Threshold Ionization Spectroscopy and Theoretical Calculations of LnO (Ln = La and Ce)

Electronic structures and spin frustration in Ln3O (Ln = Ce, Sm, Gd) neutrals and anions determined by anion photoelectron spectroscopy

The results of a combined experimental and computational study on Ln3O (Ln = Ce, Sm, and Gd) anion and neutral clusters are presented and analyzed. These three Ln's were specifically targeted because they vary in their spin state and orbital angular momentum associated with the 4fN subshell occupancies. From the anion PE spectra of Ce3O-, Sm3O-, and Gd3O- measured with 2.330 and 3.495 eV photon energies, we determine the adiabatic electron affinities of the corresponding neutrals to be 0.83 ± 0.03, 1.11 ± 0.05, and 1.17 ± 0.05 eV, respectively. The lowest energy features in all three spectra can readily be reconciled with molecular structures in which the O-atom is central to all three Ln centers, with Ce3O-/Ce3O assuming pyramidal structures and Sm3O-/Sm3O and Gd3O-/Gd3O assuming planar structures. Computationally, the lowest-energy structure of neutral Ce3O is a kite-like structure, which is not consistent with the observed spectrum. The kite-like and pyramidal structures of Ce3O- are predicted to be nearly isoenergetic. Electronic states in which all three 4fN centers are ferromagnetically coupled are predicted to be energetically favored for all species, but spin-frustrated states in which one 4fN center is antiferromagnetically coupled to the remaining centers are computed to lie 0.05 eV higher in energy than the FM-coupled states for Ce3O- and Sm3O-. The PE spectrum of Sm3O- exhibits striking anomalies in the photoelectron angular dependence. This effect is attributed to strong photoelectron-valence electron interactions that drive nominally forbidden changes in the Mf state of the remnant neutral.

Energetic and Electronic Properties of AcX and LaX (X = O and F)

The bonding and spectroscopic properties of LaX and AcX (X = O and F) diatomic molecules were studied by high-level ab initio CCSD(T) and SO-CASPT2 electronic structure calculations. Bond dissociation energies (BDEs) were calculated at the Feller-Peterson-Dixon (FPD) level. Potential energy curves and spectroscopic constants for the lowest-lying spin-orbit Ω states were obtained at the SO-CASPT2/aQ-DK level. A dense manifold of excited states was described for the monofluorides with the ground states well separated from the excited states. The spectroscopic parameters were in good agreement with those reported experimentally for LaO and LaF. For the diatomic molecules containing actinides, no experimental data of these parameters was found, but the results were consistent with other high-level calculations. The BDEs calculated at the FPD level were 791.3 (LaO), 705.2 (AcO), 650.0 (LaF), and 678.6 (AcF) kJ/mol. The NBO analysis showed that the monofluorides are essentially ionic, which explains why the BDE(AcF) is higher than BDE(LaF); for the monoxides, covalent contributions involving the d orbitals of the metal and the p orbitals of the oxygen are stronger for LaO than AcO, which explains the higher BDE for LaO. The bond orders are predicted to be 2 for LaF and AcF, 3 for AcO, and higher than 3 for LaO.

Systematic Investigation of Electronic States and Bond Properties of LnO, LnO+, LnS, and LnS+ (Ln = La-Lu) by Spin-Orbit Multiconfiguration Perturbation Theory

The electronic structures of lanthanide monoxides (LnO/LnO+) and monosulfides (LnS/LnS+) for all lanthanide series elements (Ln = La-Lu) have been systematically analyzed with sophisticated quantum chemical calculations. The ground electronic configuration has been determined to be Ln 4fn6s1 or 4fn+1 for the neutral molecules and Ln 4fn for the cations. The low-lying energy states resulting from spin-orbit coupling and ligand field effects have been resolved using spin-orbit multiconfiguration quasi-degenerate second-order perturbation theory calculations. The ionization energies of LnO (5.20-7.06 eV) are about 0.3-2.2 eV lower than those of LnS (5.54-9.22 eV) due to the difference in the Ln 6s and 4f orbital energies from which an electron is removed during the ionization process. The bond dissociation energies (BDEs) have been computed by the state-averaged general multiconfigurational perturbation theory and the completely renormalized coupled-cluster [CR-CC(2,3)] methods. The BDEs are highly dependent on the lanthanide elements as several factors of the lanthanides affect the bond dissociation. The calculated bond lengths and energies agree well with available experimental values and are systematically predicted for the series of lanthanide monoxides and monosulfides where experimental values are not available. Furthermore, the LS terms of low-lying energy states and their corresponding bond properties have been clarified in detail to systematize the similarities and differences of the lanthanide compounds.

