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arXiv:0807.4022 · Physical Review A

Stark deceleration of OH radicals in low-field-seeking and high-field-seeking quantum states

Kirstin Wohlfart, Frank Filsinger, Fabian Grätz, Jochen Küpper, Gerard Meijer

Published Sep 18, 2008• 34 citations
arXivHTMLPDF

Abstract

The Stark deceleration of OH radicals in both low-field-seeking and high-field-seeking levels of the rovibronic $^{2}\ensuremath{\Pi}_{3∕2}$, $v=0$, $J=3∕2$ ground state is demonstrated using a single experimental setup. Applying alternating-gradient focusing, OH radicals in their low-field-seeking $^{2}\ensuremath{\Pi}_{3∕2}$, $v=0$, $J=3∕2$, $f$ state have been decelerated from $345\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}239\phantom{\rule{0.3em}{0ex}}\mathrm{m}∕\mathrm{s}$, removing 50% of the kinetic energy using only 27 deceleration stages. The alternating-gradient decelerator allows one to independently control longitudinal and transverse manipulation of the molecules. Optimized high-voltage switching sequences for the alternating-gradient deceleration are applied, in order to adjust the dynamic focusing strength in every deceleration stage to the changing velocity over the deceleration process. In addition we have also decelerated OH radicals in their high-field-seeking $^{2}\ensuremath{\Pi}_{3∕2}$, $v=0$, $J=3∕2$, $e$ state from $355\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}316\phantom{\rule{0.3em}{0ex}}\mathrm{m}∕\mathrm{s}$. For the states involved, a real crossing of hyperfine levels occurs at $640\phantom{\rule{0.3em}{0ex}}\mathrm{V}∕\mathrm{cm}$, which is examined by varying a bias voltage applied to the electrodes.

