The exceptionally high density of electronic states in open d- and f-subshell molecules, particularly in the ultraviolet region of the spectrum, can be terrifying for the spectroscopist. In recent years, however, we have shown that this high density of electronic states may be exploited to allow the measurement of bond dissociation energies (BDEs) to unprecedented accuracy. In this talk I will demonstrate how we have measured BDEs for the 5d molecules ReC, ReN, ReO, and ReS. We have also been able to extend this technique to measure BDEs of triatomic molecules, including Re-C₂, SU-S, and OU-S. By identifying the threshold for two-photon ionization in a highly congested spectrum, we have also successfully measured the ionization energies of ReC, ReN, and ReO. Finally, the serendipitous production of URh allowed the BDE of this unusual bimetallic molecule to be measured. The measured values are: D₀(ReC) = 5.731±0.003 eV, D₀(ReN) = 5.635±0.003 eV, D₀(ReO) = 5.510±0.003 eV, D₀(ReS) = 3.947±0.003 eV, D₀(Re-C₂) = 5.359±0.003 eV, D₀(SU-S) = 4.910±0.003 eV, D₀(OU-S) = 5.035±0.004 eV, IE(ReC) = 8.425±0.011 eV, IE(ReN) = 8.193±0.020 eV, IE(ReO) = 8.561±0.011 eV, and D₀(URh) = 5.472±0.003 eV.

The bond dissociation energies of the An-X (An = Th, U; X = Cl, Br, I) molecules have been measured to high precision using a resonant two-photon ionization (R2PI) method. Due to the large density of electronic states in the vicinity of the ground separated atom limit, the R2PI spectra of these species become so congested in this region that they appear as continuous absorption features. Spin-orbit and nonadiabatic interactions among these states allow the molecule to find a pathway to dissociation as soon as the ground separated atom limit is exceeded. At this point, the R2PI ion signal drops to background levels, allowing a precise measurement of the bond dissociation energy (BDE) as listed below: D₀(ThCl) = 5.077 ± 0.006 eV D₀(UCl) = 4.989 ± 0.003 eV D₀(ThBr) = 4.391 ± 0.004 eV D₀(UBr) = 4.299 ± 0.013eV D₀(ThI) = 3.537 ± 0.008 eV D₀(UI) = 3.449 ± 0.008 eV For both thorium and uranium, the MCl bond is  15% stronger than the MBr bond and the MBr bond is  24% stronger than the MI bond. Ionization energies for these molecules have also been measured by the onset of two-photon, two-color ionization where the first photon is tuned to a discrete molecular transition. The measured IEs are then combined with the BDEs in a thermochemical cycle to derive the BDEs of the corresponding cations.

The chemical bond between lanthanide atoms and atomic nitrogen in the LnN diatomic molecules has received scant attention in the literature, despite numerous investigations of the corresponding solid materials. With the exceptions of LaN and CeN, it appears that no previous measurements of the bond dissociation energies (BDEs) of the LnN molecules have been made. This hinders an understanding of the electronic structure of the LnN chemical bond. Using resonant two-photon ionization (R2PI) spectroscopy, we have investigated the spectra of PrN, NdN, GdN, and TbN in the ultraviolet where the observation of a sharp predissociation threshold in a quasi-continuous optical spectrum allows the BDE to be measured to high precision and accuracy. Due to the open 4f subshell in these molecules, the density of electronic states in the vicinity of the dissociation limit is huge. Spin-orbit and nonadiabatic couplings among these states provide a rapid pathway to dissociation as soon as the ground separated atom limit is exceeded. In an R2PI spectrum, a quasi-continuous optical spectrum is observed below the BDE, but the ion signal drops sharply to background levels when the BDE is exceeded. The sharp drop in ion signal allows values of D₀(PrN) = 5.362 eV, D₀(NdN) = 4.820 eV, D₀(GdN) = 4.273 eV, and D₀(TbN) = 4.261 eV to be obtained. In each case, the assigned error limit is ±0.003 eV. As will be shown, this large variation in the LnN BDEs may be understood by considering the diabatic correlation of the LnN ground state to the appropriate separated atom limit, as has been done previously for the LnC,¹ LnS,² and LnSe² molecules. ¹D. M. Merriles, A. London, E. Tieu, C. Nielson, and M. D. Morse, Inorg. Chem. 62, 9589-601 (2023). ²J. J. Sorensen, E. Tieu, and M. D. Morse, J. Chem. Phys. 154, 124307 (2021).

The profound density of electronic states in proximity to the bond dissociation energy (BDE) of small d- and f- block molecules allows for precise and accurate measurements of the BDE by the observation of a sharp predissociation threshold in a congested or quasi-continuous optical spectrum. A resonant two-photon ionization (R2PI) scheme has been used by this group for these measurements in the past, but this imposes limitations because the dissociation limit must be reached with a single photon. To extend the method beyond the range of commercially available OPO lasers ( ∼ 6 eV), we have developed a resonant three-photon ionization method (R3PI) that was successfully applied to VO.¹ In this method, a tunable laser pulse is used to excite the molecule to a discrete vibronic level and a second tunable laser pulse is used to further excite the molecule to levels near the dissociation limit. For the method to succeed, the energetics must be such that the absorption of a second photon from the second laser pulse can ionize the molecule in an R3PI process. Under these conditions, the second laser may be scanned over the predissociation threshold and a sharp drop in ion signal occurs when the sum of the first and second photon energies matches the BDE. An advantage of this method is that the BDE can be accessed via multiple intermediate states, providing confirmation of the measurement’s validity. The BDEs of the late transition metal carbides were measured to be: RuC (6.312 eV), RhC (6.007 eV), OsC (6.427 eV), IrC (6.404 eV), and PtC (6.260 eV). For all five measurements, an uncertainty of ±0.002 eV has been assigned. ¹D. M. Merriles, A. Sevy, C. Nielson, and M. D. Morse, J. Chem. Phys. 153, 024303 (2020).

