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Non-adiabatic losses in a traveling-wave Stark decelerator

M. Buisman

Published Jan 1, 2017• 0 citations
OpenAlex

Abstract

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.

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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.
Recently, a decelerator for neutral polar molecules has been presented that operates on the basis of macroscopic, three-dimensional, traveling electrostatic traps [A. Osterwalder, S. A. Meek, G. Hammer, H. Haak, and G. Meijer, Phys. Rev. A 81, 051401 (2010)]. In the present paper, a complete description of this decelerator is given, with emphasis on the electronics and the mechanical design. Experimental results showing the transverse velocity distributions of guided molecules are shown and compared to trajectory simulations. An assessment of non-adiabatic losses is made by comparing the deceleration signals from (13)CO with those from (12)CO and with simulated signals.
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.
Physicists have observed all the elementary particles predicted by the Standard Model. These particles, however, account for only 5% of the total mass-energy of the Universe. In the quest to solve the mystery of missing matter and to extend our knowledge about nature, a large number of experimental research programs are devoted to find a glimpse of New Physics. Table-top high precision measurements on molecules can contribute to these efforts in a manner complementary to high-energy experiments at colliders. In this thesis, we describe the operation of a traveling-wave Stark decelerator. This device is designed and constructed in order to slow down heavy polar molecules for various future explorations. Deceleration is achieved by manipulation of the molecules in controllable electric fields. We demonstrate in an experiment the successful operation of the Stark decelerator by slowing down a beam of SrF molecules to one-third of their initial velocity, thus removing almost 90% of the initial kinetic energy. We also reveal limiting conditions for this process. These results are of importance for many exciting experiments using heavy molecules in the near future. One relevant and promising example of such experiments is the search for a permanent electric dipole moment of the electron.
The growing field of cold molecule physics has demonstrated many methods of quantum control for precision measurement and molecular interaction studies. One particular method is Stark deceleration, which uses electric field gradients to produce packets of slow polar molecules. Since Stark deceleration does not increase phase-space density, the efficiency of the deceleration process is critical in determining the final cold molecule density. This dissertation describes three different Stark deceleration techniques. The first type of deceleration is pulsed-pin deceleration. We use this technique to produce slow OH molecules that can be trapped for cold collision studies. A two photon state-selective detection system, which includes a 118 nm photon, is used to determine the density distribution of OH in the trap. We explore the advantages and limitations to creating 118 nm light. The second and third types of Stark deceleration are pulsed-ring and traveling-wave methods of running a ring-geometry decelerator. Ring-geometry decelerators present improved deceleration efficiencies due to the cylindrical symmetry of the electrodes and true three-dimensional confinement. The pulsed-ring mode of the ring decelerator uses commercial high-voltage switches to apply discrete pulses to the ring electrodes. The continuous operation mode of the ring decelerator uses varying sine-wave voltages to create a traveling Stark potential well and has proven to be incredibly challenging due to the analog high voltage requirements of the system. In order to bring ND3 seeded in krypton (∼415 m/s) down to rest in the laboratory, sine wave voltages with amplitudes up to ±10 kV, currents up to 500 mA, and a bandwidth of 30 kHz down to DC are required. We describe a high-voltage amplifier that can be used for traveling-wave deceleration. The traveling-wave Stark deceleration mode is then compared to the pulsed-ring Stark deceleration mode and cases where one mode is superior to another are discussed.
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.