Actinide metal and metal oxide cation-molecular complexes are studied in the gas phase to investigate their bonding, ligand coordination and solvation. Cation-molecular complexes of uranium with small molecules such as water, carbon monoxide, carbon dioxide, nitrogen or benzene are produced in a supersonic molecular beam by pulsed laser vaporization of solid metal targets. Similar methods produce metal oxide complexes. Complexes containing a metal or oxide core ion with a specific number of ligand or solvent molecules are size-selected in a time-of-flight mass spectrometer and studied with different forms of laser photodissociation measurements. Tunable infrared laser spectroscopy reveals the shifts that occur for ligand/solvent vibrations upon binding to these metals and how these vary with the charge state and the number of ligands or solvent molecules present. Additional experiments employ tunable laser UV-visible photodissociation spectroscopy to probe excited states and photodissociation thresholds. The experiments are complemented by computational chemistry, with careful attention to relativistic and spin-orbit effects. The goal of these studies is an increased understanding of the fundamental interactions and electronic structure involved in actinide bonding, coordination and solvation.

Tunable laser photodissociation spectroscopy is employed to examine uranium cation-molecular complexes such as U⁺(Benzene), UO⁺(Benzene), and UO⁺(CO₂). These ions are produced via laser vaporization, cooled by supersonic expansion, and finally mass selected by a reflectron time-of-flight mass spectrometer (RTOF-MS). The electronic photodissociation spectra for these complexes were measured at visible and near-IR wavelengths and each spectrum exhibited a threshold followed by continuous dissociation at higher energy levels. The thresholds are assigned to be the upper limit of the respective bond dissociation energies. These experimental results are compared to theoretical data obtained with DFT calculations using an effective-core potential (ECP60MDF) optimized for actinide systems.

We report on the characterization of the rovibrational structure of the ground electronic states of BaH⁺ and BaD⁺ by high-resolution pulsed-field ionization zero-kinetic-energy (PFI-ZEKE) photoelectron spectroscopy. Rotationally cold (T_rot=8 K) BaH and BaD molecules in the X ²Σ⁺ (v = 0) level were generated in a supersonic expansion of 1:10 H₂/He and 1:10 D₂/He carrier gases, respectively, following laser ablation of a barium (Ba) rod [1].The rovibrational ionization thresholds corresponding to the X^{+ 1}Σ⁺ (v⁺ = 0-5) states were reached in a resonant 1+1^′ two-photon excitation sequence via the E ²Π_1/2 (v = 0,1) rovibrational intermediate levels of BaH and BaD studied previously by Ram and coworkers [2] and Bernard and coworkers [3]. Our new results include accurate values for the adiabatic ionization energy of BaH and BaD. This work is carried out in the context of our studies of the rovibrational structure of doubly charge dications by high-resolution PFI-ZEKE spectroscopy of singly charged cations following a similar approach as recently taken to characterize the ground state of the thermodynamically stable dication MgAr²⁺ [4]. The talk will present a roadmap towards characterizing the ground state of BaH²⁺ by resonant multiphoton excitation via electronically excited states of BaH⁺. The experiments will reveal whether BaH²⁺ is thermodynamically stable as predicted in Ref. [5]. [1] J.H. Bartlett, R.A. VanGundy and M.C. Heaven, J. Chem. Phys. 143 (4), 044302 (2015). [2] R. Ram and P. Bernath, J. Mol. Spectrosc. 283, 18–21 (2013). [3] A. Bernard, C. Effantin, J. d’incan, G. Fabre, R. Stringat and R. Barrow, Mol. Phys. 67 (1), 1–18 (1989). [4] D. Wehrli, M. Génévriez and F. Merkt, Phys. Chem. Chem. Phys. 23 (18), 10978– 10987 (2021). [5] L.G. dos Santos and F.R. Ornellas, Chem. Phys. 520, 32–39 (2019).

