Gaining control over three-body recombination in an ultracold rubidium gas

Abstract

My doctoral thesis covers three areas: three-body recombination in ultracold gases, optical spectroscopy of molecules, and the characterization of a hollow-core fiber for the transmission of ultraviolet light. Three-body recombination (TBR) is a chemical process in which three atoms collide, causing two of them to form a molecule while the third atom absorbs part of the binding energy. I studied this process in an ultracold cloud of rubidium atoms in a well-defined internal quantum state at temperatures around one microkelvin. This led to the following questions: Are there preferences in how this process occurs? In which quantum states do the resulting molecules reside, and how many are produced in each state? Is it possible to control this process, and if so, how can it be experimentally implemented? To answer these questions, I developed spectroscopic techniques and investigated the molecular states in a state-resolved manner. The key results of my work are as follows: we discovered that for three-body recombination of rubidium, the spin states of the atoms tend to be preserved. Furthermore, we identified two methods to control the reaction pathway and thus the molecular product distribution. We utilized magnetic fields to precisely control the spin composition of the atomic scattering state or molecular states. For the state-resolved detection of molecules, our lab employs resonance-enhanced multiphoton ionization (REMPI). In this technique, a molecule is first excited into an intermediate state with a first photon and then ionized by a second photon. The ion can be detected with near 100% efficiency. The choice of intermediate state is crucial, as it directly affects ionization and therefore detection efficiency. To identify the optimal intermediate state, I conducted detailed optical molecular spectroscopy of excited states near the 5s + 4d asymptote. This allowed us to resolve the vibrational and rotational structure, as well as the fine structure, of the electronic states $(2)^1\Sigma_u^{+}$ and $(2)^3\Pi_g$ in an energy range from 493 to 503 THz x h. Based on these experimental results, new potential energy curves will be calculated in collaboration with Professor Tiemann from Hannover. Additionally, in collaboration with the Max Planck Institute for the Physics of Light in Erlangen, I worked on a hollow-core fiber to efficiently and robustly transmit laser light in the 300 - 320 nm range. This wavelength range is particularly relevant for the spectroscopy of highly excited Rydberg molecules, opening new possibilities for precise investigations. We achieved a transmission efficiency of over 70 % in single-mode operation with a fiber length of approximately 10m. The optical fiber showed no aging effects within the investigated time frame, which is an advantage compared to conventional fiber types that can be prone to degradation