Cold Polar Molecules for Novel Collision Experiments at Low Energies

Sawyer, Brian

Graduate Thesis Or Dissertation

Cold Polar Molecules for Novel Collision Experiments at Low Energies Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/9g54xh98g

Abstract

Research in the field of cold polar molecules is progressing rapidly. An array of interesting topics is being developed including precision measurement and fundamental tests, quantum phase transitions, and ultracold chemistry. In particular, dipolar molecules in well-defined quantum states enable exquisite control of their interactions via applied electric fields. The long-range, anisotropic interaction between dipolar molecules leads to new collision dynamics that could be used for novel collective effects, quantum state engineering, and information processing. The focus of this Thesis is the production of cold samples of neutral OH molecules via Stark deceleration and magnetic trapping for novel collision experiments. This novelty results from our combination of cold external beams and trapped target molecules which facilitates observation of dipolar effects as well as low temperature collision resonances. The large permanent electric dipole moment of OH allows us to precisely tune the lab-frame velocity of the molecular packets from ∼ 500 m/s to rest. We have magnetically trapped OH packets at the terminus of our Stark decelerator at a temperature of 70 mK and density of 10⁶ cm^-3. With improved understanding of Stark deceleration, we optimize the decelerator efficiency and its coupling to the magnetic trap. Our latest trap design uses permanent ring magnets to create a three-dimensional magnetic quadrupole field. Use of magnetically trapped OH molecules for collision experiments with external beams allow us the unique opportunity of observing both elastic and inelastic collisions. In addition, the trap confinement yields sensitivity to differential elastic cross sections at low collision energies. This open magnetic trap has allowed measurement of collision cross sections between trapped OH and external supersonic beams of He and D₂, the latter of which is of astrophysical interest due to the role of H₂-OH collisions in pumping interstellar OH megamasers. The combination of trapped OH molecules and temperature tuned beams of He and D₂ has facilitated measurement of the lowest-energy D₂-OH collision cross sections yet reported. More recently, we report the first observation of electric-field dependent cross sections between two different species of cold polar molecules - OH and ND₃ - thereby demonstrating control over molecular scattering in the cold regime. By combining for the first time the production techniques of Stark deceleration and buffer gas cooling, we increase the molecular interaction time by∼ 10⁵ over traditional crossed-beam experiments to gain enhanced sensitivity at the characteristic densities of cold molecule production.

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