Stimulated radiative association of sodium and chlorine atoms and their ions in a coupled channel treatment.

In an extension of previous work (Simsová et al., Phys. Chem. Chem. Phys., 2022, 24, 25250), we study stimulated radiative association of sodium chloride (NaCl) in an environment with a black body radiation. Colliding neutral (Na and Cl) and ionic (Na+ and Cl-) fragments are considered. The coupling between the diabatic ionic and neutral channels is accounted for. The cross sections are computed and resolved on the vibrational states of the formed NaCl molecule for detailed analysis. The thermal rate coefficients for neutral colliding fragments at kinetic temperatures, T, from 1 K to 5300 K are computed for use in astrochemical modelling. The total rate coefficient is affected by more than one order of magnitude by stimulated emission from a blackbody radiator of temperature Tb = 50 000 K. The effect from stimulated emission is largest for the lowest kinetic temperatures, where Tb of a few thousand kelvins has a significant effect. The rate coefficient for the colliding ionic fragments is calculated from 80 K to 3615 K. The blackbody radiation has little effect on this process.

Non-adiabatic dynamics in collisions of sodium and chlorine atoms and their ions.

Collisions of sodium and chlorine atoms and of their ions are studied within the diabatic two-state picture at energies below and above the ionic threshold with focus on the processes of radiative association, chemiionisation, and mutual neutralisation. The radiative-association cross sections as functions of collision energy are calculated up to 4.6 eV in the case of neutral atoms and up to 3.12 eV in the case of ions. The non-radiative charge-exchange cross sections as functions of collision energy are calculated up to 12 eV for chemiionisation and up to 10.52 eV for mutual neutralisation. The corresponding radiative-association rate coefficients are then determined up to 5300 K for the radiative association of neutral atoms and non-radiative charge-exchange and up to 3615 K for the radiative association of ions. Contribution of many Fano-Feshbach-type resonances is included to the rate coefficient of neutral-atom radiative association. The chemiionisation rate coefficients were calculated from 1000 K to 5300 K. The process of mutual neutralisation exhibits the largest cross sections and also the largest rate coefficients with values around 10-9 cm3 s-1 at all calculated temperatures, 120-5300 K.

Potential energy surface for spin-polarized rubidium trimer.

Potential energy surface for the lowest quartet state of the rubidium trimer is constructed, making use of the many-body decomposition. Interaction energies are calculated using the coupled-clusters method and interpolated using the reciprocal-power reproducing kernel Hilbert space interpolation method. Both the two-body and three-body nonadditive parts are extrapolated to exhibit the correct long-range behavior. Consequences for the low-energy scattering are briefly discussed.

Prospects for sympathetic cooling of polar molecules: NH with alkali-metal and alkaline-earth atoms--a new hope.

We explore the potential energy surfaces for NH molecules interacting with alkali-metal and alkaline-earth atoms using highly correlated ab initio electronic structure calculations. The surfaces for interaction with alkali-metal atoms have deep wells dominated by covalent forces. The resulting strong anisotropies will produce strongly inelastic collisions. The surfaces for interaction with alkaline-earth atoms have shallower wells that are dominated by induction and dispersion forces. For Be and Mg the anisotropy is small compared to the rotational constant of NH, so that collisions will be relatively weakly inelastic. Be and Mg are thus promising coolants for sympathetic cooling of NH to the ultracold regime.

On the doublet states of the potassium trimer.

The potassium trimer is investigated in its lowest electronic doublet states, employing several high-level ab initio methods (coupled cluster with single, double, and noniterative triple excitations, multiconfiguration self-consistent field, and multireference Rayleigh-Schrodinger perturbation theory of second order). One-dimensional cuts through the lowest 12 electronic states at C(2v) symmetry give insight in the complex electronic structure of the trimer, showing several (pseudo-)Jahn-Teller distortions that involve two or three excited states. Contour plots of the involved molecular orbitals are shown to prove the validity of the shell model frequently used for a qualitative description of metallic clusters.

Investigation of the benzene-dimer potential energy surface: DFT/CCSD(T) correction scheme.

A novel method, designated as the density functional theory/coupled-cluster with single and double and perturbative triple excitation [DFT/CCSD(T)] correction scheme, was developed for precise calculations of weakly interacting sp(2) hydrocarbon molecules and applied to the benzene dimer. The DFT/CCSD(T) interaction energies are in excellent agreement with the estimated CCSD(T)/complete basis set interaction energies. The tilted T-shaped structure having C(s) symmetry was determined to be a global minimum on the benzene-dimer potential energy surface (PES), approximately 0.1 kcal/mol more stable than the parallel-displaced structure. A fully optimized set of ten stationary points on the benzene-dimer PES is proposed for the evaluation of the reliability of methods for the description of weakly interacting systems.

