Photoionization spectroscopy of metal-containing diatomics

A detailed knowledge and understanding of different molecular states and their related excitations – rotational, vibrational, electronic – is crucial for many contemporary research approaches in molecular physics and beyond, such as, astrochemistry, metrology, and quantum computing. This work focuses on characterizing metal-containing diatomic molecules (aluminum monofluoride (AlF), ytterbium monofluoride (YbF) and dysprosium monoxide (DyO)) generated in molecular beam experiments with a laser ablation source. The molecules were embedded in a cold molecular beam and studied by laser spectroscopy in a nearly interaction-free manner. Various resonance-enhanced multiphoton ionization (REMPI) schemes were used to analyze the ro-vibrational structure of electronic states and to determine the adiabatic ionization energies (IE) of these molecules. One common background for the study of these molecules is the interest in the preparation of ultracold molecular aggregates, e.g. via laser cooling. To study the AlF molecule, new and existing knowledge of the electronically excited states was used to develop REMPI and mass-analyzed threshold ionization (MATI) schemes. These methods allowed accurate measurements of the IE as well as the vibrational transition frequency (ωe−2ωeχe) of the cationic ground state. A comparable methodology was used in the investigation of the YbF molecule, focusing on the IE and select higher IE’s. In YbF, the electronic ground state correlates to the 4f146s1 configuration of the Yb+ ion. However, the lowest-lying electronically excited states belong to the 4f136s2 configuration of the Yb+ ion. Both configurations are almost decoupled and transitions between them are forbidden, resulting in the formation of two separate energy ladders, each with its own ionization energy. A preliminary attempt was made to identify interactions between these configurations by measuring their respective ionization energies. For the vast majority of diatomic molecules, the bond dissociation energy (D0) is lower than the IE, as observed for AlF or a specific electronic configuration of YbF. However, special classes of diatomic molecules, certain transition metal oxides, rare-earth metal oxides and fluorides, exhibit an inverse trend, with IE < D0. The primary focus of this thesis is a detailed examination of a rare-earth metal oxide diatomic, the DyO molecule. The investigations should clarify whether the ionization energy is lower than the bond dissociation energy. In an initial study, the ionization energy of dysprosium-containing diatomic molecules, specifically the dysprosium dimer and dysprosium monoxide, was determined using single-photon ionization. In the course of further studies of low-lying electronically excited states of DyO, a three-photon one-color ionization pathway was discovered. A more detailed study of this multiphoton process led to the development of isotopologue selective REMPI excitation schemes for 161DyO and 162DyO, which permitted the study of both the IE and D0 regions. The identification of several Rydberg series below the ionization energy allowed the determination of the IE as well as the rotational constant (B) of the cationic ground state. To determine D0, a number of characteristics could be discerned, including an abrupt disappearance of transitions and an increase in line widths. The results confirm that the IE is below the D0. In further studies on the DyO molecule an infrared free-electron laser (IR-FEL) was used to study vibrational transitions by IR excitation. In an infrared-photo-induced Rydberg ionization (IR-PIRI) experiment, autoionizing Rydberg states converging to higher vibrational levels in the cationic ground state were detected to determine ωe − 2ωeχe. In a REMPI experiment, the population of the first vibrational level of the electronic ground state was studied by IR excitation, allowing the determination of the vibrational constant (ωe) and the anhamonicity (ωeχe).