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Physical Review A

Decelerating polar molecules using traveling microwave lattices

Zhong-Kun Hu, Yi Ke, Jinbo Zhao, Xiao-Bing Deng, Jun Shan Luo

Published May 29, 2014• 1 citation
Publisher

Abstract

We propose a scheme to decelerate polar molecules by using traveling microwave lattices in a waveguide. Two counterpropagating microwave fields with different frequencies create a traveling microwave lattice and decelerate polar molecules by exploiting their first-order ac Stark shift. During deceleration, molecules are always close to the field maximum, thus reducing the potential particle loss due to nonadiabatic transitions occurring in the low-field region. The method can be used for molecules in a rotational ground state, which is suitable for further sympathetic or evaporative cooling. Numerical trajectory simulations are carried out, and the results show that the phase-space acceptance area is comparable to that of some other deceleration methods.

DOI
10.1103/physreva.89.053428

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We propose a scheme to decelerate polar molecules by using traveling microwave lattices in a waveguide. Two counterpropagating microwave fields with different frequencies create a traveling microwave lattice and decelerate polar molecules by exploiting their first-order ac Stark shift. During deceleration, molecules are always close to the field maximum, thus reducing the potential particle loss due to nonadiabatic transitions occurring in the low-field region. The method can be used for molecules in a rotational ground state, which is suitable for further sympathetic or evaporative cooling. Numerical trajectory simulations are carried out, and the results show that the phase-space acceptance area is comparable to that of some other deceleration methods.
We demonstrate the deceleration of heavy polar molecules in low-field-seeking states by combining a cryogenic source and a traveling-wave Stark decelerator. The cryogenic source provides a high-intensity beam with low speed and temperature, and the traveling-wave decelerator provides large deceleration forces and high phase-space acceptance. We prove these techniques using YbF molecules and find the experimental data to be in excellent agreement with numerical simulations. These methods extend the scope of Stark deceleration to a very wide range of molecules.
Cold molecules are unique samples that can be exploited for a broad set of novel experiments, such as cold collision studies and precision spectroscopy. An important remaining challenge in this field is a widely applicable method to manipulate the motion of neutral polar molecules in high-field-seeking states, i.e., the molecular ground state and states of more complex medium- sized and large molecules. For molecules in these states, true three- dimensional field maxima in free space are required, which are an inherent property of electromagnetic waves. The experiments presented in this thesis exploit the interaction of polar molecules with near-resonant microwave fields in open Fabry–Pérot type resonators and in closed cylindrically symmetric resonators. Ammonia (14NH3) was chosen as a prototypical test molecule for these microwave manipulation experiments because well-controlled ammonia packets can be generated using Stark deceleration in single quantum states. These packets are ideal starting points for developing and characterizing novel motion manipulation techniques. In particular, the rovibronic ground state of para-ammonia has a strong interaction with microwave radiation close to 23.7 GHz corresponding to a wavelength of 12.7 mm. In a first set of experiments, transverse focusing of packets of ammonia molecules with mean forward velocities ranging from 20 m/s to 50 m/s was demonstrated in a closed cylindrically symmetric resonator. The resonator mode used for these experiments features an electric field maximum on the resonator axis that coincides with the molecular beam axis. In order to also manipulate the forward motion of the molecular packets, several consecutive electric field maxima on the molecular beam axis are required and the microwave field has to be rapidly switched at the appropriate times. In a modified closed cylindrically symmetric resonator with twelve consecutive electric field maxima on the axis, the first microwave guiding with full phase-space control was demonstrated for packets of ammonia molecules with a mean forward velocity ranging from 12 m/s to 25 m/s. With the same setup, also the first microwave acceleration and deceleration were accomplished with an extraction of as much as 30% of the initial kinetic energy and a gain by up to 60%. The main advantage of the microwave manipulation technique is its applicability to larger molecules that cannot be manipulated with conventional methods, such as Stark-deceleration. Successful deceleration is complicated for a number of reasons. For example, these larger molecules generally carry much higher kinetic energy. Furthermore, a precise theoretical calculation of the interaction with microwave fields becomes much more challenging. In the last part of this thesis, methods are proposed to increase the performance of the microwave decelerator, developed and characterized in this thesis, to eventually provide slow samples of medium-sized molecules, such as aminobenzonitrile, and trap ammonia.
Traveling-wave Stark deceleration is a wonderful tool to slow down polar molecules. Once the molecules are slowed down, the molecules can be interrogated by means of spectroscopy and information can be extracted which might lead to new discoveries. Before that can happen, the deceleration process needs to be optimized and losses such as non-adiabatic transitions must be minimized. A computer simulation can help to assist, predict and analyze particle behaviour. The simulation that was previously developed has been extended with a new non-adiabatic transition mechanism based on energy difference between energy states. With the new loss mechanism in place, results have shown that the losses can have a great impact on the spatial particle distribution within the confinement zone.
We present experiments on decelerating and trapping ammonia molecules using a combination of a Stark decelerator and a traveling wave decelerator. In the traveling wave decelerator, a moving potential is created by a series of ring-shaped electrodes to which oscillating high voltages (HV) are applied. By lowering the frequency of the applied voltages, the molecules confined in the moving trap are decelerated and brought to a standstill. As the molecules are confined in a true 3D well, this kind of deceleration has practically no losses, resulting in a great improvement on the usual Stark deceleration techniques. The necessary voltages are generated by amplifying the output of an arbitrary wave generator using fast HV amplifiers, giving us great control over the trapped molecules. We illustrate this by experiments in which we adiabatically cool trapped NH3 and ND3 molecules and resonantly excite their motion.
Cryogenic buffer-gas beams are a promising method for producing bright sources of cold molecular radicals for cold-collision and chemical-reaction experiments. In order to use these beams in studies of reactions with controlled collision energies or in trapping experiments, one needs a method of controlling the forward velocity of the beam. A Stark decelerator can be an effective tool for controlling the mean speed of molecules produced by supersonic jets, but efficient deceleration of buffer-gas beams presents new challenges due to longer pulse lengths. Traveling-wave decelerators are uniquely suited to meet these challenges because of their ability to confine molecules in three dimensions during deceleration and their versatility afforded by the analog control of the electrodes. We have created ground-state $\mathrm{CH}(X{\phantom{\rule{0.16em}{0ex}}}^{2}\ensuremath{\Pi})$ radicals in a cryogenic buffer-gas cell with the potential to produce a cold molecular beam of 1011{10}^{11}1011 molecules/pulse. We present a general protocol for Stark deceleration of beams with a large position and velocity spread for use with a traveling-wave decelerator. Our method involves confining molecules transversely with a hexapole for an optimized distance before deceleration. This rotates the phase-space distribution of the molecular packet so that the packet is matched to the time-varying phase-space acceptance of the decelerator. We demonstrate with simulations and an analytic one-dimensional model that this method can decelerate a significant fraction of the molecules in successive wells of a traveling-wave decelerator to produce energy-tuned beams for cold and controlled-molecule experiments.