Laser fluence-dependent production of molecular thorium ions in different charge states for trapped-ion experiments

Abstract: Thorium ions and molecules, recognized for their distinctive nuclear and atomic attributes, are central to numerous trapped-ion experiments globally. Our study introduces an effective, compact source of thorium ions produced via laser ablation of microgram-scale, salt-based samples. We thoroughly analyze the variety of ion species and charge states generated at varying laser fluences. Utilizing 10$\mu$g of thorium fluoride crystals and laser fluences between $100 - 700$ J$\cdot$cm$^{-2}$ we produce thorium molecular ions $^{232}$ThF$_x$$^{n+}$ (with $x= 0 - 3$ and charge states up to $n = 3+$), including ThF$^{2+}$ and ThF$^{3+}$. These species are particularly relevant for spectroscopy; ThF$^{3+}$ is valuable for its stable closed-shell configuration, while ThF$^{2+}$, which is isoelectronic to RaF, offers a unique probe for studying nuclear structure and fundamental symmetries due to its simple electronic structure with a single unpaired electron. Density functional theory calculations of the distribution of positive charge in the produced molecular cations and the simplicity of this setup indicate that this method is easily transferable to other actinide systems.