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Collisional losses of magnetically trapped cold methyl radicals

Jiahong Hu

Published Jan 1, 2021• 0 citations
Publisher

Abstract

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.

DOI
10.14288/1.0396940

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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 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.
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.
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.
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.
We present an experimental and theoretical study of atom-molecule collisions in a mixture of cold, trapped N atoms and NH molecules at a temperature of $\ensuremath{\sim}600\text{ }\text{ }\mathrm{mK}$. We measure a small N+NH\mathrm{N}+\mathrm{NH}N+NH trap loss rate coefficient of ${k}_{\mathrm{loss}}^{(\mathrm{N}+\mathrm{NH})}=9(5)(3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}\text{ }\text{ }{\mathrm{cm}}^{3}\text{ }{\mathrm{s}}^{\ensuremath{-}1}$. Accurate quantum scattering calculations based on ab initio interaction potentials are in agreement with experiment and indicate the magnetic dipole interaction to be the dominant loss mechanism. Our theory further indicates the ratio of N+NH\mathrm{N}+\mathrm{NH}N+NH elastic-to-inelastic collisions remains large (>100>100>100) into the mK regime.
We report a combined experimental and theoretical study of collision-induced dipolar relaxation in a cold spin-polarized gas of atomic nitrogen (N). We use buffer gas cooling to create trapped samples of 14N^{14}\mathrm{N}14N and 15N^{15}\mathrm{N}15N atoms with densities $(5\ifmmode\pm\else\textpm\fi{}2)\ifmmode\times\else\texttimes\fi{}{10}^{12} {\mathrm{cm}}^{\ensuremath{-}3}$ and measure their magnetic relaxation rates at milli-Kelvin temperatures. These measurements, together with rigorous quantum scattering calculations based on accurate ab initio interaction potentials for the ${}^{7}{\ensuremath{\Sigma}}_{u}^{+}$ electronic state of N2{\mathrm{N}}_{2}N2​ demonstrate that dipolar relaxation in N+N\mathrm{N}+\mathrm{N}N+N collisions occurs at a slow rate of $~{10}^{\ensuremath{-}13} {\mathrm{cm}}^{3}$/s over a wide range of temperatures (1 mK to 1 K) and magnetic fields (10 mT to 2 T). The calculated dipolar relaxation rates are insensitive to small variations of the interaction potential and to the magnitude of the spin-exchange interaction, enabling the accurate calibration of the measured N atom density. We find consistency between the calculated and experimentally determined rates. Our results suggest that N atoms are promising candidates for future experiments on sympathetic cooling of molecules.