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arXiv:1609.07735 · Physical Review Letters

Magnetic Trapping of Cold Methyl Radicals

Yang Liu, Manish Vashishta, Pavle Djuricanin, Sida Zhou, Wei Zhong, Tony Mittertreiner, David Carty, Takamasa Momose

Published Feb 27, 2017• 63 citations
arXivHTMLPDF

Abstract

We have demonstrated that a supersonic beam of methyl radicals (CH_{3}) in the ground rotational state of both para and ortho species has been slowed down to standstill with a magnetic molecular decelerator, and successfully captured spatially in an anti-Helmholtz magnetic trap for >1 s. The trapped CH_{3} radicals have a mean translational temperature of about 200 mK with an estimated density of >5.0×10^{7} cm^{-3}. The methyl radical is an ideal system for the study of cold molecules not only because of its high reactivities at low temperatures, but also because further cooling below 1 mK is plausible via sympathetic cooling with ultracold atoms. The demonstrated trapping capability of methyl radicals opens up various possibilities for realizing ultracold ensembles of molecules towards Bose-Einstein condensation of polyatomic molecules and investigations of reactions governed by quantum statistics.

DOI
10.1103/physrevlett.118.093201

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arXiv:1011.0418•New Journal of Physics•Oct 24, 2011•51 citations
We have demonstrated that a supersonic beam of methyl radicals (CH_{3}) in the ground rotational state of both para and ortho species has been slowed down to standstill with a magnetic molecular decelerator, and successfully captured spatially in an anti-Helmholtz magnetic trap for >1 s. The trapped CH_{3} radicals have a mean translational temperature of about 200 mK with an estimated density of >5.0×10^{7} cm^{-3}. The methyl radical is an ideal system for the study of cold molecules not only because of its high reactivities at low temperatures, but also because further cooling below 1 mK is plausible via sympathetic cooling with ultracold atoms. The demonstrated trapping capability of methyl radicals opens up various possibilities for realizing ultracold ensembles of molecules towards Bose-Einstein condensation of polyatomic molecules and investigations of reactions governed by quantum statistics.
Collisions between cold polyatomic molecules and atoms are essential in various processes, including interstellar chemical reactions, quantum tunneling, and precision measurements.The methyl radical was the rst polyatomic radical trapped in a magnetic trap. This was achieved by our group in 2017, where the trapped radicals were trapped at a temperature of approximately 130 mK. The methyl radical has been proposed as a potential candidate for sympathetic cooling, which can achieve temperatures below 1 mK. In order to elucidate the collisional properties of the methyl radical, collisions between magnetically trapped, cold methyl radicals, and room temperature helium and argon atoms were investigated experimentally. This work presents the results of these collisional studies and their comparison with theoretically derived trap loss rates. Quantum diffractive collisions dominate the collisions between room temperature helium atoms and cold methyl radicals in a magnetic trap with a trap depth of about 134 mK. More than 10 % of trap loss is attributed to inelastic collisions, in which the methyl radicals are excited to a rotationally excited state. On the other hand, collisions between cold methyl radicals and room temperature argon at this trap depth is near the border between quantum diffractive collisions and classical collisions.
We have demonstrated that supersonic beams of oxygen molecules (O2)({\text{O}}_{2})(O2​) in the ground rotational state were slowed to a near standstill by a linear Zeeman decelerator and successfully confined spatially in an anti-Helmholtz magnetic trap. The translational temperature of the decelerated O2{\text{O}}_{2}O2​ molecules was about 800 mK. The demonstrated trapping capability of paramagnetic species opens up various possibilities for the investigation of cold collisions and reactions of various radicals in the sub-kelvin regime by spatially confining them in a magnetic trap.
