Polar radioactive molecules have been suggested to be exceptionally sensitive systems in the search for signatures of symmetry-violating effects in their structure. Radium monofluoride (RaF) possesses an especially attractive electronic structure for such searches, as the diagonality of its Franck-Condon matrix enables the implementation of direct laser cooling for precision experiments. To maximize the sensitivity of experiments with short-lived RaF isotopologues, the molecular beam needs to be cooled to the rovibrational ground state. Due to the high kinetic energies and internal temperature of extracted beams at radioactive ion beam (RIB) facilities, in-flight rovibrational cooling would be restricted by a limited interaction timescale. Instead, cooling techniques implemented on ions trapped within a radiofrequency quadrupole cooler-buncher can be highly efficient due to the much longer interaction times (up to seconds). In this work, the feasibility of rovibrationally cooling trapped RaF+ and RaH+ cations with repeated laser excitation is investigated. Due to the highly diagonal nature between the ionic ground state and states in the neutral system, any reduction of the internal temperature of the molecular ions would largely persist through charge-exchange without requiring the use of cryogenic buffer gas cooling. Quasirelativistic X2C and scalar-relativistic ECP calculations were performed to calculate the transition energies to excited electronic states and to study the nature of chemical bonding for both RaF+ and RaH+. The results indicate that optical manipulation of the rovibrational distribution of trapped RaF+ and RaH+ is unfeasible due to the high electronic transition energies, which lie beyond the capabilities of modern laser technology. However, more detailed calculations of the structure of RaH+ might reveal possible laser-cooling pathways.
Polar radioactive molecules have been suggested to be exceptionally sensitive systems in the search for signatures of symmetry-violating effects in their structure. Radium monofluoride (RaF) possesses an especially attractive electronic structure for such searches, as the diagonality of its Franck-Condon matrix enables the implementation of direct laser cooling for precision experiments. To maximize the sensitivity of experiments with short-lived RaF isotopologues, the molecular beam needs to be cooled to the rovibrational ground state. Due to the high kinetic energies and internal temperature of extracted beams at radioactive ion beam (RIB) facilities, in-flight rovibrational cooling would be restricted by a limited interaction timescale. Instead, cooling techniques implemented on ions trapped within a radiofrequency quadrupole cooler-buncher can be highly efficient due to the much longer interaction times (up to seconds). In this work, the feasibility of rovibrationally cooling trapped RaF+ and RaH+ cations with repeated laser excitation is investigated. Due to the highly diagonal nature between the ionic ground state and states in the neutral system, any reduction of the internal temperature of the molecular ions would largely persist through charge-exchange without requiring the use of cryogenic buffer gas cooling. Quasirelativistic X2C and scalar-relativistic ECP calculations were performed to calculate the transition energies to excited electronic states and to study the nature of chemical bonding for both RaF+ and RaH+. The results indicate that optical manipulation of the rovibrational distribution of trapped RaF+ and RaH+ is unfeasible due to the high electronic transition energies, which lie beyond the capabilities of modern laser technology. However, more detailed calculations of the structure of RaH+ might reveal possible laser-cooling pathways.
The particular advantages of using the diatomic molecule radium monofluoride (RaF) as a versatile molecular probe for physics beyond the Standard Model are highlighted. i) RaF was previously suggested as being potentially amenable to direct cooling with lasers. As shown in the present work, RaF's energetically lowest electronically excited state is of 2Π symmetry (in contrast to BaF), such that no low-lying 2Δ state prevents efficient optical cooling cycles. ii) The effective electric field acting on the unpaired electron in the electronic ground state of RaF is estimated larger than in YbF, from which the best restrictions on the electron electric dipole moment (eEDM) were obtained experimentally. iii) Favourable crossings of spin-rotational levels of opposite parity in external magnetic fields exist, which are important for the measurement of the nuclear anapole moment of nuclei with a valence neutron. Thus, RaF appears currently as one of the most attractive candidates for investigation of parity-odd as well as simultaneously parity- and time-reversal-odd interactions in the realms of molecular physics.
Radioactive molecules have remained severely understudied relative to their stable counterparts, owing to difficulties associated with their radiological hazards and limited material availabilities. Despite this, their relevancy to many fields including nuclear science, chemistry, and astronomy make them attractive prospects for precision investigations. In particular, laser-coolable radioactive molecules containing octupole-deformed nuclei such as radium monohydroxide (RaOH) and radium monofluoride (RaF) promise dramatic sensitivity increases for experiments aiming to measure new physics originating from fundamental symmetry violations. Before they may be used for any of these purposes, however, new tools are needed that allow for their safe and precise study while making them accessible in a university (or similar) setting. In this thesis, we construct a tabletop cryogenic apparatus capable of studying radioactive molecules which we produce in-situ from trace quantities of starting material. We use this apparatus to perform the first spectroscopic observation of a radium-containing polyatomic molecule RaOH and its deuterated isotopologue RaOD, as well as high-resolution spectroscopy of the photon cycling transitions in RaF. To prepare for laser cooling, we develop a protocol to measure the vibrational branching ratios of these exotic species and use it to establish RaF to be among the most laser-coolable molecules known as of yet. We demonstrate the versatility of our approach by producing and detecting over twenty different atoms and molecules, ranging from transuranic radioisotopes to complex asymmetric top molecules. The species-agnostic methodology employed throughout this work allows its application to a wide class of previously ignored radioactive atoms and molecules, and makes them accessible to the many advanced experimental techniques already leveraged in stable systems.