DOI
10.1103/physreva.78.033421

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The Stark deceleration of OH radicals in both low-field-seeking and high-field-seeking levels of the rovibronic 2\ensuremathΠ3∕2^{2}\ensuremath{\Pi}_{3∕2}2\ensuremathΠ3∕2​, v=0v=0v=0, J=3∕2J=3∕2J=3∕2 ground state is demonstrated using a single experimental setup. Applying alternating-gradient focusing, OH radicals in their low-field-seeking 2\ensuremathΠ3∕2^{2}\ensuremath{\Pi}_{3∕2}2\ensuremathΠ3∕2​, v=0v=0v=0, J=3∕2J=3∕2J=3∕2, fff state have been decelerated from 345to239m∕s345\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}239\phantom{\rule{0.3em}{0ex}}\mathrm{m}∕\mathrm{s}345to239m∕s, removing 50% of the kinetic energy using only 27 deceleration stages. The alternating-gradient decelerator allows one to independently control longitudinal and transverse manipulation of the molecules. Optimized high-voltage switching sequences for the alternating-gradient deceleration are applied, in order to adjust the dynamic focusing strength in every deceleration stage to the changing velocity over the deceleration process. In addition we have also decelerated OH radicals in their high-field-seeking 2\ensuremathΠ3∕2^{2}\ensuremath{\Pi}_{3∕2}2\ensuremathΠ3∕2​, v=0v=0v=0, J=3∕2J=3∕2J=3∕2, eee state from 355to316m∕s355\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}316\phantom{\rule{0.3em}{0ex}}\mathrm{m}∕\mathrm{s}355to316m∕s. For the states involved, a real crossing of hyperfine levels occurs at 640V∕cm640\phantom{\rule{0.3em}{0ex}}\mathrm{V}∕\mathrm{cm}640V∕cm, which is examined by varying a bias voltage applied to the electrodes.
A Stark decelerator can slow down polar molecules to very low velocities. When the velocities are very low, the number of cold molecules obtained is very small. In order to obtain a higher quantity of cold molecules, inspired by the work of Reens et al. [Phys. Rev. Res. 2 (2020) 033 095], we propose an alternative method of operating a Stark decelerator. Through the trajectory simulation of OH molecules in the decelerator, we find that the number of cold molecules can be greatly increased by one order of magnitude at both low and high final velocities on a Stark decelerator consisting of around 150 electrodes. This development is due to the improved longitudinal and the transverse focusing property provided by the new switching schemes and the high-voltage configurations on the decelerator unit.
With a Stark decelerator, beams of neutral polar molecules can be accelerated, guided at a constant velocity, or decelerated. The effectiveness of this process is determined by the six-dimensional (6D) volume in phase space from which molecules are accepted by the Stark decelerator. Couplings between the longitudinal and transverse motion of the molecules in the decelerator can reduce this acceptance. These couplings are nearly absent when the decelerator operates such that only every third electric-field stage is used for deceleration, while extra transverse focusing is provided by the intermediate stages. For many applications, the acceptance of a Stark decelerator in this so-called s=3s=3s=3 mode significantly exceeds that of a decelerator in the conventionally used (s=1)(s=1)(s=1) mode. This has been experimentally verified by passing a beam of OH radicals through a $2.6\text{\ensuremath{-}}\mathrm{m}$-long Stark decelerator. The experiments are in quantitative agreement with the results of trajectory calculations, and can qualitatively be explained with a simple model for the 6D acceptance. These results imply that the 6D acceptance of a Stark decelerator in the s=3s=3s=3 mode of operation approaches the optimum value, i.e., the value that is obtained when any couplings are neglected.
We have focused and decelerated benzonitrile (C7H5N)({\mathrm{C}}_{7}{\mathrm{H}}_{5}\mathrm{N})(C7​H5​N) molecules from a molecular beam, using an array of time-varying inhomogeneous electric fields in alternating-gradient configuration. Benzonitrile is prototypical for large asymmetric top molecules that exhibit rich rotational structure and a high density of states. At the rotational temperature of 3.5K3.5\phantom{\rule{0.3em}{0ex}}\mathrm{K}3.5K in the pulsed molecular beam, many rotational states are populated. Benzonitrile molecules in their absolute ground state are decelerated from 320to289m∕s320\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}289\phantom{\rule{0.3em}{0ex}}\mathrm{m}∕\mathrm{s}320to289m∕s, and similar changes in velocity are obtained for excited rotational states. All measurements agree well with the outcome of trajectory calculations. These experiments demonstrate that such large polyatomic molecules are amenable to the powerful method of Stark deceleration.
For scientifically interesting diatomic free radical molecules, which typically possess large electric dipole moments, Stark deceleration of weak-field seeking states is an ideal method for cold sample production. We utilize a seeded, pulsed supersonic beam in combination with an electric discharge to realize an efficient free radical source that is well-suited for use with Stark deceleration. The source apparatus creates an intense OH molecular beam whose subsequent longitudinal phase-space manipulation by precisely sequenced, pulsed inhomogeneous electric fields is directly observed by in-situ laser-induced fluorescence (LIF) detection.
Abstract The traditional Stark deceleration method is difficult to apply in chemically stable polar diatomic molecules in their ground (Χ 1 Σ) state because the Χ 1 Σ state normally experiences little Stark shift and the rovibronic ground level is mostly high-field-seeking. To solve this problem, we propose a laser-assisted Stark deceleration scheme to decelerate such molecules in the present paper. Our results show that, owing to the transverse bunching effect of the applied red-detuning laser beam, the molecules of the high-field-seeking level | J = 0, M = 0〉 in the Χ 1 Σ state can be effectively decelerated. Furthermore, the present scheme is more effective because the interaction between the molecules and the combined fields can produce the pseudo-first-order Stark effect, and thus increase the depth of the effective potential. Compared to those molecules in the low-field-seeking state | J = 1, M Ω = −1〉 in the usual electrostatic Stark deceleration, a higher molecular density and lower velocity can be achieved under an equivalent initial phase angle.
Beams of polar molecules can be focused using an array of electrostatic lenses in alternating gradient (AG) configuration. They can also be accelerated or decelerated by applying an appropriate high-voltage switching sequence to the lenses. AG focusing is applicable to molecules in both low-field- and high-field-seeking states and is particularly well suited to the problem of decelerating heavy molecules and those in their ground rotational state. We describe the principles of AG deceleration and set out criteria to be followed in decelerator design, construction and operation. We calculate the longitudinal and transverse focusing properties of a decelerator, and exemplify this by 2D-imaging studies of a decelerated beam of metastable CO molecules.