Enthalpy of the Cerium (Ce) Chemi-Ionization Reaction and CeO+, CeC+, and CeCO+ Bond Energies Determined by Energy-Resolved Guided Ion Beam Mass Spectrometry Experiments

Cerium (Ce) oxide, carbide, and carbonyl cation bond energies and the exothermicity of the Ce chemi-ionization (CI) reaction with atomic oxygen were investigated using guided-ion beam tandem mass spectrometry (GIBMS). The kinetic energy dependent product cross sections for reactions of Ce+ with O2, CO, and CO2 and CeO+ with O2, CO, Xe, and Ar were measured using GIBMS. For the reactions of Ce+ with O2 and CO2, CeO+ is formed through an exothermic reaction, whereas CeO+ formation is endothermic in the reaction with CO. This reaction also forms CeC+ and CeCO+ is formed in the reaction of Ce+ with CO2, both in endothermic processes. Reactions of CeO+ with all four gases led to endothermic collision-induced dissociation as well as exchange reactions to form CeO2+ for the O2 and CO reactants. Bond dissociation energies (BDEs) at 0 K were determined through analyses of the kinetic energy dependent cross sections for all endothermic reactions. We determined that the BDEs of CeO+, CeC+, and CeCO+ are 8.46 ± 0.15, 3.93 ± 0.16, and ≥0.25 ± 0.07 eV, respectively, where the CeO+ BDE is a weighted average of 6 independent 0 K threshold measurements. The CeO+ BDE is combined with the ionization energy (IE) of Ce to determine an exothermicity of 2.91 ± 0.15 eV for the Ce + O → CeO+ + e- chemi-ionization reaction. Combined with neutral BDEs from the literature, the thermochemistry determined here also provides IE(CeO) = 5.03 ± 0.12 eV and IE(CeC) = 6.50 ± 0.16 eV. Theoretical calculations for ground and some excited states were performed for CeO+, CeC+, and CeCO+ to provide insight into the bonding and a comparison with the experimentally determined BDEs for each molecule.

Gas-phase vibrational spectroscopy of the dysprosium monoxide molecule and its cation

Rotationally resolved vibrational spectra of DyO and DyO+ in a molecular beam are obtained by IR excitation from the X8 ground state and from high-n Rydberg states of DyO using an infrared free electron laser. Vibrational excitation is detected either by resonance enhanced multiphoton ionisation from X8(v = 1) or by autoionisation of Rydberg states converging to DyO+(v = 1). For most heavy molecules, the large spectral width of an infrared free electron laser does not allow for rotational resolution. In DyO and DyO+ the P, Q, and R transitions can be resolved due to the high angular momentum in their ground states. For 164DyO a vibrational constant of ωe = 847.5(2) cm-1 and a vibrational anharmonicity of ωeχe = 2.9(1) cm-1 are deduced. For the 161DyO+ cation a transition frequency of ΔG1/2 = 907(1) cm-1 is found.

Electronic Structure of Heteronuclear Cerium-Platinum Clusters

Beyond the now well-known strong catalyst-support interactions reported for ceria-supported platinum catalysts, intermetallic Ce-Pt compounds exhibit fascinating properties such as heavy fermion behavior and magnetic instability. Small heterometallic Ce-Pt clusters, which can provide insights into the local features that govern bulk phenomena, have been less explored. Herein, the anion photoelectron spectra of three small mixed Ce-Pt clusters, Ce2OPt-, Ce2Pt-, and Ce3Pt-, are presented and interpreted with supporting density functional theory calculations. The calculations, which are readily reconciled with the experimental spectra, suggest the presence of numerous close-lying spin states, including states in which the Ce 4f electrons are ferromagnetically coupled or antiferromagnetically coupled. The Pt center is consistently in a nominal -2 charge state in all cluster neutrals and anions, giving the Ce-Pt bond ionic character. Ce-Pt bonds are stronger than Ce-Ce bonds, and the O atom in Ce2OPt- coordinates only with the Ce centers. The energy of the singly occupied Ce-local 4f orbitals relative to the Pt-local orbitals changes with cluster composition. Discussion of the results includes potential implications for Ce-rich intermetallic materials.

Probing La and Ce Excited-State Reactivity with Resonant Two-Photon Ionization Spectroscopy

Dehydrogenation and C-C bond cleavage of 1-butyne by the excited states of La and Ce atoms are investigated in laser-ablation metal molecular beams. The excited states of the metal atoms are prepared by resonant excitation, detected by resonant two-photon ionization spectroscopy, and the reaction products are monitored by photoionization time-of-flight mass spectrometry. The reactivities of La* [5d²(³F)6p (⁴G_5/2°)] and Ce* [4f5d(³F°)6s6p(³P°) (⁵H₅)] excited states are observed to be higher than those of the initial states of the corresponding metal atoms. The higher reactivities of the excited states are attributed to their higher energies and favorable electron configurations to form two covalent bonds of the metal-insertion intermediates. Although both excited La and Ce atoms show increased reactivities, the enhancement for Ce is much more pronounced than that of La, which cannot be explained by electron configurations alone. The larger reactivity enhancement from the initial states to the excited state of the Ce atom than that of La is due to the longer lifetime of the Ce excited state.

