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A cryofuge for cold-collision experiments with slow polar molecules

Wu, X., Gantner, T., Koller, M., Zeppenfeld, M., Chervenkov, S., Rempe, G.

Published Nov 3, 2017• 36 citations
OpenAlex

Abstract

Ultracold molecules represent a fascinating research frontier in physics and chemistry, but it has proven challenging to prepare dense samples at low velocities. Here we present a solution to this goal by a non-conventional approach dubbed cryofuge. It employs centrifugal force to bring cryogenically cooled molecules to kinetic energies below $1\,$K$\times k_B$ in the laboratory frame, with corresponding fluxes exceeding $10^{10}$/s at velocities below $20\,$m/s. By attaining densities higher than $10^9$/cm$^3$ and interaction times longer than $25\,$ms in samples of fluoromethane as well as deuterated ammonia, we observe cold dipolar collisions between molecules and determine their collision cross sections.

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Science•Nov 2, 2017•0 citations
Ultracold molecules represent a fascinating research frontier in physics and chemistry, but it has proven challenging to prepare dense samples at low velocities. Here we present a solution to this goal by a non-conventional approach dubbed cryofuge. It employs centrifugal force to bring cryogenically cooled molecules to kinetic energies below 1 1\,1K×kB\times k_B×kB​ in the laboratory frame, with corresponding fluxes exceeding 101010^{10}1010/s at velocities below 20 20\,20m/s. By attaining densities higher than 10910^9109/cm3^33 and interaction times longer than 25 25\,25ms in samples of fluoromethane as well as deuterated ammonia, we observe cold dipolar collisions between molecules and determine their collision cross sections.
Cooling molecules in the spin cycle A block of ice might look solid, but the molecules inside it are shaking vigorously. Slowing molecules all the way down in the laboratory offers enticing prospects to study and apply their quantum behavior. However, methods to cool dense samples to the necessary temperatures below 1 K have tended to be restricted to rather specialized diatomics. Wu et al. present a general cooling technique that applies to a wide range of conventional polar molecules such as methanol, fluoromethane, and ammonia. Their apparatus combines a preliminary cooling step in a buffer gas with a centrifuge that spins the molecules down using electric fields. Science , this issue p. 645
Molecules in the low temperature limit offer fascinating research perspectives in physics and chemistry, and molecule cooling and deceleration has developed into a research field of its own.The grand challenge today is to prepare dense samples with ample cold molecules.Towards this end, we present an elegant solution to this problem by a nonconventional approach, employing the centrifugal force in a rotating frame to decelerate molecules to a standstill.The centrifuge deceleration is a generic method since its principle is not associated with any specific internal structure of the molecules.Moreover, electrostatic guiding of molecules is applied throughout the deceleration process, which enables for the first time the deceleration of continuous molecular beams to achieve unprecedented high fluxes of slow molecules.In a proof-ofprinciple experiment, we demonstrate fluxes of ∼ 10 9 molecules/s and densities of ∼ 10 8 molecules/cm 3 below k B •1 K in the laboratory frame, originated from a roomtemperature effusive beam source.The efficiency of this decelerator is measured to be up to 20%, outperforming all the existing molecular deceleration methods.The generality of this method is demonstrated by the deceleration of several polyatomic molecular species, CH 3 F, CF 3 H, and CF 3 CCH.In addition to presenting the basic principle and the results, we also describe thoroughly in this thesis the design of the centrifuge decelerator, optimized through Monte-Carlo trajectory simulations, and the technical assembly.The second focus of this thesis is on the development and implementation of a simple and robust internal-state thermometry method for cold polyatomic molecules.The method is based on the combination of resonant radio-frequency depletion spectroscopy and Monte Carlo trajectory simulations.The radio-frequency depletion spectroscopy is capable of addressing 100% of the rotational states populated by the cold polyatomic molecules, as well as resolving their hyperfine states.With this tool, various aspects of cryogenic buffer-gas cooling used as a source of internally cold molecules are characterized.The properties of the cold molecular beams obtained at different buffer-gas temperature, densities, and cooling regimes, are analyzed.The cooling processes for different polyatomic molecules are compared.These studies reveal also two interesting phenomena in the buffer-gas cooling process.Firstly, evidence is provided that molecular rotational degrees of freedom can thermalize more efficiently than their translational ones.Secondly, the measurements provide indications for dependence of the collisional relaxation rate on the rotational states of the molecules.
