Helium dimer in its metastable triplet electronic state, He
2(a 3Σu+), is a promising candidate for direct laser cooling and precision measurements. Interactions and collisions involving He
2(a 3Σu+) remain unexplored in ultracold and cold energy domains. Here, we present the first theoretical investigation of intermolecular interactions and collision dynamics of He
2(a 3Σu+)+Ne van der Waals complex. For the ground electronic state,
X 3A′, we employ a spin-restricted coupled cluster method with single, double, and noniterative triple excitations. The resulting interaction potential exhibits an extremely shallow well with a depth of 4.612 cm
−1. We construct a three-dimensional vibrationally averaged potential energy surface. Using this surface, we perform full close-coupling calculations including fine-structure levels of He
2(a 3Σu+) to investigate collisional excitation and quenching induced by Ne. We analyze the low-energy scattering dynamics and model resonant features associated with quasi-bound states supported by interaction potential. Propensity rule for fine-structure conserving transitions is established. We additionally consider He
2+Ar collisions and compare their energy dependence and resonance structure with those of He
2+Ne. The He
2+Ar system exhibits a richer resonance structure, highlighting the sensitivity of low-energy scattering to interaction potential and reduced mass. Calculated PES and scattering properties provide a reliable benchmark for description of He
2+Ne van der Waals complex and may be useful for interpretation of future experimental spectra and collision measurements.
High Resolution Image Download MS PowerPoint Slide The recent observation of a high-spin Na 2 dimer formed on the surface of liquid helium nanodroplets raises some fundamental questions, as the ground state of Na 2 is known to have zero spin. Is it protected against spontaneous dissociation? What is its binding energy and interatomic distance? Is it stable at a higher temperature? Using calculations based on density functional theory (with and without long-range interaction) and coupled cluster methods, CCSD(T), we show that the bonding in the high-spin Na 2 dimer is governed by van der Waals interaction with binding energy (bond length) varying between −0.030 eV (5.108 Å) and −0.192 eV (4.231 Å), depending on the computational method used. Thus, the experimental method used by Kresin and co-workers can be very useful to study larger metastable high-spin clusters such as Li 4 which was predicted in 1985 to have a tetrahedral structure carrying a magnetic moment of 2 μ B, while its ground state is planar and nonmagnetic.
We have studied the long-range interaction potentials of the heteronuclear alkali-metal-atom dimers in the case when one of the atoms is in the first excited state (
nAP) and the other is in the ground state (
nBS). The presentation of the general expressions for the interaction energies in the Hund's case (c) is followed by the calculation of the corresponding
C6 constants for all heteronuclear alkali-metal-atom pairs. In the case of heavier perturbers (Rb and Cs) the
C6 constants are found to be significantly influenced by the perturber's fine structure. The van der Waals interaction in the K-Rb system is exceptionally strong resulting in
C6 constants considerably larger than in all other alkali-metal-atom heteronuclear systems. The intermediate-coupling region (ICR) is defined as the region of internuclear separation R where the interaction energy is of the order of the atomic spin-orbit splitting, while the exchange energy is still negligible. Analysis of the general form of the energy matrices in the ICR shows that the homonuclear alkali-metal-atom dimers can be treated as a special case. In the large-R limit, the ICR results reduce to those obtained in Hund's case (c). As an example, we have calculated the ICR interaction potentials of the K-Rb system, taking into account only the leading dipole-dipole interaction. Finally we have discussed the construction of the absorption coefficient in the wings of the resonance lines, both in the impact and in the quasistatic approximation.
We theoretically investigate the spin structure of weakly bound diatomic van der Waals molecules formed by two identical bosonic alkali atoms. Our studies were performed using known Born-Oppenheimer potentials while developing a reduced interaction potential model. Such reduced potential models are currently a key for solving certain classes of few-body problems of atoms as they decrease the numerical burden on the computation. Although the reduced potentials are significantly shallower than actual Born-Oppenheimer potentials, they still capture the main properties of the near-threshold bound states, including their spin structure, and the scattering states over a broad range of magnetic fields. At zero magnetic field, we find that the variation in spin structure across different alkali species originates from the interplay between electronic spin exchange and hyperfine interactions. To characterize this competition we introduce a single parameter that is a function of the singlet and triplet scattering lengths, the atomic hyperfine splitting constant, and the molecular binding energy. We show that this parameter can be used to classify the spin structure of vdW molecules for each atomic species.
The ground-state interaction potential of ytterbium dimer is investigated at eXact 2-component core-correlated CCSD(T) level of {\it ab initio} theory in the complete basis set limit with extensive augmentation by diffuse functions. For the basis set of double-
ζ quality, the comparison is made with the four-component relativistic finite-nuclei CCSD(T) calculations to identify the contraction of the dimer bond length as the main unrecoverable deficiency of the scalar-relativistic approximation. Empirical constraint on the number of bound vibrational energy levels of the
174Yb
2 dimer is accounted for by employing the model semianalytical potential function [M. Borkowski et al. Phys. Rev. A {\bf 96}, 063405 (2017)] containing the scale and shift parameters. Using the best {\it ab initio} data as its short-range part, we estimated the equilibrium parameters of Yb
2 dimer as
Re∈[4.45,4.55] A and
De=745±10 cm
−1. Furthermore, we demonstrated that the model potential provides an accurate and flexible reference for future research in constraining short-range gravity-like forces by ultracold atomic spectroscopy.