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arXiv:0806.2624 · Physical Review Letters

Molecular Beam Collisions with a Magnetically Trapped Target

Brian C. Sawyer, Benjamin Stuhl, Dajun Wang, Mark Yeo, Jun Ye

Published Nov 14, 2008• 113 citations
arXivHTMLPDF

Abstract

Cold, neutral hydroxyl radicals are Stark decelerated and efficiently loaded into a permanent magnetic trap. The OH molecules are trapped in the rovibrational ground state at a density of $\ensuremath{\sim}{10}^{6}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}3}$ and temperature of 70 mK. Collision studies between the trapped OH sample and supersonic beams of atomic He and molecular ${\mathrm{D}}_{2}$ determine absolute collision cross sections. The He-OH and ${\mathrm{D}}_{2}\mathrm{\text{\ensuremath{-}}}\mathrm{OH}$ center-of-mass collision energies are tuned from $60\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}1}$ to $230\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}1}$ and $145\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}1}$ to $510\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}1}$, respectively, yielding evidence of quantum threshold scattering and resonant energy transfer between colliding particles.

DOI
10.1103/physrevlett.101.203203

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Cold, neutral hydroxyl radicals are Stark decelerated and efficiently loaded into a permanent magnetic trap. The OH molecules are trapped in the rovibrational ground state at a density of \ensuremath∼106  cm\ensuremath−3\ensuremath{\sim}{10}^{6}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}3}\ensuremath∼106  cm\ensuremath−3 and temperature of 70 mK. Collision studies between the trapped OH sample and supersonic beams of atomic He and molecular D2{\mathrm{D}}_{2}D2​ determine absolute collision cross sections. The He-OH and D2\ensuremath-OH{\mathrm{D}}_{2}\mathrm{\text{\ensuremath{-}}}\mathrm{OH}D2​\ensuremath-OH center-of-mass collision energies are tuned from 60  cm\ensuremath−160\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}1}60  cm\ensuremath−1 to 230  cm\ensuremath−1230\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}1}230  cm\ensuremath−1 and 145  cm\ensuremath−1145\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}1}145  cm\ensuremath−1 to 510  cm\ensuremath−1510\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}1}510  cm\ensuremath−1, respectively, yielding evidence of quantum threshold scattering and resonant energy transfer between colliding particles.
A pulsed beam of ground state OH radicals is slowed down using a Stark decelerator and is subsequently loaded into an electrostatic trap. Characterization of the molecular beam production, deceleration, and trap loading process is performed via laser induced fluorescence detection inside the quadrupole trap. Depending on the details of the trap loading sequence, typically 10(5) OH (X2Pi(3/2),J=3/2) radicals are trapped at a density of around 10(7) cm(-3) and at temperatures in the 50-500 mK range. The 1/e trap lifetime is around 1.0 s.
Samples of cold and ultracold polar molecules have the potential to revolutionize physical chemistry, precision measurement, and few-body quantum physics. This thesis describes experimental results for the production of cold samples of OH radicals by Stark deceleration of a supersonic beam. Since Stark deceleration cannot increase the phase space density of the sample, the initial production stage of the OH molecule is critical. The first set of experiments describes a general methodology for the production of OH beams with maximal phase space density, as well as the subsequent coupling to a Stark decelerator. Additionally, we describe the redesign of our electrostatic trap, optimized for future collision experiments of OH with co-trapped ultracold Rb atoms. The new design resulted in a 15-fold increase in total number of trapped molecules over the previous design when tested with ND3 molecules. The second set of experiments focuses on laser-based detection of OH molecules at the exit of the decelerator based on laser induced fluorescence (LIF) and resonance enhanced multiphoton ionizaton (REMPI). The latter method uses vacuum-ultraviolet light at 118nm produced by third-harmonic generation in Xe/Ar gas mixtures as the ionizing step. The sensitivity of this latter technique is limited by the attainable photon flux of the ionizing radiation at 118nm. We present detailed measurements of the conversion efficiencies as well as absolute photon fluxes. Strategies to overcome these limitations as well as prospects for detection of OH molecules in a trap are discussed.
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.
NH radicals are magnetically trapped and their Zeeman relaxation and energy transport collision cross sections with helium are measured. Continuous buffer-gas loading of the trap is direct from a room-temperature molecular beam. The Zeeman relaxation (inelastic) cross section of magnetically trapped electronic, vibrational, and rotational ground state NH molecules in collisions with 3He^{3}\mathrm{He}3He is measured to be $3.8\ifmmode\pm\else\textpm\fi{}1.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}19}\text{ }\text{ }{\mathrm{cm}}^{2}$ at 710 mK. The NH-He energy transport cross section is also measured, indicating a ratio of diffusive to inelastic cross sections of $\ensuremath{\gamma}=7\ifmmode\times\else\texttimes\fi{}{10}^{4}$, in agreement with recent theory [R. V. Krems, H. R. Sadeghpour, A. Dalgarno, D. Zgid, J. K\l{}os, and G. Cha\l{}asi\ifmmode \acute{n}\else \'{n}\fi{}ski, Phys. Rev. A 68, 051401 (2003)].
Abstract Trapped cold molecules represent attractive systems for precision-spectroscopic studies and for investigations of cold collisions and chemical reactions. However, achieving their confinement for sufficiently long timescales remains a challenge. Here, we report the long-term trapping of Stark-decelerated OH radicals in their X2Π3/2 (ν = 0, J = 3/2, MJ = 3/2, f) state in a permanent magnetic trap. The trap environment is cryogenically cooled to a temperature of 17 K to suppress black-body-radiation-induced pumping of the molecules out of trappable quantum states and collisions with residual background gas molecules which usually limit the trap lifetime. The cold molecules are thus confined on timescales approaching minutes, an improvement of up to two orders of magnitude compared with room temperature experiments, at translational temperatures of ∼25 mK. The present results pave the way for new experiments using trapped cold molecules in precision spectroscopy, in studies of slow chemical processes at low energies and in the quantum technologies.
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.