefficients for systems with ultracold polar alkali-metal molecules

Piotr Szymon Żuchowski, Maciej B. Kosicki, Monika Kodrycka, Pavel Soldán

Published Feb 19, 2013• 52 citations
arXivHTMLPDF

Abstract

A systematic study of the leading isotropic van der Waals coefficients for the alkali-metal atom+molecule and molecule+molecule systems is presented. Dipole moments and static and dynamic dipole polarizabilities are calculated employing high-level quantum chemistry calculations. The dispersion, induction, and rotational parts of the isotropic van der Waals coefficient are evaluated. The known van der Waals coefficients are then used to derive characteristics essential for simple models of the collisions involving the corresponding ultracold polar molecules.

DOI
10.1103/physreva.87.022706

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A systematic study of the leading isotropic van der Waals coefficients for the alkali-metal atom+molecule and molecule+molecule systems is presented. Dipole moments and static and dynamic dipole polarizabilities are calculated employing high-level quantum chemistry calculations. The dispersion, induction, and rotational parts of the isotropic van der Waals coefficient are evaluated. The known van der Waals coefficients are then used to derive characteristics essential for simple models of the collisions involving the corresponding ultracold polar molecules.
The long-range part of the interatomic interactions plays a substantial role in the collisional dynamics of ultracold gases. Here, we report on the calculation of the isotropic and anisotropic C6{C}_{6}C6​ coefficients characterizing the van der Waals interaction between dysprosium or erbium atoms in the two lowest energy levels and the ground-state alkali-metal (Li, Na, K, Rb, Cs, Fr) or alkaline-earth-metal (Be, Mg, Ca, Sr, Ba) atoms. The calculations are done using the integral of dynamic dipole polarizabilities at so-called imaginary frequencies of the two interacting atoms. For all atom pairs, we find that the isotropic C6{C}_{6}C6​ coefficients are two or three orders of magnitude larger than the anisotropic ones. Those coefficients are essential for modeling collisional properties of heteronuclear quantum mixtures containing highly magnetic dysprosium or erbium atoms and alkali-metal or alkaline-earth-metal atoms.
The van der Waals coefficients for the alkali-metal atoms from Na to Fr interacting in their ground states are calculated using relativistic ab initio methods. The accuracy of the calculations is estimated by also evaluating atomic static electric-dipole polarizabilities and coefficients for the interaction of the atoms with a perfectly conducting wall. The results are in excellent agreement with the latest data from studies of magnetic field induced Feshbach resonances in ultracold collisions of Na and of Rb atoms. For Cs we provide critically needed data for ultracold collision studies.
The calculation of non-covalent interactions of highly polarizable molecules is an outstanding problem in chemistry and physics. Difficulties arise both in accurate treatments of dispersion interactions between molecules and in the evaluation of surfaces of sufficient size for use in further computations. While ``gold-standard" calculations using coupled cluster theory with singles, doubles and perturbative triples provides a reliable method for evaluating interactions the computational cost involved in calculating more than a few points of an interaction surface becomes prohibitive for even medium sized molecules. Expanding the long range interaction into a van der Waals series reduces the cost of evaluating a surface to the computation of a few parameters. We describe here the implementation of a new computer program for calculating van der Waals coefficients for arbitrary molecules using the sum over states method. The laboratory-frame transformation of the computed van der Waals surface and the inclusion of rotational state dressing on the surface is derived. Analytic approximations for the interactions of two linear molecules in the presence of a small DC electric field are also derived. Recent achievements in the formation and manipulation of ultracold polar molecules have opened the door to exciting new studies in cold chemical reactions. To characterize the energetics and reaction pathways of trimer and tetramer formation and reactions, we have computed the structure and thermochemistry of the model trimer Li3_33​ and every X2_22​Y2_22​ alkali tetramer through Cs. Related interest in the control of molecular ion reactions is also investigated with recent theoretical results for the rubidium hydroxide reaction presented here.
Progress in ultracold experiments with polar molecules requires a clear understanding of their interactions and reactivity at ultralow collisional energies. Two important theoretical steps in this process are the characterization of interaction potentials between molecules and the modeling of the reactive scattering mechanism. Here, we report on the ab initio calculation of isotropic and anisotropic van der Waals interaction potentials for polar KRb and RbCs colliding with each other or with ultracold atoms. Based on these potentials and two short-range scattering parameters, we then develop a single-channel scattering model with flexible boundary conditions. Our calculations show that, at low temperatures (and in the absence of an external electric field), the reaction rates between molecules or between molecules and atoms have a resonant character as a function of the short-range parameters. We also find that both the isotropic and anisotropic van der Waals coefficients have significant contributions from dipole coupling to excited electronic states. Their values can differ dramatically from those solely obtained from the permanent dipole moment. A comparison with recently obtained reaction rates of fermionic 40 K 87 Rb shows that the experimental data cannot be explained by a model where the short-range scattering parameters are independent of the relative orbital angular momentum or partial wave.
The van der Waals coefficients, C6,C8,{C}_{6},{C}_{8},C6​,C8​, and C10{C}_{10}C10​ for the alkali-metal (Li, Na, K, and Rb) and alkaline-earth-metal (Be, Mg, Ca, and Sr) atoms are estimated by a combination of ab initio and semiempirical methods. Polarizabilities and atom-wall coefficients are given as a diagnostic check, and the lowest order nonadiabatic dispersion coefficient, D8{D}_{8}D8​ and the three-body coefficient, C9{C}_{9}C9​ are also presented. The dispersion coefficients are in agreement with the available relativistic many-body perturbation theory calculations. The contribution from the core was included by using constrained sum rules involving the core polarizability and Hartree-Fock expectation values to estimate the f-value distribution.
The C3{C}_{3}C3​ coefficients and their ratios for the alkali-metal atoms are determined very accurately by taking into account the optical properties of the atoms and four distinct types of trapping materials such as Au (metal), Si (semiconductor), vitreous SiO2{\text{SiO}}_{2}SiO2​ (dielectric), and SiNx{\text{SiN}}_{x}SiNx​ (dielectric). Dynamic dipole polarizabilities are calculated precisely for the alkali-metal atoms that produce C3{C}_{3}C3​ coefficients in a perfectly conducting medium within 0.2% accuracy. Thus, uncertainties from the atomic polarizabilities in the evaluation of the material-dependent C3{C}_{3}C3​ coefficients are small and accuracies of these C3{C}_{3}C3​ values mainly rely on the used optical data which are not taken into account in the present work. Our findings are in very good agreement with the available measurements, but they differ by around 10% from the previously reported high-precision calculations in heavy atoms for the perfect conductors. We also revisit the dispersion coefficients for the alkali-metal dimers and evaluate them using the above dynamic dipole polarizabilities which provide slightly different error bars than the other reported precise results. These coefficients are fitted into a ready-to-use functional form to aid the experimentalists in finding out the interaction potential only with the knowledge of atom-wall distance.