Observation of magnetically tunable Feshbach resonances in ultracold ²³ Na ⁴⁰ K + ⁴⁰ K collisions

Abstract

Resonances in ultracold collisions involving heavy molecules are difficult to understand, and have proven challenging to detect. Here we report the observation of magnetically tunable Feshbach resonances in ultracold collisions between 23 Na 40 K molecules in the rovibrational ground state and 40 K atoms. We prepare the atoms and molecules in various hyperfine levels of their ground states and observe the loss of molecules as a function of the magnetic field. The atommolecule Feshbach resonances are identified by observing an enhancement of the loss rate coefficients. We have observed three resonances at approximately 101 G in various atom-molecule scattering channels, with the widths being a few hundred milliGauss. The observed atom-molecule Feshbach resonances at ultralow temperatures probe the three-body potential energy surface with an unprecedented resolution. Our work will help to improve the understanding of complicated ultracold collisions, and open up the possibility of creating ultracold triatomic molecules.Understanding collisions involving molecules at the quantum level has been a long-standing goal in chemical physics [1]. Scattering resonance is one of the most remarkable quantum phenomena and plays a critically important role in the study of collisions. It is very sensitive to both the long-range and short-range parts of the molecule interaction potential, and thus offers a unique probe of the potential energy surface (PES) governing the collision dynamics. Although scattering resonances are well known and have been the main features in ultracold atomic gases and nuclear collisions [2], they have proven challenging to observe in molecule systems. Recently, significant progress has been achieved in experimentally studying resonances in cold molecular collisions involving the light particles, e.g., H 2 , HD molecule or He atom, by means of molecular beam techniques. In the crossed-beam or merged-beam experiments, shape resonances or Feshbach resonances have been observed in atom-molecule chemical reactions [3][4][5][6][7][8], atom-molecule inelastic collisions [9][10][11], and molecule-molecule inelas- * These authors contributed equally to this work. tic collisions [12,13]. However, in these experiments, the collision energies are still high (at Kelvin or sub Kelvin), and thus a few partial waves contribute to the scattering cross sections.Ultracold molecules offer great opportunities to study molecular collisions in the quantum regime. At ultralow temperatures, the de Broglie wavelength of the collision partners is much larger than the range of molecular interaction potential, and only the lowest possible partial wave dominates the collision process [14,15]. Consequently, the collisions at ultracold temperatures are highly quantum mechanical. Due to the anisotropy of the PES, the collisions involving ultracold molecules may support many resonances that are contributed by the rotational and vibrational excited states [16,17]. Therefore, it is expected that scattering resonances can be routinely obse...

Loss of Ultracold Rb87Cs133 Molecules via Optical Excitation of Long-Lived Two-Body Collision Complexes

“…However, in their proposal, the complex lifetimes are predicted to be sufficiently long that a further collision with a third molecule is possible, leading to loss of all three molecules from the trap [48]. Crucially, if the formation of collision complexes is the rate-limiting step, then the loss would appear to be two-body in nature, consistent with experimental observations [43][44][45][46].…”

“…Thankfully not all bialkali molecules have energetically allowed two-body reactive collisions [42], offering hope that stable molecular gases may be produced. However, experiments with nonreactive molecules such as bosonic 87 Rb 133 Cs [4,43] and 23 Na 87 Rb [44,45], and fermionic 23 Na 40 K [6,46], have all observed fast losses from optical traps, characterised by two-body loss rates comparable to those found in the reactive case. Understanding the mechanism for this loss is of paramount importance to the development of the field.…”

Universal Scattering of Ultracold Atoms and Molecules in Optical Potentials

“…Elastic resonant scattering between alkali-metal dimers and its heaviest constituent atom has been explored in Reference [16]. In fact, some Feshbach resonances have been observed in 500 nK 23 Na 40 K+ 40 K collisions [17]. The KRb + K and other reaction-rate measurements [18][19][20][21][22] have focused on total reaction rate coefficients.…”

Precise Feshbach resonance spectroscopy using tight anharmonic traps

“…Collisions of lanthanide atoms with high magnetic moments such as erbium and dysprosium involve hundreds of coupled channels due to large interaction anisotropy, leading to multiple resonances per gauss [15,16]. Similar situation occurs for atom-molecule and molecule-molecule collisions [17], where resonances have recently been observed [18]. Coupled multichannel systems require extensive numerical resources and cannot provide classification of bound states in terms of quantum numbers.…”