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arXiv:1104.3976 · Physical Chemistry Chemical Physics

Stark deceleration of CaF molecules in strong- and weak-field seeking states

Thomas E. Wall, J. F. Kanem, J. M. Dyne, J. J. Hudson, B. E. Sauer, E. A. Hinds, Michael R. Tarbutt

Published Jan 1, 2011• 21 citations
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Abstract

We report the Stark deceleration of CaF molecules in the strong-field seeking ground state and in a weak-field seeking component of a rotationally-excited state. We use two types of decelerator, a conventional Stark decelerator for the weak-field seekers and an alternating gradient decelerator for the strong-field seekers, and we compare their relative merits. We also consider the application of laser cooling to increase the phase-space density of decelerated molecules.

DOI
10.1039/c1cp21254k

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We report the Stark deceleration of CaF molecules in the strong-field seeking ground state and in a weak-field seeking component of a rotationally-excited state. We use two types of decelerator, a conventional Stark decelerator for the weak-field seekers and an alternating gradient decelerator for the strong-field seekers, and we compare their relative merits. We also consider the application of laser cooling to increase the phase-space density of decelerated molecules.
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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=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 $^{2}\ensuremath{\Pi}_{3∕2}$, 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 $^{2}\ensuremath{\Pi}_{3∕2}$, 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.
We describe the production of cold, slow-moving LiH molecules. The molecules are produced in the ground state using laser ablation and supersonic expansion, and 68% of the population is transferred to the rotationally excited state using narrowband radiation at the rotational frequency of 444 GHz. The molecules are then decelerated from 420 to 53 m s −1 using a 100 stage Stark decelerator. We demonstrate and compare two different deceleration modes, one where every stage is used for deceleration, and another where every third stage decelerates and the intervening stages are used to focus the molecules more effectively. We compare our experimental data to the results of simulations and find good agreement. These simulations include the velocity dependence of the detection efficiency and the probability of transitions between the weak-field seeking and strong-field seeking quantum states. Together, the experimental and simulated data provide information about the spatial extent of the source of molecules. We consider the prospects for future trapping and sympathetic cooling experiments.
We explore a technique for decelerating molecules using a static magnetic field and optical pumping. Molecules travel through a spatially varying magnetic field and are repeatedly pumped into a weak-field seeking state as they move towards each strong field region, and into a strong-field seeking state as they move towards weak field. The method is time-independent and so is suitable for decelerating both pulsed and continuous molecular beams. By using guiding magnets at each weak field region, the beam can be simultaneously guided and decelerated. By tapering the magnetic field strength in the strong field regions, and exploiting the Doppler shift, the velocity distribution can be compressed during deceleration. We develop the principles of this deceleration technique, provide a realistic design, use numerical simulations to evaluate its performance for a beam of CaF, and compare this performance to other deceleration methods.
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.
In a Stark decelerator, polar molecules are slowed down and focused by an inhomogeneous electric field which switches between two configurations. For the decelerator to work, it is essential that the molecules follow the changing electric field adiabatically. When the decelerator switches from one configuration to the other, the electric field changes in magnitude and direction, and this can cause molecules to change state. In places where the field is weak, the rotation of the electric field vector during the switch may be too rapid for the molecules to maintain their orientation relative to the field. Molecules that are at these places when the field switches may be lost from the decelerator as they are transferred into states that are not focused. We calculate the probability of nonadiabatic transitions as a function of position in the periodic decelerator structure and find that for the decelerated group of molecules the loss is typically small, while for the un-decelerated group of molecules the loss can be very high. This loss can be eliminated using a bias field to ensure that the electric field magnitude is always large enough. We demonstrate our findings by comparing the results of experiments and simulations for the Stark deceleration of LiH and CaF molecules. We present a simple method for calculating the transition probabilities which can easily be applied to other molecules of interest.