Abstract (eng)
The topics of this thesis describe the effort that was made towards laser cooling of anions, with the goal to use them as a sympathetic coolant for antiprotons in a Penning trap. It covers a theoretical analysis of the requirements and the expected performance of different cooling schemes, as well as a report on efforts towards an experimental realisation. The focus is on the molecular anion species C2−, which was chosen due to a recent survey of possible molecular candidates and the challenges which arise in the use of atomic anions (Yzombard et al., 2015).
After an introduction to the topic, some general properties of anionic molecules are discussed, including their electronic level structure. The theory behind the interaction of light with molecules is presented, to the extent relevant for the later sections. This then leads to a more detailed presentation of the specific case of C2−, with a focus on the levels and transitions which are interesting for laser cooling. Simulations of different laser cooling schemes are then presented. These were done on the basis of a GPU accelerated C++ code, which is designed for the simulation of particles in a Penning trap (Van Gorp et al., 2011; Van Gorp and Dupre, 2013), and which was customised to include the interaction of the molecules with the laser. The simulations led to two publications (Fesel et al., 2017; Gerber et al., 2018), which are reproduced in sections 2.5.1 and 2.5.2. A discussion and comparison of the results which were found is presented in section 2.6. The discussion aims at giving the reader an overview of the challenges and prospects, which are to be expected with laser cooling of C2−.
Chapter 3 then describes the experimental work that was done. The main topic is the setup for a C2− source which was built. This is based on the use of a commercial supersonic expansion valve (SSEV) and a subsequent dielectric barrier discharge (DBD) (Even, 2015). The DBD is then used to ionise a gas mixture of acetylene in a noble carrier. The produced anions are accelerated and then mass selected with the help of a Wien filter. A detailed description of the setup is given (see figure 3.1 for an overview), followed by a presentation of the results. Here, a mass spectrum is presented, showing the successful production of anions with a mass of 24u, which corresponds to C2−.
Additionally, the setup foresees a possibility for photodetachment spectroscopy on the produced anions. To this end, an optical resonator was built, which allows to enhance the light intensity of a 399nm laser, while also allowing the transmission of 2.54μm light. This allows for resonant stimulation of C2−. The optical setup for the frequency stabilisation of the laser to the resonator is presented (see figure 3.6 for an overview), followed by a presentation of the experimental results on the device. This includes a determination of the resonators finesse to F = (12800 ± 700) and a verification of the stability of the laser lock during the operation of the C2− source. Finally in chapter 4, a conclusion is given, which summarises the work described by this thesis and includes an outlook on the upcoming steps.