New Insight into the Gas Phase Reaction Dynamics in Pulsed Laser Deposition of Multi-Elemental Oxides

The gas-phase reaction dynamics and kinetics in a laser induced plasma are very much dependent on the interactions of the evaporated target material and the background gas. For metal (M) and metal–oxygen (MO) species ablated in an Ar and O₂ background, the expansion dynamics in O₂ are similar to the expansion dynamics in Ar for M⁺ ions with an MO⁺ dissociation energy smaller than O₂. This is different for metal ions with an MO⁺ dissociation energy larger than for O₂. This study shows that the plume expansion in O₂ differentiates itself from the expansion in Ar due to the formation of MO⁺ species. It also shows that at a high oxygen background pressure, the preferred kinetic energy range to form MO species as a result of chemical reactions in an expanding plasma, is up to 5 eV.

Bonding properties of molecular cerium oxides tuned by the 4f-block from ab initio perspective

Probing chemical bonding in molecules containing lanthanide elements is of theoretical interest, yet it is computationally challenging because of the large valence space, relativistic effects, and considerable electron correlation. We report a high-level ab initio study that quantifies the many-body nature of Ce–O bonding with the coordination environment of the Ce center and particularly the roles of the 4 f orbitals. The growing significance of the overlap between Ce 4 f and O 2 p orbitals with the increasing coordination of Ce atoms enhances Ce–O bond covalency and in return directs the molecular geometry. Upon partial reduction from neutral to anionic ceria, the excessive electrons populate the Ce-centered localized 4 f orbital. The interplay between the admixture and localization of the 4 f-block dually modulates bonding patterns of cerium oxide molecules, underlying the importance of many-body interactions between ligands and various lanthanide elements.

Excited states of lutetium oxide and its singly charged cation

Vibronic spectra of lutetium oxide (LuO) seeded in supersonic molecule beams are investigated with mass-analyzed threshold ionization (MATI) spectroscopy and second-order multiconfigurational quasi-degenerate perturbation (MCQDPT2) theory. Six states of LuO and four states of LuO+ are located by the MCQDPT2 calculations, and an a3Π(LuO+) ← C2Σ+ (LuΟ) transition is observed by the MATI measurement. The vibronic spectra show abnormal vibrational intervals for both the neural and cation excited states, and the abnormality is attributed to vibrational perturbations induced by interactions with neighboring states.

Predissociation measurements of the bond dissociation energies of EuO, TmO, and YbO

The observation of a sharp predissociation threshold in the resonant two-photon ionization spectra of EuO, TmO, and YbO has been used to measure the bond dissociation energies of these species. The resulting values, D₀(EuO) = 4.922(3) eV, D₀(TmO) = 5.242(6) eV, and D₀(YbO) = 4.083(3) eV, are in good agreement with previous values but are much more precise. In addition, the ionization energy of TmO was measured by the observation of a threshold for one-color two-photon ionization of this species, resulting in IE(TmO) = 6.56(2) eV. The observation of a sharp predissociation threshold for EuO was initially surprising because the half-filled 4f⁷ subshell of Eu in its ground state generates fewer potential energy curves than in the other molecules we have studied by this method. The observation of a sharp predissociation threshold in YbO was even more surprising, given that the ground state of Yb is nondegenerate (4f¹⁴6s², ¹S_g) and the lowest excited state of Yb is over 2 eV higher in energy. It is suggested that these molecules possess a high density of electronic states at the energy of the ground separated atom limit because ion-pair states drop below the ground limit, providing a sufficient electronic state density to allow predissociation to set in at the thermochemical threshold.

Lanthanide Oxides: From Diatomics to High-Spin, Strongly Correlated Homo- and Heterometallic Clusters

Small lanthanide (Ln) oxide clusters present both experimental and theoretical challenges because of their partially filled, core-like 4f ⁿ orbitals, a feature that results in a plethora of close-lying and fundamentally similar electronic states. These clusters provide a bottom-up approach toward understanding the electronic structure of defective or doped bulk material but also can offer a challenge to the theorists to find a method robust enough to capture electronic structure patterns that emerge from within the 4f ⁿ (0 < n < 14) series. In this Feature Article, we explore the electronic structures of small lanthanide oxide clusters that deviate from bulk stoichiometry using anion photoelectron spectroscopy and supporting density functional theory calculations. We will describe the evolution of electronic structure with oxidation and how Ln_xO_y^- cluster reactivities can be correlated with specific Ln-local orbital occupancies. These strongly correlated systems offer additional insights into how interactions between electrons and electronically complex neutrals can lead to detachment transitions that lie outside of the sudden one-electron detachment approximation generally assumed in anion photoelectron spectroscopy. With a better understanding of how we can control nominally forbidden transitions to sample an array of spin states, we suggest that more in-depth studies on the magnetic states of these systems can be explored. Extending these studies to other Ln-based materials with hidden magnetic phases, along with sequentially ligated single molecule magnets, could advance current understanding of these systems.