The electronic energy level structure of yttrium monoxide (YO) provides a long-lived, low-lying 2 Δ state ideal for high-precision molecular spectroscopy, narrowline laser cooling at the single photon-recoil limit, and studying dipolar physics with unprecedented interaction strength. High-resolution laser spectroscopy of ultracold laser-cooled YO molecules is used to study the Stark effect in the A ′ 2 Δ 3 / 2 J = 3 / 2 state. An immediate onset of the linear Stark effect is observed in the presence of weak applied electric fields due to the near-degenerate Λ doublet and the large electric dipole moment. By applying a small electric field the Stark-insensitive state is spectroscopically isolated and the absolute transition frequency to the X 2 Σ + electronic ground state is determined with a fractional frequency uncertainty of 9 × 10 − 12 . This electric field control is necessary to implement a quasi-closed photon-cycling scheme that preserves parity. With this scheme the first narrowline laser cooling of a molecule is demonstrated, reducing the temperature of sub-Doppler cooled YO in two dimensions.
The electronic energy level structure of yttrium monoxide (YO) provides a long-lived, low-lying 2 Δ state ideal for high-precision molecular spectroscopy, narrowline laser cooling at the single photon-recoil limit, and studying dipolar physics with unprecedented interaction strength. High-resolution laser spectroscopy of ultracold laser-cooled YO molecules is used to study the Stark effect in the A ′ 2 Δ 3 / 2 J = 3 / 2 state. An immediate onset of the linear Stark effect is observed in the presence of weak applied electric fields due to the near-degenerate Λ doublet and the large electric dipole moment. By applying a small electric field the Stark-insensitive state is spectroscopically isolated and the absolute transition frequency to the X 2 Σ + electronic ground state is determined with a fractional frequency uncertainty of 9 × 10 − 12 . This electric field control is necessary to implement a quasi-closed photon-cycling scheme that preserves parity. With this scheme the first narrowline laser cooling of a molecule is demonstrated, reducing the temperature of sub-Doppler cooled YO in two dimensions.
Laser cooling and magneto-optical trapping of neutral atoms has been a driving force in physics over the last few decades by providing an efficient method to produce cold and dense samples of cold atoms. Polar molecules which can have strong, long-range dipolar interactions and possess more rich internal structure promise a wide range of new physics and chemistry studies if they can be controlled efficiently. Until recently, the only way to achieve sub-milliKelvin level temperatures in molecules was to associate two precooled atomic species, which while very effective, has a firm bound on the chemical diversity accessible. While laser cooling of molecules relies on fortuitous molecular properties, it will significantly increase the choices of species available for molecular cooling.In this thesis I present the improved optical deceleration of the molecule yttrium monoxide (YO) and the loading of molecules into a 3-D magneto-optical trap (MOT). By making an improved choice of repumping scheme from our groups’ previous work, our photon scattering rate has been increased. By also improving our buffer gas cell source, we produce a much larger flux (2000 times higher) of slowed molecules at 5 m/s to reach the trapping region. I have designed and implemented 3-D MOT field coils operating at 5 MHz, and demonstrated a successful operation of a 3-D MOT. This is the first 3-D laser trapping of an oxide and the first of a molecule with an electronic state intermediate to the main cycling transition states.I have also performed a study of the transition from the ground state to the intermediate electronic state, which has a narrower linewidth of 150 kHz, in contrast to the main cycling transition’s linewidth of 5 MHz. This transition is well suited to second-stage narrow line cooling after loading molecules with the main MOT transition, which will allow direct laser cooling of a molecule to the 10 uK regime.
Complex molecular structure demands customized solutions to laser cooling by extending its general set of principles and practices.Compared with other laser-cooled molecules, yttrium monoxide (YO) exhibits a large electron-nucleus interaction, resulting in a dominant hyperfine interaction over the electron spinrotation coupling.The YO ground state is thus comprised of two manifolds of closely spaced states, with one of them possessing a negligible Landé g factor.This unique energy level structure favors dual-frequency dc magneto-optical trapping (MOT) and gray molasses cooling (GMC).We report exceptionally robust cooling of YO at 4 μK over a wide range of laser intensity, detunings (one-and two-photon), and magnetic field.The magnetic insensitivity enables the spatial compression of the molecular cloud by alternating GMC and MOT under the continuous operation of the quadrupole magnetic field.A combination of these techniques produces a laser-cooled molecular sample with the highest phase space density in free space.
The YbF molecule is a sensitive system for measuring the electron’s electric dipole moment. The precision of this measurement can be improved by direct laser cooling of the molecules to ultracold temperature. However, low-lying electronic states arising from excitation of a 4f electron may hinder laser cooling. One set of these “4f hole” states lies below the A2Π1/2 excited state used for laser cooling, and radiative decay to these intermediate levels, even with branching ratios as small as 10−5, can be a hindrance. Other 4f hole states lie very close to the A2Π1/2 state, and a perturbation results in states of mixed character that are involved in the laser cooling cycle. This perturbation may enhance the loss of molecules to states outside of the laser cooling cycle. We model the perturbation of the A2Π1/2 state to determine the strength of the coupling between the states, the de-perturbed potential energy curves, and the radiative branching ratios to various vibrational levels of the ground state, X2Σ+. We use electronic structure calculations to characterize the 4f hole states and the strengths of transitions between these states and the A2Π1/2 and X2Σ+ states. We identify a leak out of the cooling cycle with a branching ratio of roughly 5×10−4, dominated by the contribution of the ground state configuration in a 4f hole state. Finally, we assess the impact of these results for laser cooling of YbF and molecules with similar structure.
