Mark Yeo

2D Magneto-Optical Trapping of Diatomic Molecules

We demonstrate one- and two-dimensional transverse laser cooling and magneto-optical trapping of the polar molecule yttrium (II) oxide (YO). In a 1D magneto-optical trap (MOT), we characterize the magneto-optical trapping force and decrease the transverse temperature by an order of magnitude, from 25 to 2 mK, limited by interaction time. In a 2D MOT, we enhance the intensity of the YO beam and reduce the transverse temperature in both transverse directions. The approach demonstrated here can be applied to many molecular species and can also be extended to 3D.

Evaporative cooling of the dipolar hydroxyl radical

Atomic physics was revolutionized by the development of forced evaporative cooling, which led directly to the observation of Bose–Einstein condensation, quantum-degenerate Fermi gases and ultracold optical lattice simulations of condensed-matter phenomena. More recently, substantial progress has been made in the production of cold molecular gases. Their permanent electric dipole moment is expected to generate systems with varied and controllable phases, dynamics and chemistry. However, although advances have been made in both direct cooling and cold-association techniques, evaporative cooling has not been achieved so far. This is due to unfavourable ratios of elastic to inelastic scattering and impractically slow thermalization rates in the available trapped species. Here we report the observation of microwave-forced evaporative cooling of neutral hydroxyl (OH•) molecules loaded from a Stark-decelerated beam into an extremely high-gradient magnetic quadrupole trap. We demonstrate cooling by at least one order of magnitude in temperature, and a corresponding increase in phase-space density by three orders of magnitude, limited only by the low-temperature sensitivity of our spectroscopic thermometry technique. With evaporative cooling and a sufficiently large initial population, much colder temperatures are possible; even a quantum-degenerate gas of this dipolar radical (or anything else it can sympathetically cool) may be within reach.

Molecular beam collisions with a magnetically trapped target.

Cold, neutral hydroxyl radicals are Stark decelerated and confined within a magnetic trap consisting of two permanent ring magnets. The OH molecules are trapped in the ro-vibrational ground state at a density of ∼10 cm−3 and temperature of 70 mK. Collisions between the trapped OH sample and supersonic beams of atomic He and molecular D2 are observed and absolute collision cross sections measured. The He–OH and D2–OH center-of-mass collision energies are tuned from 60 cm−1 to 230 cm−1 and 145 cm−1 to 510 cm−1, respectively, yielding evidence of reduced He–OH inelastic cross sections at energies below 84 cm−1, the OH ground rotational level spacing.

Cold heteromolecular dipolar collisions

Cold molecules promise to reveal a rich set of novel collision dynamics in the low-energy regime. By combining for the first time the techniques of Stark deceleration, magnetic trapping, and cryogenic buffer gas cooling, we present the first experimental observation of cold collisions between two different species of state-selected neutral polar molecules. This has enabled an absolute measurement of the total trap loss cross sections between OH and ND(3) at a mean collision energy of 3.6 cm(-1) (5 K). Due to the dipolar interaction, the total cross section increases upon application of an external polarizing electric field. Cross sections computed from ab initio potential energy surfaces are in agreement with the measured value at zero external electric field. The theory presented here represents the first such analysis of collisions between a (2)Π radical and a closed-shell polyatomic molecule.

Rotational State Microwave Mixing for Laser Cooling of Complex Diatomic Molecules

We demonstrate the mixing of rotational states in the ground electronic state using microwave radiation to enhance optical cycling in the molecule yttrium (II) monoxide (YO). This mixing technique is used in conjunction with a frequency modulated and chirped continuous wave laser to slow longitudinally a cryogenic buffer-gas beam of YO. We generate a flux of YO below 10 m/s, directly loadable into a three-dimensional magneto-optical trap. This technique opens the door for laser cooling of diatomic molecules with more complex loss channels due to intermediate states.

Microwave state transfer and adiabatic dynamics of magnetically trapped polar molecules

Cold and ultracold polar molecules with nonzero electronic angular momentum are of great interest for studies in quantum chemistry and control, investigations of novel quantum systems, and precision measurement. However, in mixed electric and magnetic fields, these molecules are generically subject to a large set of avoided crossings among their Zeeman sublevels; in magnetic traps, these crossings lead to distorted potentials and trap loss from electric bias fields. We have characterized these crossings in OH by microwave-transferring trapped OH molecules from the upper |f; M = +3/2> parity state to the lower |e; +3/2> state and observing their trap dynamics under an applied electric bias field. Our observations are very well described by a simple Landau-Zener model, yielding insight to the rich spectra and dynamics of polar radicals in mixed external fields.

Electric-field-induced inelastic collisions between magnetically trapped hydroxyl radicals

Inelastic collisions are observed between magnetically trapped neutral hydroxyl (OH•) radicals at a temperature of 45 mK in the presence of an electric field. The collision rate is measured over a range of electric fields from 0.2 to 10 kV/cm. However, the two-body collision rates must be deconvolved from a novel electric-field induced non-adiabatic single particle loss, which arises from Landau–Zener crossings between energy surfaces at the plane where the electric field vector is transverse to the trapping magnetic field. The observed inelastic collision rate follows an approximate quadratic power law in the effective dipole moment.