Collisions in an ultracold gas of 23Na39K molecules

Abstract

The creation of a quantum degenerate ensemble of heteronuclear molecules with dominant dipole-dipole interactions, is considered to be instrumental for studying and understanding strongly correlated many-body systems. The latter might reveal new quantum states of matter and substantially advance quantum metrology, computation and ultracold chemistry. A handful of experiments have been successful in creating ultracold bialkali molecular ensembles with large phase-space densities, assembled from different fermionic or bosonic alkali constituents. Every new species promises to improve the knowledge about molecular many-body systems interacting through anisotropic long-range dipole-dipole interactions. Nevertheless, only recently two groups succeeded in observing a degenerate gas of fermionic molecules, while a Bose-Einstein condensate is yet to be demonstrated. A substantial obstacle is the unexpected two-body loss rate close to the universal limit, observed in every experiment. New theoretical concepts attempted to explain them by introducing so-called sticky collisions and subsequent photo-excitation. Providing experimental data on ultracold molecule and also atom-molecule collisions is crucial for the understanding of the yet unknown and complicated molecular collision processes. This thesis reports on the progress achieved in the development of a new bialkali molecular platform. New Feshbach resonances in 23Na+39K collisions are revealed, which enable the production of weakly bound molecules. Spectroscopic measurements of 23Na39K are performed and possible pathways to the rovibronic ground state discussed. Finally, for the first time, an ultracold ensemble of 23Na39K ground-state molecules is created. To study the possible photoexcitation of metastable complexes, a chopped optical dipole trap has been realized. A large parameter space of different modulation frequencies, laser intensities and wavelengths as well as dark to bright time ratios has been probed. As the key findings of this thesis, surprisingly, the expected decrease of molecular losses has not been observed. Instead, all measurements show a null result. Together with similar results from a group in Hong Kong work- ing with 23Na87Rb this suggests that the current theory describing ultracold molecular collisions is incomplete. Subsequent studies of collisions between 39K atoms prepared in different hyperfine states, with 23Na39K molecules in a single hyperfine state, show intriguing scattering properties. In particular, the non spin-stretched state |F = 1, mF = −1> exhibits a strong suppression of the two-body losses far below the universal limit.