Cold molecules are attractive for a wide range of scientific applications, including quantum computation, the study of chemical reactions, and tests of fundamental physics. Laser cooling has proved to be an invaluable technique in the cooling of atoms. This technique was once thought to be infeasible for molecules, because it is difficult to find a closed cycling transition due to their vibrational structure. Recently, laser cooling of several diatomic species has been demonstrated. These molecules possess electronic transitions with highly diagonal Franck-Condon matrices, which make it possible to drive a quasi-closed cycling transition. Ytterbium fluoride (YbF) molecules are amenable to laser cooling and are especially interesting because they are used to measure the electron's electric dipole moment (eEDM). Measurements of the eEDM test the prediction of theories that extend the Standard Model of particle physics. The sensitivity of an eEDM experiment could be greatly increased by using ultracold molecules produced by direct laser cooling. This thesis presents work done towards producing a laser-cooled beam of YbF for an eEDM experiment. This work includes the construction of the cooling experiment, a novel method for efficiently combining laser beams of very similar frequencies, results of spectroscopic measurements to find the required transitions for laser cooling, the results of initial optical cycling experiments, and the first laser cooling results of YbF. Using a one-dimensional optical molasses, a beam of molecules is Doppler cooled in one transverse direction to a temperature of approximately 70 mK. Preliminary evidence of cooling to lower temperatures through a Sisyphus mechanism is also presented. Finally, paths towards improving the laser cooling are suggested. The work opens the door to improved measurements of the eEDM using ultracold YbF molecules.
While cold molecule experiments are rapidly moving towards their promised benefits of precision spectroscopy, controllable chemistry, and novel condensed phases, heretofore the field has been greatly limited by a lack of methods to cool and compress chemically diverse species to temperatures below ten millikelvin. While in atomic physics these needs are fulfilled by laser cooling, magneto-optical trapping, and evaporative cooling, until now none of these techniques have been applicable to molecules. In this thesis, two major breakthroughs are reported. The first is the observation of evaporative cooling in magnetically trapped hydroxyl (OH) radicals, which potentially opens a path all the way to Bose-Einstein condensation of dipolar radicals, as well as allowing cold- and ultracold-chemistry studies of fundamental reaction mechanisms. Through the combination of an extremely high gradient magnetic quadrupole trap and the use of the OH Λ-doublet transition to enable highly selective forced evaporation, cooling by an order of magnitude in temperature was achieved and yielded a final temperature no higher than 5mK. The second breakthrough is the successful application of laser cooling and magneto-optical trapping to molecules. Motivated by a proposal in this thesis, laser cooling of molecules is now known to be technically feasible in a select but substantial pool of diatomic molecules. The demonstration of not only Doppler cooling but also two-dimensional magneto-optical trapping in yttrium (II) oxide, YO, is expected to enable rapid growth in the availability of ultracold molecules--just as the invention of the atomic magneto-optical trap stimulated atomic physics twenty-five years ago.
Slowing down atoms using laser cooling, together with techniques like magnetic trapping and evaporative cooling, has led to numerous exciting results over the last decade [1], with the realization of Bose-Einstein condensation as the most spectacular achievement [2]. Laser-cooling schemes are applicable to atoms since closed multilevel systems can be realized and many consecutive absorptionemission cycles can be induced. This leads to a significant momentum transfer from the photons of the laser to the atom, resulting in a change of the velocity of the atom. There currently is a great interest in producing cold molecules for the study of cold molecule-molecule collisions, molecular quantum collective effects, Doppler-free spectroscopy, frequency standards, as well as many other applications already realized for atoms [3]. Slowing down of neutral molecules, a prerequisite for being able to trap them, has proven to be considerably more difficult to be achieved, however. The above-mentioned laser-cooling scheme cannot be applied to slow down molecules as no simple closed-level systems can be found; the unavoidable off-resonant radiative decay of molecules from the excited state to other vibrational levels in the electronic ground state hampers efficient momentum transfer [4]. Once molecules are present at sufficiently low kinetic energies, below a fraction of a cm21, they can be trapped in various ways. Trapping of neutral molecules in inhomogeneous magnetic [3] and electric fields [5] and trapping based on the use of evanescent fields close to the dielectricvacuum interface have been proposed [6]. It has also been suggested that extremely high laser powers might be used to induce a dipole moment in polarizable molecules, leading to trapping of these molecules in the laser beam [7]. Other methods to produce slow molecules have been put forward and have recently been successfully applied to produce the first samples of trapped molecules. By injecting molecules in a cold buffer gas, sufficiently low kinetic energies of the injected molecules can be obtained after thermalization that they can subsequently be trapped in an inhomogeneous magnetic field. This has recently been demonstrated for CaH injected in 3He buffer gas at temperatures of around 0.3 K [8]. Associative collisions between ultracold trapped atoms [9] have proven to be