Over the past few decades, radiofrequency ion traps have become an attractive platform for studying chemical reactions as they enable a high degree of control over ion-molecule dynamics. In this review, we summarize techniques for the trapping and cooling of atomic and molecular ions in radiofrequency traps, including Doppler and resolved-sideband laser cooling, sympathetic cooling, and cryogenic buffer-gas methods. We discuss strategies for controlling key reaction parameters: the preparation of specific internal quantum states by internal cooling, optical pumping, state-selective photoionization and quantum logic spectroscopy; the manipulation of collision energies through micromotion control, dynamic trapping, and combination with molecular beams; and the selection of molecular structure via isotopic substitution, conformational separation, and isomer-specific ion generation. We illustrate applications of these approaches by discussing studies on quantum-state-dependent kinetics, quantum resonance effects, and structure-sensitive reactivity in ion-neutral collisions. We conclude by outlining future challenges, including full state-to-state reaction mapping, reaching the ultracold quantum regime free of micromotion, and the exploration of complex and chiral systems.
The unstable radium nucleus is appealing for probing new physics due to its high mass, octupole deformation, and energy level structure. Ion traps, with long hold times and low particle numbers, are excellent for work with radioactive species, such as radium and radium-based molecular ions, where low activity, and hence low total numbers, is desirable. We address the challenges associated with the lack of stable isotopes in a tabletop experiment with a low-activity (∼10 μCi) source where we laser-cool trapped radium ions. With a laser-cooled radium ion we measured the 7p^{2}P_{1/2}^{o} state's branching fractions to the ground state, 7s^{2}S_{1/2}, and a metastable excited state, 6d^{2}D_{3/2}, to be p=0.9104(7) and 0.0896(7), respectively. With a nearby tellurium reference line we measured the 7s^{2}S_{1/2}→7p^{2}P_{1/2}^{o} transition frequency, 640.096 63(6) THz.
Direct laser-coolability and a comparatively simple electronic structure render alkaline-earth metal monofluoride molecules versatile laboratories for precision tests of fundamental physics. In this theoretical work, a route for efficient stopping and cooling of high-energy hot beams of mass-selected alkaline-earth metal monofluorides via their anions is explored to facilitate subsequent precision experiments with trapped molecules. It is shown that these molecular anions possess an unusually strong chemical bond and that RaF− features properties favourable for efficient pre-cooling, indicating the applicability of direct laser-cooling of the anion.
Ultracold polar molecules have emerged as promising platforms for quantum simulation, quantum information science, precision measurements of fundamental symmetry violations, and quantum chemistry. Over the past decade, rapid progress in direct laser cooling of molecules has enabled new capabilities such as magneto-optical traps (MOTs), sub-Doppler cooling, and optical dipole traps (ODTs), expanding the quantum toolbox with ultracold molecules of complex internal structures and strong dipolar interactions. This thesis presents the design, construction, and operation of an experiment on ultracold strontium monofluoride (88Sr19F) molecules, demonstrating state-of-the-art techniques for molecular laser cooling and trapping. The apparatus integrates a broad range of technical sub-systems, including cryogenics, ultra-high vacuum, optics, electronics, and automated laboratory control software. SrF molecules produced in a slowed cryogenic buffer gas beam are captured into a dual-frequency MOT. Subsequent Λ-enhanced gray molasses cooling reduces the molecule temperature to as low as 5 μK. A novel blue-detuned MOT is also implemented to further compress the molecular cloud, increasing the density by a factor of ∼ 400. At this high density, SrF molecules are efficiently loaded into an ODT, a tight, conservative trap with long lifetimes. Population transfer between different rotational levels is achieved via optical pumping and coherent microwave control, yielding ∼ 0.8 ms coherence time in Rabi oscillation and ∼ 90 % transfer efficiency. This thesis also describes the development of an ultracold rubidium (87Rb) atom experiment, in which a loading rate of ∼ 10^10 atoms/s into a radio-frequency (RF) MOT is demonstrated. A modular two-dimensional MOT serves as the Rb source and can be seamlessly integrated into the SrF apparatus, opening the possibility for future studies of co-trapping and sympathetic cooling of SrF molecules with atoms. The results presented in this thesis establish an experimental foundation for exploring new frontiers in ultracold molecular physics.