Interactions between ultracold molecules controlled by physicists

Amazing stuff! Controlling chemical reactions down to the level of a single molecule! Mind boggling!

"A way of colliding ultracold molecules while controlling the rate at which they react has been developed by physicists at the Massachusetts Institute of Technology (MIT) in the US. Researchers at Germany’s Max Planck Institute for Quantum Optics have made a similar discovery using an different experimental technique. Their research opens new pathways for enhanced control of chemical reactions. ..."

From the abstract of the first research article:

"Collisional resonances are important tools that have been used to modify interactions in ultracold gases, for realizing previously unknown Hamiltonians in quantum simulations, for creating molecules from atomic gases and for controlling chemical reactions. So far, such resonances have been observed for atom–atom collisions, atom–molecule collisions and collisions between Feshbach molecules, which are very weakly bound. Whether such resonances exist for ultracold ground-state molecules has been debated owing to the possibly high density of states and/or rapid decay of the resonant complex. Here we report a very pronounced and narrow (25 mG) Feshbach resonance in collisions between two triplet ground-state NaLi molecules. This molecular Feshbach resonance has two special characteristics. First, the collisional loss rate is enhanced by more than two orders of magnitude above the background loss rate, which is saturated at the p-wave universal value, owing to strong chemical reactivity. Second, the resonance is located at a magnetic field where two open channels become nearly degenerate. This implies that the intermediate complex predominantly decays to the second open channel. We describe the resonant loss feature using a model with coupled modes that is analogous to a Fabry–Pérot cavity. Our observations provide strong evidence for the existence of long-lived coherent intermediate complexes even in systems without reaction barriers and open up the possibility of coherent control of chemical reactions."

From the abstract of the second research article:

"Scattering resonances are an essential tool for controlling the interactions of ultracold atoms and molecules. However, conventional Feshbach scattering resonances, which have been extensively studied in various platforms, are not expected to exist in most ultracold polar molecules because of the fast loss that occurs when two molecules approach at a close distance. Here we demonstrate a new type of scattering resonance that is universal for a wide range of polar molecules. The so-called field-linked resonances occur in the scattering of microwave-dressed molecules because of stable macroscopic tetramer states in the intermolecular potential. We identify two resonances between ultracold ground-state sodium–potassium molecules and use the microwave frequencies and polarizations to tune the inelastic collision rate by three orders of magnitude, from the unitary limit to well below the universal regime. The field-linked resonance provides a tuning knob to independently control the elastic contact interaction and the dipole–dipole interaction, which we observe as a modification in the thermalization rate. Our result provides a general strategy for resonant scattering between ultracold polar molecules, which paves the way for realizing dipolar superfluids and molecular supersolids, as well as assembling ultracold polyatomic molecules."

Interactions between ultracold molecules controlled by physicists – Physics World

A Feshbach resonance in collisions between triplet ground-state molecules (no public access, but article above contains a link to the PDF file)

Field-linked resonances of polar molecules (open access)

Fig. 1: Interaction potentials and bound states of microwave-dressed ground-state molecules

Physicists detect a new type of molecular bond with an unusual bond length of several micrometers

Amazing stuff! At thefrontier of ultracold chemistry and physics!

"... Using a specially designed microscope, the team observed a binding mechanism between a charged ion and a neutral Rydberg atom ... The extent of the bond length in the new molecule is as wide as a few micrometres, which is at least 1000 times larger than in usual molecules. ...
To verify the molecule’s formation, the researchers devised a special ion microscope. ... in this microscope an electric field separates the molecule and ionizes the Rydberg atom. The now separated ion and Rydberg core are then guided along the microscope and onto a detector. Due to their different charge-mass ratios, the Rydberg core and the ion will arrive at this detector at different times, allowing each of them to be detected individually. ..."

"Ultracold temperatures in atomic and molecular gases have allowed for a new branch of chemistry, where novel weak binding mechanisms between atoms have been observed. ..."

From the abstract:

"Atoms with a highly excited electron, called Rydberg atoms, can form unusual types of molecular bonds. The bonds differ from the well-known ionic and covalent bonds not only by their binding mechanisms, but also by their bond lengths ranging up to several micrometres. Here we observe a new type of molecular ion based on the interaction between the ionic charge and a flipping-induced dipole of a Rydberg atom with a bond length of several micrometres. We measure the vibrational spectrum and spatially resolve the bond length and the angular alignment of the molecule using a high-resolution ion microscope. As a consequence of the large bond length, the molecular dynamics is extremely slow. These results pave the way for future studies of spatio-temporal effects in molecular dynamics (for example, beyond Born–Oppenheimer physics)."

Physicists detect a new type of molecular bond – Physics World

Observation of a molecular bond between ions and Rydberg atoms (no public access)

Spatial imaging of a novel type of molecular ions (corresponding preprint article)

Physicists fine tune chemical reaction rates for ultracold molecules

Amazing stuff!

"A new technique to cool reactive molecules to temperatures low enough [close to absolute zero Kelvin] to achieve quantum degeneracy – something not generally possible before – has been created by researchers in the US. In this temperature regime, the dominance of quantum effects over thermal fluctuations should allow researchers to study new quantum properties of molecules. As a first example, the researchers demonstrated how a slight change in applied electric field can alter the reaction rate between molecules by three orders of magnitude. ...
This has led to some fascinating discoveries. In ultracold quantum bosonic or fermion-pair quantum gases, for example, all the atoms in a trap can simultaneously occupy the quantum ground state, resulting in a wavefunction that is macroscopic. ...
Cooling and trapping molecules is much trickier because they are inherently more complex than atoms. Whereas atoms can only contain quanta of energy in electronic excitations, the chemical bonds in molecules can stretch, rotate and bend – and cooling molecules involves removing energy from all of these degrees of freedom. ...
they applied an electric field to compress potassium and rubidium atoms in a 3D optical trap, inducing the atoms to pair up and thereby forming a 2D cloud of polarized potassium-rubidium molecules. Side-to-side collisions were elastic, whereas the head-to-tail ones were inelastic. As the molecules were polarized and confined to two dimensions, they were much more likely to collide side-to-side than head-to-tail. This allowed the researchers to achieve about 200 elastic collisions for every inelastic one, driving out the hotter molecules and cooling their sample to quantum degeneracy. ..."

"... It could also be used for creating long-lived quantum molecular gases of other polar molecules under strong electric fields"

Physicists fine tune chemical reaction rates for ultracold molecules – Physics World

Here are the respective research papers:

Dipolar evaporation of reactive molecules to below the Fermi temperature

Resonant collisional shielding of reactive molecules using electric fields