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Instruments and Experimental Techniques

An Experimental Setup for Production of Polarized H2 and D2 Molecules

Dmitri K. Toporkov, S. A. Zevakov, D. M. Nikolenko, Igor A. Rachek, Yu. V. Shestakov, A. V. Yurchenko

Published Jan 1, 2019• 2 citations
Publisher

Abstract

Abstract— The setup is designed to produce spin-polarized hydrogen and deuterium molecules. Superconducting sextupole magnets with a magnetic induction of 3.4 T near the poles and a field gradient of 320 T/m were used for the spatial separation of molecules with different magnetic moments by spin filtration in an inhomogeneous magnetic field. The choice of the location of the polarized-molecule source elements is justified. Cryogenic pumps are used in the molecular-beam detection chamber in order to obtain an ultrahigh vacuum. At a nozzle temperature of 7 K, the measured flux of spin-polarized hydrogen molecules is 3 × 1012 molecules/s. For deuterium, the measured flux is lower by more than seven times due to the small magnetic moments.

DOI
10.1134/s0020441219010184

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Physical Review A•Oct 18, 2018•6 citations
Abstract— The setup is designed to produce spin-polarized hydrogen and deuterium molecules. Superconducting sextupole magnets with a magnetic induction of 3.4 T near the poles and a field gradient of 320 T/m were used for the spatial separation of molecules with different magnetic moments by spin filtration in an inhomogeneous magnetic field. The choice of the location of the polarized-molecule source elements is justified. Cryogenic pumps are used in the molecular-beam detection chamber in order to obtain an ultrahigh vacuum. At a nozzle temperature of 7 K, the measured flux of spin-polarized hydrogen molecules is 3 × 1012 molecules/s. For deuterium, the measured flux is lower by more than seven times due to the small magnetic moments.
Polarized atomic beam sources have been in operation for many years to produce either nuclear polarized atomic hydrogen or deuterium beams. In recent experiments, such a source was used to polarize both isotopes independently at the same time. By recombination of the atoms, hydrogen-deuterium molecules with all possible nuclear spin combinations can be created. Those spin isomers are useful for further applications, like precision spectroscopy, as polarized targets for laser-particle acceleration, polarized fuel for fusion reactors, or as an option for future measurements of electric dipole moments.
The electron polarization of hydrogen (H) and deuterium (D) atoms in a spin-exchange optical-pumping apparatus has been measured as a function of H (D) density and magnetic-field strength. The atomic density was varied between 4\ifmmode\times\else\texttimes\fi{}1013{10}^{13}1013 and 2.6\ifmmode\times\else\texttimes\fi{}1014{10}^{14}1014 atoms/cm3{\mathrm{cm}}^{3}cm3 and the magnetic-field strength from 120 to 400 mT. The results suggest that the atoms approach spin-temperature equilibrium even though the magnetic fields are large compared to the critical fields of the atoms.
For molecular systems, spin relaxation is expected to be suppressed compared to the case of atoms, since the paired electrons in a hydrogen or deuterium molecule are chemically stable, and only weakly interact with the spin of the nucleus. Such systems would be largely insensitive to polarization losses due to spin-exchange collisions, to the interaction of the electron spins with external fields (e.g. the RF-field of a bunched charged-particle beam), and/or to the presence of container walls. Here, we discuss the results of a recent experiment where we obtained evidence that nuclear polarization is maintained, when polarized atoms recombine to molecules on a copper surface (in a magnetic field of 23 mT and at a density of about 1012 molecules⋅cm−3).
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 study the polarization dynamics of the molecular rotation and the nuclear spin in the presence of magnetic fields, in the hydrogen isotope molecules, H2,D2{\mathrm{H}}_{2},\phantom{\rule{0.16em}{0ex}}{\mathrm{D}}_{2}H2​,D2​, and HD. If only the rotational or the nuclear angular momentum is polarized, polarization is transferred from one to the other and back in a oscillatory fashion, due to hyperfine quantum beats. Application of magnetic fields is shown to increase the frequencies of these oscillations and reduce their amplitude so that the polarization exchange can be strongly suppressed. We derive the time-averaged polarization as a function of the (static) magnetic field. Finally, we show, using continuous lasers, molecular beams, and static magnetic fields, that the molecular rotation polarization can be transferred and maintained in the nuclear spin, providing highly nuclear-spin-polarized molecules.