Tuning ab initio data to scattering length: the a (3)Sigma(+) state of KRb.

Interaction energies for the lowest triplet state a (3)Sigma(+) of KRb are calculated using high level ab initio methods. The interaction energies are then morphed so that the resulting potential energy curve yields 32 bound states and the correct scattering length for (40)K(87)Rb. Calculated vibrational spacings are shown to be in very good agreement with the available experimental Fourier transform and photoassociation vibrational data, but a different numbering scheme has to be used for the experimental vibrational assignment.

Interactions and dynamics in Li+Li2 ultracold collisions.

A potential energy surface for the lowest quartet electronic state ((4)A(')) of lithium trimer is developed and used to study spin-polarized Li+Li(2) 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 Li(2). 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.

Ultracold Rb-OH collisions and prospects for sympathetic cooling.

We compute ab initio cross sections for cold collisions of Rb atoms with OH radicals. We predict collision rate constants of order 10(-11) cm3/s at temperatures in the range 10-100 mK at which molecules have already been produced. However, we also find that in these collisions the molecules have a strong propensity for changing their internal state, which could make sympathetic cooling of OH in a Rb buffer gas problematic in magnetostatic or electrostatic traps.

Ultracold collisions involving heteronuclear alkali metal dimers.

We carry out the first quantum dynamics calculations on ultracold atom-diatom collisions in isotopic mixtures. The systems studied are spin-polarized 7Li + 6Li7Li, 7Li + 6Li2, 6Li + 6Li7Li, and 6Li + 7Li2. Reactive scattering can occur for the first two systems even when the molecules are in their ground rovibrational states, but is slower than vibrational relaxation in homonuclear systems. Implications for sympathetic cooling of heteronuclear molecules are discussed.

Ultracold Li + Li2 collisions: bosonic and fermionic cases.

We have carried out quantum dynamical calculations of vibrational quenching in Li + Li(2) collisions for both bosonic (7)Li and fermionic (6)Li. These are the first ever such calculations involving fermionic atoms. We find that for the low initial vibrational states considered here (v < or = 3), the quenching rates are not suppressed for fermionic atoms. This contrasts with the situation found experimentally for molecules formed via Feshbach resonances in very high vibrational states.

Spectroscopy of K+ x Rg and transport coefficients of K+ in Rg (Rg=He-Rn).

Ab initio calculations employing the coupled-cluster method, with single and double substitutions and accounting for triple excitations noniteratively [CCSD(T)], are used to obtain accurate potential energy curves for the K(+)He, K(+)Ne, K(+)Ar, K(+)Kr, K(+)Xe, and K(+)Rn cationic complexes. From these potentials, rovibrational energy levels and spectroscopic parameters are calculated. In addition, mobilities and diffusion coefficients for K(+) cations moving through the six rare gases are calculated, under conditions that match previous experimental determinations. A detailed statistical comparison of the present and previous potentials is made with available experimental data, and critical conclusions are drawn as to the reliability of each set of data. It is concluded that the present ab initio potentials match the accuracy of the best model potentials and the most reliable experimental data.

Interaction of NH(X3Sigma-) molecules with rubidium atoms: implications for sympathetic cooling and the formation of extremely polar molecules.

The Rb-NH interaction is investigated as a prototype for interactions between alkali-metal atoms and stable molecules. For spin-aligned Rb and NH that interact on a quartet surface (4A"), the interaction is relatively weak, with a well depth of 0.078 eV. However, there are also doublet surfaces of ion-pair character that are very much deeper (well depth 1.372 eV). They may be important for atom-molecule collision rates and offer the possibility of forming strongly dipolar molecules by photoassociation. Similar deeply bound ion-pair states are likely to exist for other alkali atom-molecule pairs.

Quantum dynamics of ultracold Na+ Na2 collisions.

Ultracold collisions between spin-polarized Na atoms and vibrationally excited Na2 molecules are investigated theoretically, using a reactive scattering formalism (including atom exchange). Calculations are carried out on both pairwise additive and nonadditive potential energy surfaces for the quartet electronic state. The Wigner threshold laws are followed for energies below 10(-5) K. Vibrational relaxation processes dominate elastic processes for temperatures below 10(-3)-10(-4) K. For temperatures below 10(-5) K, the rate coefficients for vibrational relaxation (v=1-->0) are 4.8x10(-11) and 5.2x10(-10) cm(3) s(-1) for the additive and nonadditive potentials, respectively. The large difference emphasizes the importance of using accurate potential energy surfaces for such calculations.