The quenching of uranyl luminescence was studied in an aqueous solution at low pH. Solutions with different nitrate salts, held at constant uranyl nitrate and nitric acid concentrations, were tested to examine the quenching effects caused by the cations in the nitrate salts. Alkali metals (Li⁺, Na⁺, Rb⁺) and quaternary ammonium (NH₄⁺, (CH₃)₄N⁺, (C₂H₅)₄N⁺) cations were investigated, and the solution containing (CH₃)₄N⁺ significantly increased the uranyl fluorescence decay rate. Lifetime measurements for most solutions under these conditions produced lifetimes between 1.4–1.9 μs at 20^oC, while the presence of (CH₃)₄N⁺ reduced the lifetime of uranyl fluorescence (measured at 510 nm) to 0.6μs. Concentration effects on fluorescence quenching (Stern-Volmer) indicated a dynamic process with decreasing lifetimes at higher cation concentrations. Temperature effects on quenching were also studied through the Arrhenius relationship. Solutions containing alkali metal ions had activation energies indicative of quenching via H₂ONagaishi, R. et al., Journal of Alloys and Compounds, 271 –273 (1998) 794 –798. while the (CH₃)₄N⁺ solution had a greatly reduced activation energy. These experiments showed that (CH₃)₄N⁺ quenched uranyl fluorescence more effectively than all other cations in this study. Nagaishi, R. et al., Journal of Alloys and Compounds, 271 –273 (1998) 794 –798.,

Uranium hexafluoride (UF₆) is a heavily utilized material feedstock for uranium enrichment processes. UF₆ is also highly reactive and undergoes spontaneous, rapid hydrolysis when exposed to water in the atmosphere. The resulting reaction intermediates and products are key signatures for nuclear safeguards and nonproliferation activities; however, the fundamental reaction dynamics of the hydrolysis are still debated in the literature. Here we have investigated the hydrolysis of several binary hexafluorides, including UF₆, using cryogenic layering and FTIR spectroscopy to better understand the underlying reaction dynamics. The hydrolysis reactions also produce dynamic hydrogen bonding network which provide insight into proton and metal ion transport mechanisms. l0pt Figure

In this presentation, we will briefly overview a suite of high-resolution spectroscopic and quantum control techniques currently employed in our laboratory, which could be tailored for studying heavy element chemistry. These methodologies include: (1) velocity-modulated dual-comb spectroscopy, (2) precise determination of electric dipole moments of actinide molecular ions within a rotating framework, and (3) single-molecule catalytic processes via optical mass spectrometry. Our objective is to illuminate the potential of leveraging atomic, molecular, and optical (AMO) technologies to unravel the complex questions surrounding actinide chemistry. As a research team dedicated to harnessing the power of precision measurements with small molecules for probing new physics beyond the Standard Model, we are eager to broaden our investigative horizons. Through sharing our techniques and insights, we aim to contribute to and serve the vibrant community engaged in heavy element chemistry research.

Affiliations: 1 Department of Chemistry and PRISMA Cluster of Excellence, Johannes Gutenberg University Mainz, Germany 2 GSI Helmholtzzentrum für Schwerionenforschung GmbH, Darmstadt, Germany 3 Helmholtz Institute Mainz, Germany 4 PRISMA+ Cluster of Excellence, Johannes Gutenberg University Mainz, Germany Production and studies of atomic and molecular systems with heavy radioactive elements are a rapidly emerging field [1]. Systems from the actinides to the superheavies are the focus of the work in our group. We study table-top production of thorium atomic ions [2] and small thorium-containing molecules [3] for ion trapping studies with relevance for the search for effects of physics beyond the standard model and for quantum logic spectroscopy, together with the Budker and the Schmidt-Kaler groups in Mainz, Germany. Studies of thorium and uranium clusters are performed in collaboration with the Schweikhard group in Greifswald, Germany [4]. We explore the production of small molecules for production of low-energy ion beams of the early actinides (Ac-Pu) at CERN-ISOLDE, Geneva, Switzerland (e.g., [5]). At the GSI accelerator lab in Darmstadt, Germany, we study the superheavy elements [6], with a current focus on carbonyl compounds of seaborgium (element 106) [7] and bohrium (element 107) [8], and on pushing the boundaries of the heaviest chemically studied elements from flerovium (element 114) [9] over moscovium (element 115) towards livermorium (element 116). At the symposium, I will give an overview of these activities. [1] G. Arrowsmith-Kron et al., arXiv 2302.02165 (2023). [2] K. Groot-Berning et al., Phys. Rev. A 99, 023420 (2018). [3] J. Stricker et al., contribution to this conference. [4] P. Fischer et al., contribution to this conference. [5] M. Au et al., Nucl. Instrum. Meth. B 541, 375 (2023). [6] O. Smits et al., Nat. Rev. Phys. 6, 86 (2024). [7] J. Even et al., Science 345, 1491 (2014). [8] V. Pershina et al., J. Chem. Phys. 149, 204306 (2018) [9] A. Yakushev et al., Front. Chem. 10, 976635 (2022).

