Cold Molecules

JILA experimental and theoretical physicists work closely together on investigations of ultracold molecules. Their efforts to create and investigate these exotic forms of matter build on the Institute's ultracold atom expertise. Researchers use tools such as Feshbach resonances, laser cooling, and Stark deceleration to produce cold molecules. They study these molecules under rigorously controlled conditions, seeking to understand how to produce them in specific quantum states, track the flow of energy among molecular constituents, and control chemical reactions. JILA scientists have recently undertaken major initiatives to explore cold polar molecules with complex internal structures, the production of "cold molecule soups" containing different types of molecules, and the study of chemical reactions in ultracold environments. In the latter studies, they are working on controlling the internal states of the original atoms and molecules and monitoring the reactions between them.

Ultracold Polar Molecules

When the Deborah Jin and Jun Ye group collaboration wanted to investigate the creation of stable ultracold polar molecules, it initially created ultracold Feshbach molecules of KRb and studied their collision behavior with atoms of K and Rb in the same ultracold gas cloud. The goal was to understand the new system and prepare for future experiments to drive the KRb molecules into more stable molecular states. Theorist José D’Incao, a senior research associate in the Chris Greene group, assisted the experimentalists in understanding the physics governing the interactions (collisions) between the atoms and molecules.

These early efforts bore fruit in the fall of 2008 when the two groups crafted tens of thousands of ultracold polar molecules in their lowest energy state. These ground-state molecules are too cold to exist naturally anywhere in the Universe. To make them, the groups started with a cloud of 40K and 87Rb atoms, which they cooled to less than a millionth of a degree about absolute zero (see Figure I). The atoms in the ultracold cloud were far apart. For atoms to form even the most loosely associated molecules, they had to move about 30 times closer together in the ultracold gas.

To bring the atoms closer together, the researchers lowered the magnetic field around the ultracold gas cloud, causing the formation of large, fluffy, and loosely bound Feshbach KRb molecules (see Figure II). This process, known as Feshbach association, brought the atomic nuclei in these molecules just close enough to barely stay together; the electron clouds around the nuclei didn’t interact with each other as they would in a “normal” molecule. The mechanism holding the Feshbach molecules together was similar to that of clusters of water molecules found in nature.

For the atomic nuclei in the new molecules to form tightly bound ground-state polar molecules, they had to move another 30 times closer to each other — close enough for their electron clouds to overlap and form true chemical bonds. The researchers accomplished this feat by using two laser beams to produce a perfectly tuned light pulse that coherently transferred the molecules into one of two low-energy ground states (see Figure III).

The higher-energy ground state molecules had electron clouds that overlapped slightly, and they had small, but short-lived, electric dipole moments. In contrast, the lowest-energy ground-state KRb molecules (see Figure IV) had electron clouds that overlapped to a greater degree. Their electric dipole moments were more than tenfold stronger, and they lasted longer.

The Jin and Ye groups have been working around the clock since 2008 to characterize these exotic dipole molecules. They plan to use them in future studies of dipolar Fermi gases and Bose-Einstein condensation. Jin, Ye, and Cornell are collaborating with theorist John Bohn, who is working on theoretical models of cold molecule behavior and candidate molecules for eEDM experiments. Theorist Chris Greene contributes to this field with his studies of the quantum mechanics of ultracold systems.

The Ye group is also investigating collisions of low-energy (80 K) ground-state polar molecules such as hydroxyl (OH) free radicals and formaldehyde (H₂CO) molecules. These experiments should bring new opportunities for control over intermolecular interactions at cold temperatures. The group recently studied collisions between OH radicals and supersonic beams of either helium atoms or deuterium molecules. The more complex D₂ molecules were excited by some of the OH molecules via a collisional resonance that set both molecules spinning. Similar resonant energy transfer may also occur in interstellar clouds between H₂ and OH molecules, leading to the emission of coherent radio-wavelength photons, or masing.

