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Design and Characterization of a New Pulsed Molecular-Beam Valve

Ian W Collett

Published Jan 1, 2016• 1 citation
OpenAlex

Abstract

A common technique for producing samples of cold molecules that can be trapped and studied is electromagnetic deceleration of a fast moving, but internally cold, supersonic beam. The quality of these samples, which are ideally as cold and dense as possible, is fundamentally limited by the quality of the beam coupled to the decelerator. This thesis outlines the design, construction, and characterization of a new pulsed valve that produces samples of molecules as short as fifty microseconds.

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The work depicted in this thesis demonstrate the successful operation of a 4 meter long traveling-wave Stark decelerator. This is a novel tool for deceleration of heavy diatomic molecules to arbitrarily slow beams with a very small velocity spread which can serve as ideal starting point for precision tests of fundamental physics. The first part of the thesis discusses the characterization of a new Supersonic beam source for creation of SrF molecules which serves as an ideal candidate for precision test of nuclear spin dependent parity violation. The later part of the thesis discusses the the results of deceleration of a SrF molecules in the N = 1 rotation state of the SrF(v = 0) electronic ground state from a supersonic beam with an initial velocity of 340 m/s to 265 m/s using a deceleration potential with 3.25 k V voltage amplitude using a the 4 m long traveling-wave decelerator built in our research lab. Increasing the voltage amplitude of the potential to 4.8 kV, we were able to decelerate molecules from 340 m/s to 250 m/s which corresponds to a deceleration strength of 6.6 km /s2. By cooling the valve by flushing cold nitrogen gas through the valve enclosure we were able to decrease the initial velocity of the supersonic beam. With the valve cooled to ~190 K, we demonstrated deceleration of SrF molecules from 290 m/s to 120 m/s which corresponds to the removal of almost 85% of the initial kinetic energy. The thesis described the operation of the decelerator and end with an outlook for precision tests using this novel tool.