Cold molecules promise to reveal a rich set of novel collision dynamics in the low-energy regime. By combining for the first time the techniques of Stark deceleration, magnetic trapping, and cryogenic buffer gas cooling, we present the first experimental observation of cold collisions between two different species of state-selected neutral polar molecules. This has enabled an absolute measurement of the total trap loss cross sections between OH and ND(3) at a mean collision energy of 3.6 cm(-1) (5 K). Due to the dipolar interaction, the total cross section increases upon application of an external polarizing electric field. Cross sections computed from ab initio potential energy surfaces are in agreement with the measured value at zero external electric field. The theory presented here represents the first such analysis of collisions between a (2)Π radical and a closed-shell polyatomic molecule.
Cold polar molecules offer fascinating prospects for ultracold chemistry and quantum physics. However, their inherent collision properties remain largely unknown, as it seems fundamentally impossible to reach sufficiently low energies by merging two beams of molecules with substantial dipole moments. Here we report measurements of state-to-state cross-sections for collisions between ammonia isotopologues at energies between 0.3 and 100 cm−1 using an advanced beam-merging protocol. We experimentally observed a local maximum in the cross-sections, which indicates that the dipole moments can switch off at low collision energies. Scattering calculations reproduced this maximum in good agreement and explained the observed scaling with the molecule’s parity splitting energies. Measurements of the correlated energy transfer in both collision partners yielded direct evidence of the suppression of the dipole–dipole interaction at low energies. Our results demonstrate how collisions between an important class of polar molecules evolve from high temperatures towards the ultracold regime in a counterintuitive way, and offer distinctive opportunities to control cold molecular collisions with external fields. Interactions between polar molecules are expected to become increasingly strong at low collision energies through dipole–dipole forces. Now it has been shown that ammonia molecules instead exhibit suppressed dipolar interactions in the cold regime as the dipole moment effectively switches off, as shown by local maxima in the collision cross-sections.
Reaching high densities is a key step toward cold-collision experiments with polyatomic molecules. We use a cryofuge to load up to $2\ifmmode\times\else\texttimes\fi{}{10}^{7}$ CH3F{\mathrm{CH}}_{3}\mathrm{F}CH3​F molecules into a boxlike electric trap, achieving densities up to 107/cm3{10}^{7}/{\mathrm{cm}}^{3}107/cm3 at temperatures around 350 mK where the elastic dipolar cross section exceeds $7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}\text{ }\text{ }{\mathrm{cm}}^{2}$. We measure inelastic rate constants below $4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}\text{ }\text{ }{\mathrm{cm}}^{3}/\mathrm{s}$ and control these by tuning a homogeneous electric field that covers a large fraction of the trap volume. Comparison to ab initio calculations gives excellent agreement with dipolar relaxation. Our techniques and findings are generic and immediately relevant for other cold-molecule collision experiments.
Cold Molecules A block of ice might look solid, but the molecules inside it are shaking vigorously. Slowing molecules all the way down in the laboratory offers enticing prospects to study and apply their quantum behavior. However, methods to cool dense samples to the necessary temperatures below 1 K have tended to be restricted to rather specialized diatomics. Wu et al. present a general cooling technique that applies to a wide range of conventional polar molecules such as methanol, fluoromethane, and ammonia. Their apparatus combines a preliminary cooling step in a buffer gas with a centrifuge that spins the molecules down using electric fields. Science , this issue p. [645][1] [1]: /lookup/doi/10.1126/science.aan3029