In similar studies, Heather Lewandowski's group uses a two-step process to prepare ultracold molecules of NH, a simple free radical. First, NH molecules are forced through a small opening into a vacuum system where intermolecular collisions cool the rapidly expanding gas (

400 m/s) to less than 1 K. Next, the Lewandowski group uses time-varying electric fields (Stark deceleration) to slow the cold molecules to rest. Once the molecules are cold and stopped, they are subjected to magnetic trapping, electrostatic trapping, laser cooling, or sympathetic cooling. Lewandowski works with theorist Chris Greene to investigate collisions of cold polar NH radicals (produced in this process) with each other and other ultracold atoms such as rubidium (Rb).

The Lewandowski group is using metastable NH molecules to investigate collisional quenching, in which collisions knock a molecule into a lower energy state with a release of energy as heat rather than light. The researchers have found that an excited state of this molecule can form a resonant state with atoms of ⁸⁷Rb in which the energy of the Rb atoms increases as the energy in the NH molecules decreases and vice versa. Theoretical analysis of this observation by Chris Greene’s group has predicted enhanced quenching of NH with Rb because of the resonance. The scientists have also noted that excited NH molecules have a virtually identical structure to excited molecules of oxygen (O₂) in the atmosphere. Both the experimentalists and theorists are interested in learning more about how to de-excite both these molecules via a resonance with another species. However, this interaction might be considerably more complicated in the atmosphere.

The Lewandowski group also studies the interactions of ammonia molecules (NH₃) with ultracold atoms of ⁸⁷Rb. The group

uses Stark deceleration to cool the NH₃ molecules for interaction studies with ⁸⁷Rb atoms brought to ultracold temperatures by laser cooling and trapping. The group has begun a new project to study charge-exchange reactions between NH3+ ions and Rb atoms. By monitoring cold, controlled collisions of these two species, the group hopes to "see" how an NH₃⁺ ion rips an electron off a Rb atom at the low energies of a system at 1–100 mK. For more information on this experiment, please see The Quantum Mechanics of Chemistry.

Theoretical Models of Cold Molecule Behavior

Theorists John Bohn and Chris Greene are engaged in wide-ranging studies of the behavior of ultacold molecules. Bohn is studying the collision physics of pairs of dipolar KRb molecules in low-temperature gases. He found that at ultracold (<1 µK) temperatures, collisions in a molecular dipole system consisting of a gas of ultracold KRb molecules could be readily modeled (using simplifying assumptions) with quantum mechanics. The KRb collisions could also be readily modeled at "merely cold" temperatures (1 K–1 mK), but with semiclassical approximations. However, the terrain in between ultracold and merely cold (1 mK–1 µK) was impossible to model simply. Collision details varied from one molecule to the next, and no single theory applied. Nevertheless, the Jin and Ye groups have already observed molecular collisions at these temperatures in ultracold gas clouds of KRb molecules.

Bohn is working on the very practical problem of being able to explain what Jin and Ye are seeing, despite the fact this system is "shockingly complicated." For now, he’s focusing on whether KRb molecule collisions can be used for evaporative cooling.

Bohn also plans to investigate ultracold chemistry of these and other cold polar molecules. For instance, would it be possible to guide ultracold atoms or molecules as they approach each other? Would it be possible to control ultracold chemical reactions by switching electric fields on and off, or even by making them weaker or stronger? Eventually, Bohn wants to be able to control every aspect of ultracold collisions.

Bohn has also begun to explore the many-body physics of ultracold polar molecules. There are different kinds of ultracold dipolar molecules that could be made, and they’re all really complicated. They can attract or repel one another, depending on their orientations and shapes. Plus their interactions are strong. In a BEC, dipoles push on each other from very far away. These studies of the many-body physics of ultracold dipolar molecules will allow Bohn to determine whether cold dipolar molecular systems could serve as a model for particle and condensed-matter physics.

Candidate Molecules for eEDM Experiments

Bohn is working with Eric Cornell and Jun Ye on the design of an experiment to measure the electron electric dipole moment (eEDM) in trapped molecular ions. The proposed precision measurement experiment is described in measurements of fundamental parameters of nature. Bohn has identified a family of individual molecular ions that are particularly well suited for experiments to see whether the electron has a tiny eEDM. These ions are capable of creating an enormous electric field on the ions’ valence electrons — a much bigger field than would be possible to apply in the lab. The valence electrons in these molecules also have a high angular momentum around the molecular axis, and they’re close enough to the nucleus of the molecule to experience the big electric field between the ions’ constituent atoms. All these properties are necessary in experiments attempting to measure an eEDM in ultracold tapped molecular ions.

