Experiments on cold molecules produced via Stark deceleration.

摘要

The work described in this thesis is the construction and operation of a Stark deceleration apparatus. Specifically, this work represents only the second successful implementation of the method1 and the first deceleration of the hydroxyl radical (OH) and the formaldehyde molecule (H 2CO). Experimentally, much of the work in the beginning of our experiments was the construction of a suitable molecular beam. Though molecular beams have been used for decades in physical chemistry, the design goals of the typical physical chemistry experiment are very different than ours, and as such, much work was required to adapt these sources for Stark deceleration. Since the maximum-attainable, decelerated molecular density is set at this stage, the importance of a good molecular beam source cannot be understated. Once a stable decelerated beam became routine in our laboratory, we performed the first precision measurement on cold molecules leading to the best ever values for the frequencies of the main lines in the lowest OH Λ-doublet. These measurements, when coupled with the appropriate astrophysical measurements, can be used to put the most stringent constraints on the variations of the fundamental constants. Excitingly, we have recently been informed that the required astrophysical measurements may be performed as early as the latter part of this year.; Our most recent work, not covered in this thesis, 2 has been the construction and implementation of a magnetic trap for OH molecules. To prepare decelerated molecules for loading into this trap we have built a second-generation decelerator, which has more stages than our first, to provide a higher flux of decelerated molecules. In the construction of this decelerator, several interesting phenomena, which currently limit the decelerator efficiency, have come to our attention. Specifically, these are transverse over-focussing of the decelerated molecular packets in the decelerator and the possibility of Majorana type transitions during deceleration. These effects (especially transverse over-focussing) extremely limit the number of molecules decelerated to below 15 m/s in our apparatuses. Since molecules of this speed or less are required for proper loading into our magnetic trap, both of these phenomena must be addressed in order for the optimal deceleration performance to be reached. Currently, as detailed in this thesis, experiments are already underway to learn how to eliminate them. Thus, it is my opinion that the technique of Stark deceleration is very close to blossoming into the best technique for producing cold molecules,3 since addressing these issues will lead to several orders of magnitude gain in decelerated molecule number. Furthermore, with ideas for further cooling of trapped samples being developed, e.g. cavity-assisted Doppler cooling, the possibilities for cold molecule physics are staggering.; 1The Berlin group, whose collaboration has been fruitful (for both sides, I hope), was first. 2This topic will likely be the crux of the dissertation of Brian Sawyer. 3Perhaps this argument could already be made.