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A common technique for producing samples of cold molecules that can be trapped and studied is electromagnetic deceleration of a fast moving, but internally cold, supersonic beam. The quality of these samples, which are ideally as cold and dense as possible, is fundamentally limited by the quality of the beam coupled to the decelerator. This thesis outlines the design, construction, and characterization of a new pulsed valve that produces samples of molecules as short as fifty microseconds.
The work depicted in this thesis demonstrate the successful operation of a 4 meter long traveling-wave Stark decelerator. This is a novel tool for deceleration of heavy diatomic molecules to arbitrarily slow beams with a very small velocity spread which can serve as ideal starting point for precision tests of fundamental physics. The first part of the thesis discusses the characterization of a new Supersonic beam source for creation of SrF molecules which serves as an ideal candidate for precision test of nuclear spin dependent parity violation. The later part of the thesis discusses the the results of deceleration of a SrF molecules in the N = 1 rotation state of the SrF(v = 0) electronic ground state from a supersonic beam with an initial velocity of 340 m/s to 265 m/s using a deceleration potential with 3.25 k V voltage amplitude using a the 4 m long traveling-wave decelerator built in our research lab. Increasing the voltage amplitude of the potential to 4.8 kV, we were able to decelerate molecules from 340 m/s to 250 m/s which corresponds to a deceleration strength of 6.6 km /s2. By cooling the valve by flushing cold nitrogen gas through the valve enclosure we were able to decrease the initial velocity of the supersonic beam. With the valve cooled to ~190 K, we demonstrated deceleration of SrF molecules from 290 m/s to 120 m/s which corresponds to the removal of almost 85% of the initial kinetic energy. The thesis described the operation of the decelerator and end with an outlook for precision tests using this novel tool.
With the Stark deceleration technique, packets of molecules with a tunable velocity, a narrow velocity spread, and a high state purity can be produced. These tamed molecular beams find applications in high resolution spectroscopy, cold molecule trapping, and controlled scattering experiments. The quality and purity of the packets of molecules emerging from the decelerator critically depend on the specifications of the decelerator, but also on the characteristics of the molecular beam pulse with which the decelerator is loaded. We consider three frequently used molecular beam sources, and discuss their suitability for molecular beam deceleration experiments, in particular with the application in crossed beam scattering in mind. The performance of two valves in particular, the Nijmegen Pulsed Valve and the Jordan Valve, is illustrated by decelerating ND 3 molecules in a 2.6 meter-long Stark decelerator. We describe a protocol to characterize the valve, and to optimally load the pulse of molecules into the decelerator. We characterize the valves regarding opening time duration, optimal valve-to-skimmer distance, mean velocity, velocity spread, state purity, and relative intensity.
This dissertation describes a general approach of generating a high flux of cold atoms that can be confined in a magneto-optical trap. As an alternative to the state-of-the-art laser cooling method, this approach widens our ability to cool and control atoms without relying on a specific atomic transition and availability of laser. In this dissertation I will discuss the design, construction of the experiment and characterization of a pulsed cold atom source in detail. This work is based on a new paradigm: entrainment of atoms in the carrier gas of a supersonic beam, followed by the magnetic deceleration and trapping. Our methodology is based on the supersonic beam created by the expansion of a dense carrier gas from a pulsed release of gas through a small aperture. Cold noble gas emerging from this pulsed high pressure supersonic nozzle acts as a carrier, into which atoms of interest are then entrained. In our studies with the lithium atoms, up to 10¹¹ atoms per pulse could be entrained into a supersonic beam of helium at a translation temperature of below 100 mK. The supersonic valve is typically operated at a rate of below 1 Hz in the experiment. A much larger flux can be achieved at a higher repetition rate. These atoms are moving at a speed of 500 ms⁻¹ and need to be decelerated almost to a complete stop, in order to be trapped and cooled to quantum degeneracy. A 2.5-meter long moving-trap magnetic decelerator with 480 coil pairs was built and characterized. Atoms moving at a speed around 500 ms⁻¹ were trapped and decelerated to various final velocities ranging from 400 ms⁻¹ to 50 ms⁻¹, at a resulting temperature of 30 mK and a flux of 10⁸ ~ 10⁹ atoms per pulse. This whole process takes place within only 10 ms, at a repetition rate of 100 Hz 10¹⁰ ~ 10¹¹ atoms can be delivered per second. This approach is very general compared to the laser cooling, since most atoms in the periodic table have a magnetic moment in their ground state or can be optically pumped to a long-lived para-magnetic metastable state. In this dissertation, I show the working principle of each component of our experiment and characterize the atom beam at multiple stages. I describe in detail the construction and optimization of our magnetic decelerator, and demonstrate the control and monitoring system with the experiment results. Also implemented in this work is a flexible laser system that is composed of a reference diode laser and two tapered amplifiers to control and probe the internal states of atoms, as well as provide the trapping force. I also explore the optical molasses and chirped cooling techniques which help load the atoms into a magneto-optical trap. The successful demonstration of this method of creating a cold atom source leads us to believe that the magnetically decelerated supersonic atoms will play an important role in the area of cold atom physics
The observed molecule signal in cold molecule experiments is limited by the molecule source and the efficiency of the detection method. The purpose of my thesis work was to improve the scientific apparatus physicists use to trap and detect cold molecules. The first part of my thesis focuses on improving a new cold molecule source known as a Nijmegen Pulsed Valve by optimizing the parameters used to run it. The valve worked best when operating at a voltage of 8-12V, valve pulse duration of 70-100 microseconds, and valve frequency of 1Hz. The new Neijmegen Pulsed Valve, run with optimized parameters, was then compared to the standard cold molecule source known as the Piezoelectric Valve. The two valves showed similar performance. The second part of my thesis focuses on investigating a method used to detect cold molecules, known as photoionization spectrometry with vacuum ultraviolet (VUV) light, with the overall goal of improving the method. A common VUV light source is 118 nm light produced by third harmonic generation (THG) in a mixture of xenon and argon. According to theory, it is possible to increase the amount of 118 nm light from THG by increasing the pressure of the phased matched Xe-Ar mixture, however the amount of 118 nm light decreases with increasing mixture pressure. One untested factor thought to be causing this discrepancy is linear absorption of 118 nm light in xenon. However, the tests I performed show that linear absorption in xenon is not the issue.