The Bohn group has found seven such molecules: HfH⁺, PtH⁺, HfF⁺, ThO⁺, ThF⁺, WC⁺, and OsC⁺. Eric Cornell’s eEDM experiment will use HfF+, and the ACME (Advanced Cold Molecule Electric dipole moment search) collaboration from Yale and Harvard is considering the use of ThO. Aaron Leanhardt of the University of Michigan is using WC⁺. Meanwhile, theorist Bohn is not only looking for more candidate molecules, but he’s also begun to investigate their properties. He wants to identify in advance any issues involved in doing spectroscopy on them.

Quantum Mechanics of Ultracold Systems

Theorist Chris Greene and his group study ultracold molecules to better understand the quantum mechanics of these systems. Research projects include the development of new methods for describing the interface between few-body systems like ultracold gases and condensed matter systems, the study of three-particle systems in which three particles can form multiple bound states — even though no two of them can form a stable pair, and the properties of unusually large, weakly bound molecules that form in BECs.

In 2006, Greene was happy to see experimental confirmation of predictions he and his collaborators made in the late 1990s addressing both the characteristics of and possible ways to observe Efimov states in three-body recombination. These states, originally predicted by Vitaly Efimov in 1970, derive from a bizarre quantum-mechanical effect occurring in three-particle systems. Under certain circumstances, the three particles can produce an infinite number of bound levels even though no two of them are capable of forming a stable pair.

The Greene group recently began a study of the interactions of three particles in a four-body system to see whether Efimov effects occur there. The group has identified very interesting physics in the four-body system, but has not seen evidence of a four-body Efimov effect per se. However, the group did discover two four-body states attached to every three-body Efimov state: (1) a low-energy–tightly bound state in which all four particles are approximately equidistant from one another and (2) a high-energy–quasi-bound state in which three particles exist in an Efimov triangle and the fourth particle is relatively far away. It is now possible to predict the four-body states attached to an Efimov trimer, provided the Efimov state is known.

The new theoretical research on Efimov states recently led to the discovery of experimental evidence for at least one four-body state in data from experiments done in 2006 in Rudy Grimm’s laboratory at the Universität Innsbruck. (These were the landmark experiments that confirmed the existence of three-body Efimov states.) In 2009, new experiments at Innsbruck confirmed the existence of both predicted four-body states attached to Efimov trimers.

The year 2009 was a banner year for experimental verification of the Greene group’s theoretical work with cold molecules. Researchers Tilman Pfau’s laboratory at the University of Stuttgart created the world’s first long-range Rydberg molecules from an ultracold cloud of Rb atoms. The new diatomic molecules of rubidium (Rb₂) are as big as a virus. They are held together by a ghostly quantum mechanical force field with the energy of about 100 billionths of an electron volt.

The Stuttgart work followed on the heels of work done in 2006 at Northwestern University, Croatia’s Institute of Physics, and Germany’s University of Dortmund. In these efforts, Greene and his colleagues reported the first indirect experimental evidence for the existence of Rydberg molecules. This evidence confirmed the basic theory developed by Greene and his collaborators in 2000 and 2002 that predicted the existence of this bizarre class of Rydberg molecules in BECs. The theorists had predicted that these molecules would be characterized by high electronic excitations. They would also be their extraordinarily huge molecules, i.e., 50–500 times larger than garden-variety molecules such as N₂ or O₂.

According to Greene’s theory, the defining characteristic of another class of even more highly excited and strongly bound Rydberg molecules (than those just discovered) would be their quantum mechanical probability density. These probability densities exhibit striking resemblances to either a butterfly or the trilobite that ruled Earth's seas 300 million years ago. The creation of the first Rydberg molecules in the laboratory has opened up the possibility of creating one of the more strongly bound Rydberg molecules by further exciting the molecules minted at Stuttgart.