Abstract 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 (CH3F and CF3CCH). Comparison of the rotational and translational temperatures provides evidence of faster rotational thermalisation for the CH3F/He system in the limit of low He density. In addition, the relaxation rates for different rotational states appear to be different.
Abstract 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 (CH3F and CF3CCH). Comparison of the rotational and translational temperatures provides evidence of faster rotational thermalisation for the CH3F/He system in the limit of low He density. In addition, the relaxation rates for different rotational states appear to be different.
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.
The measurement of the rotational state distribution of a velocity-selected, buffer-gas-cooled beam of ND3 is described. In an apparatus recently constructed to study cold ion-molecule collisions, the ND3 beam is extracted from a cryogenically cooled buffer-gas cell using a 2.15 m long electrostatic quadrupole guide with three 90° bends. (2+1) resonance enhanced multiphoton ionization spectra of molecules exiting the guide show that beams of ND3 can be produced with rotational state populations corresponding to approximately T(rot) = 9-18 K, achieved through manipulation of the temperature of the buffer-gas cell (operated at 6 K or 17 K), the identity of the buffer gas (He or Ne), or the relative densities of the buffer gas and ND3. The translational temperature of the guided ND3 is found to be similar in a 6 K helium and 17 K neon buffer-gas cell (peak kinetic energies of 6.92(0.13) K and 5.90(0.01) K, respectively). The characterization of this cold-molecule source provides an opportunity for the first experimental investigations into the rotational dependence of reaction cross sections in low temperature collisions.
We propose a method to study the rotational relaxation of polar molecules [here taking fluoromethane (CH3F) as an example] in collisions with 3.5 K buffer-gas helium (He) atoms by using an electrostatic guiding technique. The dependence of the guiding signal of CH3F on the injected He flux and the dependence of the guiding efficiency of CH3F on its rotational temperature are investigated both theoretically and experimentally. By comparing the experimental and simulated results, we find that the translational and rotational temperatures of the buffer-gas cooled CH3F molecules can reach to about 5.48 and 0.60 K, respectively, and the ratio between the translational and average rotational collisional cross sections of ${\mathrm{CH}}_{3}\mathrm{F}\ensuremath{-}\mathrm{He}$ is $\ensuremath{\gamma}={\ensuremath{\sigma}}_{t}/{\ensuremath{\sigma}}_{r}=36.49\ifmmode\pm\else\textpm\fi{}6.15$. In addition, the slowing, cooling, and boosting effects of the molecular beam with different injected He fluxes are also observed and their forming conditions are investigated in some detail. Our study shows that our proposed method can not only be used to measure the translational and rotational temperatures of the buffer-gas cooled molecules, but also to measure the ratio of the translational collisional cross section to the average rotational collisional cross section, and even to measure the average rotational collisional cross section when the translational collisional cross section is measured by fitting the lifetime of molecule signal to get a numerical solution from the diffusion equation of buffer-gas He atoms in the cell.
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.
This thesis reports on the production and characterisation of the first slow and cold beam of YbF molecules using buffer gas cooling. These molecules are being used to measure the electron’s electric dipole moment, and an intense source of slow-moving molecules is desirable for this experiment. The molecules are loaded into a buffer gas cell via laser ablation where they thermalise with cold helium buffer gas. They are then detected inside the cell using laser absorption imaging and spectroscopy on the X2Σ+→A2II1/2 transition. The formation, diffusion and thermalisation dynamics of the molecules inside the cell are studied. Measurements of laser absorption versus intensity reveal that saturation of the absorption is due to a competition between optical pumping into dark states and repopulation of the addressed level by inelastic and velocity-changing collisions. A beam of YbF molecules is extracted through an aperture in the buffer gas cell and characterised using laser induced fluorescence detection. Peak fluxes of 1010 molecules per steradian per pulse, in the rotational and vibrational ground state, are obtained. The translational and rotational temperatures are in equilibrium with the cell temperature of 4 K. The forward velocity of the pulses can be varied between 130 m/s and 200 m/s by changing the buffer gas pressure. This source is an order of magnitude brighter and more than three times slower than a supersonic source of YbF molecules and provides an excellent starting point for improving the measurement of the electron’s electric dipole moment and for deceleration and trapping experiments. In order to reduce the helium load on the vacuum system and to shorten the molecular pulses, a second set-up, delivering the buffer gas into an open copper cylinder in pulses rather than in a continuous flow,is characterised and shows promising first results.
This thesis reports on the use of buffer gas cooling to produce slow and intense molecular beams, with the aim of creating a source of molecules that can be used to measure the electron electric dipole moment. A beam of Yb is extracted from a buffer gas cell by entrainment in a flow of helium. The beam is characterised and simulations are performed of the helium flow through the cell to better understand the effect this has on the beam characteristics. The velocity of the beam is found to vary between 65 m/s[superscript -1] and 204 m/s[superscript -1] depending on the helium flow through the cell, in agreement with the predictions of the simulations. The temperature of the beam is 2.4 K and the minimum divergence is 12°. The percentage of Yb that is extracted from the cell is ~10%. Sources which use a pulsed valve to produce a supersonic jet are also characterised at cryogenic temperatures. The addition of a thermalisation cell is found to produce beams that are slower, colder and more intense. The peak beam intensity is 4.5×10[superscript 10] molecules per seradian per pulse, the velocity (206 ± 5) m/s and the translational temperature is (12.3 ± 0.5) K. An electric decelerator is used to decelerate a pulse of YbF molecules from 300 m/s[superscript -1] to 276 m/s[superscript -1] by confining them to a travelling trap. Simulations of the molecules in the decelerator show that its acceptance is greater than those demonstrated previously. A detailed understanding of this deceleration is developed. The Franck-Condon factors for the A[superscript 2]Π[subscript 1/2] (v = 0) − X[superscript 2]Σ[superscript +] (v - 0, 1, 2) transition of YbF are measured. The results show that, using three lasers, the molecule could be laser-cooled.