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A Centrifuge Decelerator and a Thermometer for Cold Polar Molecules

Xing Yang Wu

Published Jan 1, 2017• 1 citation
OpenAlex

Abstract

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.

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A comprehensive characterisation of cold molecular beams from a cryogenic buffer-gas cell, providing insight into the physics of buffer-gas cooling, is presented. Cold molecular beams are extracted from a cryogenic cell by electrostatic guiding, which is also used to measure their velocity distribution. The rotational-state distribution of the molecules is probed by radio-frequency resonant depletion spectroscopy. With the help of complete trajectory simulations, yielding the guiding efficiency for all of the thermally populated states, it is possible to determine both the rotational and the translational temperature of the molecules at the output of the buffer-gas cell. This thermometry method is demonstrated for various regimes of buffer-gas cooling and beam formation as well as for molecular species of different sizes (CH3 F and CF3 CCH). Comparison of the rotational and translational temperatures provides evidence of faster rotational thermalisation for the CH3 F/He system in the limit of low He density. In addition, the relaxation rates for different rotational states appear to be different.

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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.
A comprehensive characterisation of cold molecular beams from a cryogenic buffer-gas cell, providing insight into the physics of buffer-gas cooling, is presented. Cold molecular beams are extracted from a cryogenic cell by electrostatic guiding, which is also used to measure their velocity distribution. The rotational-state distribution of the molecules is probed by radio-frequency resonant depletion spectroscopy. With the help of complete trajectory simulations, yielding the guiding efficiency for all of the thermally populated states, it is possible to determine both the rotational and the translational temperature of the molecules at the output of the buffer-gas cell. This thermometry method is demonstrated for various regimes of buffer-gas cooling and beam formation as well as for molecular species of different sizes (CH3 F and CF3 CCH). Comparison of the rotational and translational temperatures provides evidence of faster rotational thermalisation for the CH3 F/He system in the limit of low He density. In addition, the relaxation rates for different rotational states appear to be different.
Inspired by the spectacular successes in the field of cold atoms, there is currently great interest in cold molecules. Cooling molecules is useful for various fundamental physics studies and gives access to an exotic regime in chemistry where the wave property of the molecules becomes important. Although cooling molecules has turned out to be considerably more difficult than cooling atoms, a number of methods to produce samples of cold molecules have been demonstrated over the last few years. This paper aims to review the application of cold molecules and the methods to produce them. Emphasis is put on the deceleration of polar molecules using time-varying electric fields. The operation principle of the array of electrodes that is used to decelerate polar molecules is described in analogy with, and using terminology from, charged-particle accelerators. It is shown that, by applying an appropriately timed high voltage burst, molecules can be decelerated while the phase-space density, i.e. the number of molecules per position-velocity interval, remains constant. In this way the high density and low temperature in the moving frame of a pulsed molecular beam can be transferred to the laboratory frame. Experiments on metastable CO in states that are either repelled by or attracted to high electric fields are presented. Loading of slow molecules into traps and storage rings is discussed.
We describe the combination of buffer-gas cooling with electrostatic velocity filtering to produce a high-flux continuous guided beam of internally cold and slow polar molecules. In a previous paper (L.D. van Buuren et al., Phys. Rev. Lett., 2009, 102, 033001) we presented results on density and state purity for guided beams of ammonia and formaldehyde using an optimized set-up. Here we describe in more detail the technical aspects of the cryogenic source, its operation, and the optimization experiments that we performed to obtain the best performance. The versatility of the source is demonstrated by the production of guided beams of different molecular species.
Abstract Manipulating the motion of large polyatomic molecules, such as benzonitrile (C6H5CN), presents significant difficulties compared to the manipulation of diatomic molecules. Although recent impressive results have demonstrated manipulation, trapping, and cooling of molecules as large as CH3F, no general technique for trapping such molecules has been demonstrated, and cold neutral molecules larger than 5 atoms have not been trapped (M. Zeppenfeld, B. G. U. Englert, R. Glöckner, A. Prehn, M. Mielenz, C. Sommer, L. D. van Buuren, M. Motsch, G. Rempe, Nature 2012, 491, 570–573 ). In particular, extending Stark deceleration and electrostatic trapping to such species remains challenging. Here, we propose to combine a novel “asymmetric doublet state” Stark decelerator with recently demonstrated slow, cold, buffer‐gas‐cooled beams of closed‐shell volatile molecules to realize a general system for decelerating and trapping samples of a broad range of volatile neutral polar prolate asymmetric top molecules. The technique is applicable to most stable volatile molecules in the 100–500 AMU range, and would be capable of producing trapped samples in a single rotational state and at a motional temperature of hundreds of mK. Such samples would immediately allow for spectroscopy of unprecedented resolution, and extensions would allow for further cooling and direct observation of slow intramolecular processes such as vibrational relaxation and Hertz‐level tunneling dynamics.
This thesis reports on the combination of buffer-gas cooling with electric velocity filtering to produce a high-density guided beam of slow and internally cold polar molecules. In the continuous source, warm molecules are first cooled by collisions with a cryogenic helium buffer gas. Cold molecules are then extracted by means of an electric quadrupole guide. For formaldehyde the population of individual rotational states in the guide is monitored by depletion spectroscopy, resulting in single-state populations up to 82 %. This thesis also reports on a technique to produce pulses of slow guided molecules. The pulses exhibit a narrow velocity distribution around a tunable velocity.
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.