The actinides comprise fifteen metallic elements with atomic numbers Z = 89-103. The chemistry and electronic properties of actinide compounds are rich in variety due to the Z dependence of the 5f and 6d valence orbital energies [1,2]. The large nuclear charges result in strong relativistic effects acting on the atomic orbitals. Theoretical calculations of actinide compounds require treatment of relativistic and many-electron interactions for which quantitative modeling is challenging to achieve. Theory and sensitive experimental methods are needed to gain understanding of their electronic structures. We have used a high-resolution x-ray beamline at the Advanced Photon Source and a multi-crystal von Hamos x-ray spectrometer to record x-ray emission spectra across the L₃ edges of four uranimum compounds: UO₂, UO₃, Cs₂UCl₆, and Cs₂UO₂Cl₄. The combined beamline bandwidth and von Hamos resolution was  ∼ 2 eV, which is smaller than the  ∼ 8 eV L₃ lifetime. This allowed the recording of high-resolution x-ray emission spectra (RXES) to observe resonance and threshold features in the L₃ edge x-ray absorption spectra. In addition to energy variations of the L₃-N_4,5 x-ray fluorescence lines, inelastic scattering was recorded that we tentatively assign to resonant excitation of unoccupied 6d orbitals. The RXES spectra are correlated with prominent features in the x-ray absorption spectra. The measurements are compared with calculations that treat relativistic and many-electron interactions that have been developed to investigate electronic structures of compounds containing heavy elements [3,4]. [1] T. Vitova et al., Nat. Commun. 8, 16053 (2017). [2] R. E. Wilson et al., Nat. Commun. 9, 622 (2018). [3] L. Cheng, J. Chem. Phys. 151, 104103 (2019). [4] S. H. Southworth et al., Phys. Rev. A 100, 022507 (2019).

Extreme ultraviolet (XUV) spectroscopy accesses shallow-core, d- and f-orbitals in lanthanides and actinides, revealing information on bonding and the oxidation states in the electronic structure containing these elements. We adapt tabletop laser-driven high harmonic generation (HHG) to study the N_4,5- (predominantly 4d to 5f) and O_4,5-absorption (5d to 5f) edges of several lanthanides and uranium oxide crystals, respectively, in the 40-150 eV range. Reflection and absorption measurements are compared to calculations using density functional theory (DFT) to interpret and assign underlying transitions. These results yield insights into the fundamental electronic structure and demonstrate chemical sensitivity to similar compounds, which may be interesting for understanding crystalline and molecular f-electron systems for applications ranging from surface chemistry, photochemistry, and electronic or chemical structure determination to nuclear forensics.

r0pt Figure In this work, we report ¹⁹F nuclear magnetic resonance (NMR) measurements of electronic structure in AnF₄ (An = Th, U, Np, Pu) in an 11.74 T magnetic field. The actinide tetrafluorides allow the study of the distinctive evolution of electronic structure across the actinide row in a single structurally isomorphous series (Figure 1). Our analysis of the spectra shows that atomic susceptibility calculated with the valence electrons in 5f orbitals is not sufficient to explain the width and position of the anisotropic lineshapes. Electron sharing between the metal and ligand results in a partial hole is at the ligand and nonzero local electron spin density. A finite isotropic and anisotropic hyperfine coupling exists between the hole and nucleus, which indicates ligand-orbital-resolved charge transfers involving 5f-2s and 5f-2p orbital pairs, respectively. The charge transfers show a rapid increase as atomic number increases. The overall increase of both charge transfers indicates enhanced electron sharing between metal 5f and ligand orbitals, consistent with energy-degeneracy driven covalency. This work was supported by the U.S. Department of Energy Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences, Heavy Element Chemistry Program, FWP 73200.

A series of four lanthanide thenoyltrifluoroacetone (TTA) complexes consisting of two f0 (La3+ and Ce4+) and two f1 (Ce3+) complexes were examined using steady-state and time-resolved spectroscopic techniques. The wide range of spectroscopic techniques presented herein has enabled us to discern the nature of the excited states (charge transfer, CT vs. ligand localized, LL) as well as construct a Jablonski diagram for detailing the excited state reactivity within the series of molecules. The wavelength and excitation power dependence for these series of complexes are the first direct verification for the presence of simultaneous competing, non-interacting CT and LL excited states. Additionally, a computational framework is described that can be used to support spectroscopic assignments as a guide for future studies. Finally, the relationship between the obtained photophysics and possible photochemical separation mechanisms is described.

Cerium clusters are at the focus of current research interests due to their unique physical properties such as magnetism, superconductivity and quantum phase transitions. They also represent the entry-point for the heavy element clusters. The study on size-selected cerium clusters isolated in the gas phase provides information on the inherent cluster properties in the absence of perturbing environments. Here, we report on the electronic-state-selected vibrational wave-packet dynamics for the mass-selected neutral cerium clusters Ce₂₋₄ probed by two-color femtosecond pump-probe spectroscopy involving the negative-neutral-positive excitation scheme (fs NeNePo). The diatomic cerium anion were found for the first time and studied in the pump photodetachment energy range of 0.5 eV to 0.9 eV. CASSCF calculations and quantum dynamic simulations are performed to disentangle the observed oscillatory wave-packet dynamics of Ce₂, which is found that the oscillations arise mainly from the electronic states belonging to 4f²π_u³σ_g²σ_u¹ superconfigurations. The fs NeNePo transients of Ce₃ and Ce₄ show vibrational wave-packet dynamics that decay rapidly within 2 picoseconds.