Collisional Cooling and Magnetic Control of Reactions in Ultracold Spin-polarized NaLi+Na Mixture

Abstract

This thesis describes two studies with a mixture of ultracold triplet ground state 23Na6Li molecules and 23Na atoms. The first study is to realize the cooling of ultracold NaLi molecules with Na atoms that efficiently exchange kinetic energy when colliding with molecules and bring them to thermal equilibrium with lower temperatures. Such cooling based on inter-particle collisions, which is the so-called collisional cooling, has not been successful in other molecular systems at micro-Kelvin temperatures or below before the work described in this thesis. By harnessing the good-to-bad collision ratio of the NaLi+Na mixture around 200, we increase the phase space density of the molecules by a factor of 20, achieving the temperatures as low as 220 nano-Kelvin. This collisional property arises from the suppression of reactive and inelastic collisions by the polarization of the spin in a chemically non-reactive quartet state, possibly together with the small size and the low mass of the NaLi+Na mixture. The loss rate in the spin-polarized NaLi+Na mixture is more than an order of magnitude lower than the so-called universal rate, which occurs in most molecular collisions as particles are lost or react at short range with unity probability. The second study is about the control of the reactive collisions by magnetically-tunable scattering resonances, so-called Feshbach resonances, between molecules and atoms. By controlling the phase of the scattering wavefunction via a Feshbach resonance at 978 G, we modify the loss rate by more than two orders of magnitude, and such a large dynamic range of tunability in loss is possible only in a molecular system in which the background loss is far below the universal limit, like the spin-polarized NaLi+Na mixture. We explain the observed tunability in loss by a single-channel model based on quantum interference of the scattering wavefunction reflected at long and short ranges, which is analogous to the optical interference in a Fabry-Perot resonator. By fitting our Fabry-Perot model to the precisely measured lineshape of the loss feature at 978 G, we measure short range parameters, such as a background phase shift parameter and a quantum-defect parameter, which indicates that the spin-polarized NaLi+Na has about 4% loss probability at short range. By extending the single-channel model to consider the lifetime of a bound state of a closed channel, we treat a weak Feshbach resonance at 1030 G as a lossy phase shifter. With this extended two-channel model, we measure that the lifetime of a three-body collision complex associated with the resonance at 1030 G is about 60 ns. The work of this thesis shows that the spin-polarization is a simple yet powerful method to suppress reactive collisions in a molecular system and establishes the light triplet molecule as a useful system to study quantum chemistry, molecular scattering resonances, and possibly short range physics.