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arXiv:1706.02806 · Physical Review A

Controlling spin flips of molecules in an electromagnetic trap

David Reens, Hao Wu, Tim Langen, Jun Ye

Published Dec 27, 2017• 31 citations
arXivHTMLPDF

Abstract

Doubly dipolar molecules exhibit complex internal spin dynamics when electric and magnetic fields are both applied. Near magnetic trap minima, these spin dynamics lead to enhancements in Majorana spin-flip transitions by many orders of magnitude relative to atoms and are thus an important obstacle for progress in molecule trapping and cooling. We conclusively demonstrate and address this with OH molecules in a trap geometry where spin-flip losses can be tuned from over 200 ${\mathrm{s}}^{\ensuremath{-}1}$ to below our 2 ${\mathrm{s}}^{\ensuremath{-}1}$ vacuum-limited loss rate with only a simple external bias coil and with minimal impact on trap depth and gradient.

DOI
10.1103/physreva.96.063420

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Doubly dipolar molecules exhibit complex internal spin dynamics when electric and magnetic fields are both applied. Near magnetic trap minima, these spin dynamics lead to enhancements in Majorana spin-flip transitions by many orders of magnitude relative to atoms and are thus an important obstacle for progress in molecule trapping and cooling. We conclusively demonstrate and address this with OH molecules in a trap geometry where spin-flip losses can be tuned from over 200 s\ensuremath−1{\mathrm{s}}^{\ensuremath{-}1}s\ensuremath−1 to below our 2 s\ensuremath−1{\mathrm{s}}^{\ensuremath{-}1}s\ensuremath−1 vacuum-limited loss rate with only a simple external bias coil and with minimal impact on trap depth and gradient.
Cold and ultracold polar molecules with nonzero electronic angular momenta 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 $|f;\phantom{\rule{0.16em}{0ex}}M=+\frac{3}{2}\ensuremath{\rangle}$ parity state to the lower $|e;\phantom{\rule{0.16em}{0ex}}+\frac{3}{2}\ensuremath{\rangle}$ state and observing their trap dynamics under an applied electric bias field. Our observations are very well fit by a simple Landau-Zener model, yielding insight to the rich spectra and dynamics of polar radicals in mixed external fields.
We analyze the dynamics of a paramagnetic, dipolar molecule in a generic ``magneto-electrostatic'' trap where both magnetic and electric fields may be present. The potential energy that governs the dynamics of the molecules is found using a reduced molecular model that incorporates the main features of the system. We discuss the shape of the trapping potentials for different field geometries, as well as the possibility of nonadiabatic transitions to untrapped states, i.e., the analog of Majorana transitions in a quadrupole magnetic atomic trap. Maximizing the lifetime of molecules in a trap is of great concern in current experiments, and we assess the effect of nonadiabatic transitions on obtainable trap lifetimes.
Molecular physics has experienced groundbreaking progress in the fields of precision spectroscopy, chemical reaction kinetics, quantum state engineering and many-body physics. In order to better observe these phenomena, there is an insatiable pursuit of larger trapped molecular densities and longer lifetime. In this thesis, several key milestones that we have recently achieved towards these goals for hydroxyl radicals (OH) are reported. First, we discovered an enhanced spin-flip behavior of dipolar molecules due to the existence of dual (electric and magnetic) dipole moments and obtained a better understanding of complex spin-dynamics for both Hund's case (a) and (b) molecules in mixed electric and magnetic fields. Second, we demonstrated that the skimmer cooling technique can be applied to radicals and the production of OH can be enhanced by factor of 30 due to both clogging mitigation and more favorable skimmer-valve distance. Finally, we showed some preliminary results that it is possible to use a newly built Stark decelerator to study electric field controlled sub-kelvin collisions in an intra-beam.
We present a theoretical study of atom-molecule collisions in superimposed electric and magnetic fields and show that dynamics of electronic spin relaxation in molecules at temperatures below 0.5 K can be manipulated by varying the strength and the relative orientation of the applied fields. The mechanism of electric field control of Zeeman transitions is based on an intricate interplay between intramolecular spin-rotation couplings and molecule-field interactions. We suggest that electric fields may affect chemical reactions through inducing nonadiabatic spin transitions and facilitate evaporative cooling of molecules in a magnetic trap.
Research in the field of cold polar molecules is progressing rapidly. An array of interesting\ntopics 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.
We report magnetic confinement of neutral, ground state OH at a density of approximately 3 x 10(3) cm(-3) and temperature of approximately 30 mK. An adjustable electric field sufficiently large to polarize the OH is superimposed on the trap in various geometries, making an overall potential arising from both Zeeman and Stark effects. An effective molecular Hamiltonian is constructed, with Monte Carlo simulations accurately modeling the observed single-molecule dynamics in various trap configurations. Magnetic trapping of cold polar molecules under adjustable electric fields may enable study of low energy dipolar interactions.