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The Journal of Chemical Physics

Production of cold beams of ND3 with variable rotational state distributions by electrostatic extraction of He and Ne buffer-gas-cooled beams

Kathryn S. Twyman, Martin T. Bell, Brianna R. Heazlewood, T. P. Softley

Published Jul 14, 2014• 23 citations
Publisher

Abstract

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.

DOI
10.1063/1.4885855

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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.
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.
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.
We present a method which delivers a continuous, high-density beam of slow and internally cold polar molecules. In our source, warm molecules are first cooled by collisions with a cryogenic helium buffer gas. Cold molecules are then extracted by means of an electrostatic quadrupole guide. For ND3{\mathrm{ND}}_{3}ND3​ the source produces fluxes up to $(7{\ifmmode\pm\else\textpm\fi{}}_{4}^{7})\ifmmode\times\else\texttimes\fi{}{10}^{10}\text{ }\text{ }\mathrm{\text{molecules}}/\mathrm{s}$ with peak densities up to $(1.0{\ifmmode\pm\else\textpm\fi{}}_{0.6}^{1.0})\ifmmode\times\else\texttimes\fi{}{10}^{9}\text{ }\text{ }\mathrm{\text{molecules}}/{\mathrm{cm}}^{3}$. For H2CO{\mathrm{H}}_{2}\mathrm{CO}H2​CO the population of rovibrational states is monitored by depletion spectroscopy, resulting in single-state populations up to $(82\ifmmode\pm\else\textpm\fi{}10)%$.
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.
We have produced translationally cold ammonia (ND3) molecules in various quantum states by kinematic cooling. In these experiments, ND3 molecules are brought nearly to rest in the (J, K) = (2,0), (2,1), (2,2), (3,1), (3,2), and (3,3) rotational levels of the ground vibronic state by rotationally-inelastic collisions with Ne atoms. The cold molecules are produced in quantum-state-dependent velocity distributions whose laboratory frame velocities are measured to be between 21 m s(-1) (E(trans)/k = 530 mk) and 32 m s(-1) (E(trans)/k = 1.2 K), and are calculated to be between 7.5 m s(-1) (E(trans)/k = 70 mK) and 27 m s(-1) (E(trans)/k = 880 mK). Due to systematic experimental effects, the measured velocities are upper limits to the actual velocities. These temperatures are low enough that it should be possible to use electrostatic traps to confine cold molecules in many of these quantum states.
We propose a method to study the rotational relaxation of polar molecules [here taking fluoromethane (CH3F)({\mathrm{CH}}_{3}\mathrm{F})(CH3​F) 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{\mathrm{CH}}_{3}\mathrm{F}CH3​F on the injected He flux and the dependence of the guiding efficiency of CH3F{\mathrm{CH}}_{3}\mathrm{F}CH3​F 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{\mathrm{CH}}_{3}\mathrm{F}CH3​F 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.