In this dissertation, substantial headway towards the Zeeman deceleration and magnetic trapping of methyl radicals, and AC Stark trapping of ammonia are presented. Notably, the first ever trapping of gaseous methyl radicals within a permanent magnetic trap is discussed in detail. Additionally, the use of a counter rotating nozzle to produce slow polyatomic molecular beam is discussed. The methyl radical is the smallest and most stable paramagnetic intermediate hydrocarbon species, and as such, the physicochemical properties of this species are of substantial interest to astrochemists and organic chemists alike. Methyl radical, being a paramagnetic molecule, is decelerated using an 85 coil Zeeman decelerator from 320 m/s to 60 m/s. Notably, the first ever trapping of methyl radicals in the gas phase inside a permanent magnetic trap is described here. The trapped radicals had a temperature of 200 mK, and a lifetime of greater than one second. Lastly, the collisional loss cross sections for the methyl radical with helium and argon are also reported. In addition to the magnetic trapping of the methyl radical, progress towards the trapping of polar molecules with microwave radiation is discussed in detail. A quality (abbr. Q) factor of 5 x 105 was achieved by cooling a Fabry Perot cavity with superconducting mirror surfaces to 3 K. Resulting from the large Q factor, the maximum electric field inside the cavity is 1 MV/m when a power input of 10 W is used. At this electric field, ammonia molecules travelling slower than 10 m/s can be successfully trapped. Additionally, the AC stark shift of ammonia molecules within the trap is reported as the first step towards their trapping. Lastly, a counter rotating nozzle was constructed based on the design presented by Strebel et al. at the University of Freiburg. The technical modifications presented in this thesis yielded an increase in the density of the molecular beam by two orders of magnitude, in comparison to the Freiburg design. This nozzle was used to decelerate acetone from 430 m/s to 110 m/s, and unlike the Stark and Zeeman decelerators, it can be used to decelerate any gaseous molecules.
Control of the translational motion of methyl radicals is demonstrated by using pulsed magnetic fields created by a series of solenoid coils. By synchronously switching the magnetic fields corresponding to the motion of the radicals, we have observed the deceleration and focusing of a supersonic radical beam. The present result shows that Zeeman deceleration of polyatomic free radicals in the doublet state can be realized. This opens possibilities for trapping cold polyatomic free radicals in a spatially confined area leading to further studies of cold collisions and reactions.
We present a rigorous theoretical study of low-temperature collisions of polyatomic molecular radicals with (1)S(0) atoms in the presence of an external magnetic field. Accurate quantum scattering calculations based on ab initio and scaled interaction potentials show that collision-induced spin relaxation of the prototypical organic molecule CH(2)(X(3)B(1)) (methylene) and nine other triatomic radicals in cold (3)He gas occurs at a slow rate, demonstrating that cryogenic buffer-gas cooling and magnetic trapping of these molecules is feasible with current technology. Our calculations further suggest that it may be possible to create ultracold gases of polyatomic molecules by sympathetic cooling with alkaline-earth atoms in a magnetic trap.
We present an experimental realization of a moving magnetic trap decelerator, where paramagnetic particles entrained in a cold supersonic beam are decelerated in a co-moving magnetic trap.Our method allows for an efficient slowing down of both paramagnetic atoms and molecules to near stopping velocities.We show that under realistic conditions we will be able to trap and decelerate a large fraction of the initial supersonic beam.We present our first results on deceleration in a moving magnetic trap by bringing metastable neon atoms to near rest.Our estimated phase space volume occupied by decelerated particles at a final velocity of 50 m s -1 shows an improvement of two orders of magnitude as compared to currently available deceleration techniques.Cold chemistry (at temperature of ∼1 K) is a fast developing field with many interesting theoretical predictions yet only a few experiments [1][2][3][4].It has been predicted that in the cold chemistry regime, reactions might proceed through resonance states [5] and both reaction rate and path could be controlled [6] via external magnetic or electrostatic fields.Low-temperature chemistry is not limited to artificially created conditions alone but occurs naturally at the low temperature of interstellar space [7] and the atmospheres of remote planets and moons [8].It is anticipated that cold chemistry will follow the enormous progress in molecular chemistry that was spurred by the advent of supersonic beams more than half a century ago [9].However, the progress in experimental cold chemistry is rather slow partly due to the absence of a general method of atomic and molecular cooling.Although the low-temperature environment created within a supersonic expansion has been used to observe many reactions [10], a direct 1 Author to whom any correspondence should be addressed.