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
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
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
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 1K×kB in the laboratory frame, with corresponding fluxes exceeding 1010/s at velocities below 20m/s. By attaining densities higher than 109/cm3 and interaction times longer than 25ms in samples of fluoromethane as well as deuterated ammonia, we observe cold dipolar collisions between molecules and determine their collision cross sections.
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 gases of atoms and molecules provide a system for the exploration of a diverse set of physical phenomena. For example, cold gasses of magnetically and electrically polar atoms and molecules are ideal systems for quantum simulation and quantum computation experiments, and cold gasses of large polar molecules allow for novel spectroscopic techniques. Buffer-gas cooling is a robust and widely applicable method for cooling atoms and molecules to temperatures of approximately 1 Kelvin. In this thesis, I present novel applications of buffer-gas cooling to obtaining gases of trapped, ultracold atoms and diatomic molecules, as well as the study of the cooling of large organic molecules. In the first experiment of this thesis, a buffer-gas beam source of atoms is used to directly load a magneto-optical trap. Due to the versatility of the buffer-gas beam source, we obtain trapped, sub-milliKelvin gases of four different lanthanide species using the same experimental apparatus. In the second experiment of this thesis, a buffer-gas beam is used as the initial stage of an experiment to directly laser cool and magneto-optically trap the diatomic molecule CaF. In the third experiment of this thesis, buffer-gas cooling is used to study the cooling of the conformational state of large organic molecules. We directly observe conformational relaxation of gas-phase 1,2-propanediol due to cold collisions with helium gas. Lastly, I present preliminary results on a variety of novel applications of buffer-gas cooling, such as mixture analysis, separation of chiral mixtures, the measurement of parity-violation in chiral molecules, and the cooling and spectroscopy of highly unstable reaction intermediates.
Cold molecules are an invaluable tool for modern physics. Cold atoms have already revolutionized precision metrology, allowed for spectacular findings in quantum physics and opened the door for the creation and observation of novel states of matter. Cold molecules bring with them additional degrees of freedom which give rise to unique properties that enable new kinds of experiments. For these experiments it is of vital importance to start with a sufficiently large number of cold molecules. There are multiple paths for the slowing and cooling of atoms and most of the slowing techniques utilizing lasers for direct cooling have been transferred to the molecular case with its specific requirements. The Zeeman slowing technique is advantageous for loading a trap with cold particles but had not yet been transferred from atoms to the molecular case. In this thesis I will show the first realization of the Zeeman slowing technique for diatomic molecules. This thesis will discuss several preparatory steps: Our group has had some ideas to collimate the molecular beam with a conical nozzle outlet at the buffer gas cell which I investigated on potassium atoms generated by the ablation of 39K35Cl inside the buffer gas cell. I also realized a fluorescence spectroscopy of the |XΣ1/2, N = 1, v = 0⟩ → |AΠ1/2, N = 0, v = 0⟩ transition in magnetic fields up to 300 G and confirmed simulations regarding our two laser cycling scheme in the Paschen-Back regime by measuring the scattering rate of light on one molecule. Finally, I created pre-cooled 40Ca19F with a forward velocity of around 150 m s−1 from a 40Ca19F2 target with laser ablation inside a cryogenic buffer gas cell. With the Zeeman slowing technique I slowed the molecules down to 25 m s−1, a velocity where trapping can be possible. I have also shown the characteristic compression into specific velocity classes for not yet trappable velocities. Together these two findings demonstrate that molecular Zeeman slowing is possible and a real promising tool for future experiments.
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.