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Physical Review A

Rotational relaxation of fluoromethane molecules in low-temperature collisions with buffer-gas helium

Xingjia Li, Liang Xu, Yanning Yin, Supeng Xu, Yong Xia, Jianping Yin

Published Jun 6, 2016• 9 citations
Publisher

Abstract

We propose a method to study the rotational relaxation of polar molecules [here taking fluoromethane $({\mathrm{CH}}_{3}\mathrm{F})$ 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 ${\mathrm{CH}}_{3}\mathrm{F}$ on the injected He flux and the dependence of the guiding efficiency of ${\mathrm{CH}}_{3}\mathrm{F}$ 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 ${\mathrm{CH}}_{3}\mathrm{F}$ 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.

DOI
10.1103/physreva.93.063407

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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 CH3F\ensuremath−He{\mathrm{CH}}_{3}\mathrm{F}\ensuremath{-}\mathrm{He}CH3​F\ensuremath−He is \ensuremathγ=\ensuremathσt/\ensuremathσr=36.49\ifmmode±\else\textpm\fi6.15\ensuremath{\gamma}={\ensuremath{\sigma}}_{t}/{\ensuremath{\sigma}}_{r}=36.49\ifmmode\pm\else\textpm\fi{}6.15\ensuremathγ=\ensuremathσt​/\ensuremathσr​=36.49\ifmmode±\else\textpm\fi6.15. 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.
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.
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.
The anisotropic interactions of three polar molecules CO, HF, and LiH with He atoms (in their ground electronic states), obtained from accurate ab initio calculations that explicitly include their vibration-to-translation coupling terms are analyzed in detail, to compare their relative features. The quantum scattering calculations of their rotovibrational inelastic cross sections are conducted using a recently proposed multichannel treatment, the modified variable phase method, that has been implemented by the authors and applied here to ultralow collision energies. A comparison of the different relaxation efficiencies exhibited by the three title molecules in releasing their internal vibrational energy during ultracold collisions with 4 He and 3 He buffer gas is performed in detail and specific suggestions for experimental choices are extracted from these findings.
NH radicals are magnetically trapped and their Zeeman relaxation and energy transport collision cross sections with helium are measured. Continuous buffer-gas loading of the trap is direct from a room-temperature molecular beam. The Zeeman relaxation (inelastic) cross section of magnetically trapped electronic, vibrational, and rotational ground state NH molecules in collisions with 3He^{3}\mathrm{He}3He is measured to be $3.8\ifmmode\pm\else\textpm\fi{}1.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}19}\text{ }\text{ }{\mathrm{cm}}^{2}$ at 710 mK. The NH-He energy transport cross section is also measured, indicating a ratio of diffusive to inelastic cross sections of $\ensuremath{\gamma}=7\ifmmode\times\else\texttimes\fi{}{10}^{4}$, in agreement with recent theory [R. V. Krems, H. R. Sadeghpour, A. Dalgarno, D. Zgid, J. K\l{}os, and G. Cha\l{}asi\ifmmode \acute{n}\else \'{n}\fi{}ski, Phys. Rev. A 68, 051401 (2003)].
We report on the buffer-gas cooling and trapping of CrH and MnH molecules in a magnetic quadrupole trap with densities on the order of ${10}^{6}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ at a temperature of 650mK650\phantom{\rule{0.3em}{0ex}}\mathrm{mK}650mK. Storage times of up to 180ms180\phantom{\rule{0.3em}{0ex}}\mathrm{ms}180ms have been observed, corresponding to a 20-fold lifetime enhancement with respect to the field-free diffusion through the 3He^{3}\mathrm{He}3He buffer-gas. Using Monte Carlo trajectory simulations, inelastic molecule-3He^{3}\mathrm{He}3He collision cross sections of $1.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}18}$ and $3.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}17}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{2}$ are extracted for CrH and MnH, respectively. Furthermore, elastic molecule-3He^{3}\mathrm{He}3He collision cross sections of $1.4(\ifmmode\pm\else\textpm\fi{}0.5)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{2}$ are determined for both species. We conclude that the confinement time of these molecules in a magnetic trapping field is limited by inelastic collisions with the helium atoms leading to Zeeman relaxation.