Formation and dynamics of van der Waals molecules in buffer-gas traps

Abstract

We show that weakly bound He-containing van der Waals molecules can be produced and magnetically trapped in buffer-gas cooling experiments, and provide a general model for the formation and dynamics of these molecules. Our analysis shows that, at typical experimental parameters, thermodynamics favors the formation of van der Waals complexes composed of a helium atom bound to most open-shell atoms and molecules, and that complex formation occurs quickly enough to ensure chemical equilibrium. For molecular pairs composed of a He atom and an S-state atom, the molecular spin is stable during formation, dissociation, and collisions, and thus these molecules can be magnetically trapped. Collisional spin relaxations are too slow to affect trap lifetimes. However, 3 He-containing complexes can change spin due to adiabatic crossings between trapped and untrapped Zeeman states, mediated by the anisotropic hyperfine interaction, causing trap loss. We provide a detailed model for Ag 3 He molecules, using ab initio calculation of Ag-He interaction potentials and spin interactions, quantum scattering theory, and direct Monte Carlo simulations to describe formation and spin relaxation in this system. The calculated rate of spin-change agrees quantitatively with experimental observations, providing indirect evidence for molecular formation in buffer-gas-cooled magnetic traps.The ability to cool and trap molecules holds great promise for new discoveries in chemistry and physics [1][2][3][4] . Cooled and trapped molecules yield long interaction times, allowing for precision measurements of molecular structure and interactions, tests of fundamental physics 5,6 , and applications in quantum information science 7 . Chemistry shows fundamentally different behavior for cold molecules 8 , and can be highly controlled based on the kinetic energy, external fields 9 , and quantum states 10 of the reactants.Experiments with cold molecules have thus far involved two classes of molecules: Feshbach molecules and deeply bound ground-state molecules. Feshbach molecules are highly vibrationally excited molecules, bound near dissociation, which interact only weakly at long range, due to small dipole moments. They are created by binding pairs of ultracold atoms using laser light (photoassociation), ac magnetic fields (rf as- . By contrast, ground-state heteronuclear molecules often have substantial dipole moments and are immune to spontaneous decay and vibrational relaxation, making these molecules more promising for applications in quantum information processing 7 and quantum simulation 11 . These molecules are either cooled from high temperatures using techniques such as buffer-gas cooling or Stark deceleration, or are created via coherent deexcitation of Feshbach molecules.This article introduces a third family to the hierarchy of trappable molecules -van der Waals (vdW) complexes. VdW molecules are bound solely by long-range dispersion interactions, leading to the weakest binding energies of any groundstate molecules, on the order of a wavenum...

Dissociation energies of Ag–RG (RG = Ar, Kr, Xe) and AgO molecules from velocity map imaging studies

“…A better understanding of the nature of these fundamental interactions would inform fields as diverse as molecule surface interactions [1] and buffer gas cooling. [2] The study of such molecules has been lent added impetus in recent years, by the popular use of rare gas atoms as messenger tags in action spectroscopy studies of gas-phase metal-ligand [3,4] naked metal [5][6][7][8] and metal oxide clusters. [9][10][11] In such studies, the structures of interest are widely assumed to be affected minimally by the presence of the inert tag with the result that the photodissociation action spectrum mirrors that of the naked species.…”

Cold Anisotropically Interacting van der Waals Molecule: TiHe

“…Recent groundbreaking advances in the production and trapping of translationally cold molecules [16] made it possible to create trapped ensembles of cold polar molecules with high enough densities to study collisions and chemical reactions [16][17][18] and carry out ultra-precise spectroscopic measurements to probe the physics beyond the Standard Model [19]. The production and trapping of cold vdW molecules would similarly enable highly sensitive spectroscopic detection of heretofore unobserved clusters, as well as the study and control of their quantum dynamics [7,[20][21][22].We have recently observed the formation of cold, ground-state LiHe molecules in a cryogenic He buffer gas [23]. The LiHe molecule has a single near-threshold bound state with a binding energy of 0.024 cm −1 [24] comparable to that of the He 2 dimer [11,21].…”

“…In condensed-phase chemical physics, vdW clusters serve as a model to study the mechanisms of solvation, nucleation, and chemical reactivity [6][7][8]. In the context of quantum many-body physics, vdW molecules can be used to explore the formation of exotic quasiparticles in superfluid helium nanodroplets [9].…”

“…Due to their vanishingly small binding energies, these molecules belong to an exotic class of quantum halo dimers characterized by extremely delocalized wavefunctions, universal properties, and enormously large three-body formation rates [3,25]. As the binding energies of most other * weinstein@physics.unr.edu; http://www.physics.unr.edu/xap/ atoms and molecules with He are much larger than those of LiHe and He 2 [7], it is far from obvious whether atomHe vdW clusters would form upon immersing the parent atoms into cryogenic He buffer gas. In fact, no such clusterization was observed in recent buffer gas cooling experiments involving large hydrocarbon molecules [26].…”

“…Lf, 33.15.Fm, Weakly bound complexes of atoms and molecules held together by long-range van der Waals (vdW) forces are key to understanding a wide range of phenomena in physics and chemistry, ranging from classical and quantum chaos [1] and phase transitions [2] to the universal physics of Efimov trimers and quantum droplets [3][4][5]. In condensed-phase chemical physics, vdW clusters serve as a model to study the mechanisms of solvation, nucleation, and chemical reactivity [6][7][8]. In the context of quantum many-body physics, vdW molecules can be used to explore the formation of exotic quasiparticles in superfluid helium nanodroplets [9].…”