A foil of ²³²Th produced roughly thirty years ago is investigated by high-vacuum laser-ablation and multi-reflection time-of-flight (MR-ToF) mass analysis. Produced cat- and anionic molecules are identified through precision mass measurements by reflecting them back and forth between two electrostatic mirrors and thus greatly increasing their flight time. A number of thorium-based molecules are identified, including atomic clusters Th_n⁺ with up to n=13 atoms. While large clusters are predominantly found as pure Th_n⁺, smaller species are more abundant with attached carbon, nitrogen, oxygen, or fluorine atoms (Fig. 1). Additionally, a small uranium contamination leads to compound molecules incorporating both Th and U. Selected species are excited with a 532-nm laser pulse to probe their photodissociation behavior and determine relative fragment abundances. r0pt Figure

Thorium isotopes became of high interest in for fundamental physics studies, e. g., for testing the standard model of particle physics because of their unique nuclear and atomic properties [1,2]. In the project Trapping And Cooling of Thorium Ions via Calcium (TACTICa), state of the art ion trapping and spectroscopic techniques are developed for a precise determination of nuclear moments, hyperfine intervals, and isotope shifts with different Th isotopes [3]. For this purpose, atomic Th ions are trapped in a Paul trap inside a calcium ion crystal and sympathetically cooled to below the Doppler limit to perform quantum logic spectroscopy [4]. The atomic Th ions are implemented into the crystal using two production methods, i. e., laser ablation of macroscopic thorium samples [3] and ultra-thin layers of alpha-decaying uranium isotopes which produce thorium daughter nuclei expectedly in a rather broad charge-state distribution that recoil from the sample with the momentum imparted by the alpha decay [5]. We have recently started to investigate producing thorium molecular ions including thorium monofluoride. Molecules like thorium fluoride [6] are of interest in the search for scalar dark matter [7] and appears interesting for use as quantum sensor to investigate CP violation [8]. We demonstrated the direct production of singly charged thorium mono- to tetrafluoride ions as well as up to triply charged thorium mono fluoride ions (ThF³⁺) by laser ablation[9]. Latest results on the production of thorium atomic and molecular ions [9] and on the spectroscopy of polarization gradient cooled thorium-232 ions inside of a linear calcium crystal [10] will be presented. [1] V. V. Flambaum, Physical Review Letters 97, 1–3 (2006). [2] V. V. Flambaum et al., Physical Review A 97, 1–12 (2018). [3] K. Groot-Berning et al., Phys. Rev. A 99, 023420 (2019). [4] W. Li et al., New J. Phys. 24, 043028 (2022) . [5] R. Haas et al., Hyperfine Interact. 241, 25 (2020). [6] V. V. Flambaum, Phys. Rev. C 99, 35501 (2019). [7] D. Antypas et al., Quantum Sci. Technol. 6, 034001 (2021). [8] N. R. Hutzler et al., https://arxiv.org/abs/2010.08709 (2020). [9] J. Stricker et al., in preparation (2024). [10] C. Leichtweiß et al., in preparation (2024).