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Low-kinetic energy photoelectron detachment experiments have revealed the unexpected dependence of transition intensities on photon energy, which is hypothesized to result from time-dependent coupling between low-kinetic energy photoelectrons and the remnant molecule. This study explores how the kinetic energy and detachment axis of the photoelectron influence the interaction and modify the final remnant electronic structure. Using real-time simulations on several model systems (H2, NO, N2, and C2 hydrocarbons), this study demonstrates that electron-remnant interactions are strongly dependent on the detachment orientation, electron kinetic energy, and remnant electronic structure. The results reveal that higher kinetic energies lead to significant nonadiabatic transitions, while lower kinetic energies yield more adiabatic behavior. While generally lower kinetic energies show prolonged electron-remnant interactions, the extent of temporal and spatial interactions does not necessarily vary linearly with the kinetic energy, and the final remnant electronic structure is found to be very sensitive to the exact nature of the photoelectron-remnant interactions. In addition, the point charge model employed for the photoelectron provides a useful approach for the deconvolution of more complete simulations to provide deeper insights into the specific photoelectron-remnant interactions that determine the eventual remnant wavefunction. The findings underscore the importance of considering both temporal and spatial electron dynamics in understanding low-kinetic energy photodetachment processes and provide a foundation for a further exploration of electron-molecule interactions in the low-energy regime., (© 2025 Author(s). Published under an exclusive license by AIP Publishing.)

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., (© 2025 Author(s). Published under an exclusive license by AIP Publishing.)

Stoichiometric Zn x O x clusters in the subnanometer size regime have been the topic of several computational and mass spectrometry studies that showed the particular stability of the stoichiometric species relative to nonstoichiometric species (Zn x O y , x ≠ y ). In the current study, we present the angle-resolved anion photoelectron (PE) spectra of stoichiometric Zn x O x - clusters (2 ≤ x ≤ 5), which are interpreted with supporting computational studies that include natural orbital ionization calculations on detachment transition cross sections. All spectra show evidence of D x h ring structures, which had been predicted to be the most stable structures in previous computational studies. However, a new lowest energy isomer is reported for the Zn 2 O 2 anion and neutral, a bent chain, which is readily reconciled with the most intense feature in the Zn 2 O 2 - PE spectrum. The computed PE angular distributions (PADs) associated with the lowest energy cluster structures identified computationally agree with the experimental results, with the exception of Zn 5 O 5 - , the experimental PAD of which suggests that strong vibronic coupling may be introducing anomalies. While the lowest lying electronic state of the Zn 2 O 2 chain structure is a triplet state, all neutral ring structures (including Zn 2 O 2 , the anion of which also populates the ion beam), favor a singlet electronic state. The computed singlet-triplet splitting of the D x h structures increases monotonically with x . Overall, we find that the properties of the ring structures evolve smoothly, rather than in the punctuated manner typically seen in the small cluster size regime.

The photoelectron (PE) spectra of C 6 F 5 X - (X = Cl, Br, I) and computational results on the anions and neutrals are presented and compared to previously reported results on C 6 F 6 - [McGee, C. J. J. Phys. Chem. A 2023, 127, 8556-8565.]. The spectra all exhibit broad, vibrationally unresolved detachment transitions, indicating that the equilibrium structures of the anions are significantly different from the neutrals. The PE spectrum of C 6 F 5 Cl - exhibits a parallel photoelectron angular distribution (PAD), similar to that of the previously reported C 6 F 6 - spectrum, while the PE spectra of C 6 F 5 Br - and C 6 F 5 I - have isotropic PADs, and also exhibit a prominent X - PE feature due to photodissociation of C 6 F 5 X - resulting in X - formation. Identification of the C 6 F 5 X - detachment transition origins, which is equivalent to the neutral electron affinity (EA), in all three cases is difficult, since the broadness of the detachment feature is accompanied by vanishingly small detachment cross section near the origin. Upper limits on the EAs were determined to be 1.70 eV for C 6 F 5 Cl, 2.10 eV for C 6 F 5 Br, and 2.00 eV for C 6 F 5 I, all significantly higher than the 0.76 eV upper limit determined for C 6 F 6 with the same experiment. The broad detachment transitions are consistent with computational results, which predict very large differences between the neutral and anionic C-X (X = Cl, Br, I) bond lengths. Based on differences between the MBIS atom charges in the anions and neutrals, the excess charge in the anion is on the unique C atom and X, in contrast to the nonplanar C 2 v structured C 6 F 6 - anion, for which the charge is delocalized over the molecule. In C 6 F 5 Cl - , the C-Cl bond is predicted to be bent out of the plane, while both C 6 F 5 Br - and C 6 F 5 I - are predicted to be planar on average. The impact of the interruption of the symmetry in the hexafluorobenzene neutral and anion on the molecular and electronic structure of C 6 F 5 X/C 6 F 5 X - is considered, as well as the possible dissociative state leading to X - (X = Br, I) formation, and the nature of the C-X bond.

Di-, tri-, and tetrafluorophenyl radicals each have three regioisomers, several of which can form multiple distinct radical structures. We present the photoelectron spectra of the di-, tri-, and tetrafluorophenide regioisomer anions generated from their associated fluorobenzene precursors. By comparing the spectra to the results of density functional theory calculations, we determine that in cases where more than one possible radical isomer is possible for a given regioisomer (radicals formed from 1,2-difluorobenzene, 1,3-difluorobenzene, 1,2,3-trifluorobenzene, and 1,2,4-trifluorobenzene) the most stable anion corresponds to a less stable neutral, suggesting that the reactive C-center on these fluorine-substituted phenyl groups can be controlled by charge state. Full analyses of the spectra and computational results yield further insights into the differences between the electronic and molecular structures of the fluorophenyl radicals and their associated anions.

Substituents have a profound effect on the electronic structure of the benzene molecule. In this paper, we present new photoelectron spectra of the C 5 HF 5 - molecular anion, to test predictions [ Int. J. Quant. Chem. 2017, 188, e25504] that pentafluorobenzene has a positive electron affinity, as hexafluorobenzene was already known to have. The PE spectrum of C 6 HF 5 - exhibits a broad and vibrationally unresolved band due to significant differences between the structure of the anion and the neutral. The vertical detachment energy (VDE) of C 5 HF 5 - is determined to be 1.33 ± 0.05 eV, and the lowest binding energy at which the signal is observed is 0.53 ± 0.05 eV, which, if taken as the electron affinity, is in good agreement with the computed value. In addition, we attempted to generate intact C 6 H 2 F 4 - molecular ions using the 1,2,3,4-tetrafluorobenzene, 1,2,3,5-tetrafluorobenzene, and 1,2,4,5-tetrafluorobenzene precursors, as tetrafluorobenzene was predicted to have a near-zero but marginally positive electron affinity. Using a photoemission anion source, we were not able to produce the intact tetrafluorobenzene anion. Density functional theory calculations support a more detailed discussion of the impact of fluorine substitution on the electronic structure of these species.

The electron affinities (EAs) of a series of ·C 6 H 5- x F x (1 ≤ x ≤ 4) fluorophenyl radicals are determined from the photoelectron spectra of their associated fluorophenide anions generated from C 6 H 6- x F x (1 ≤ x ≤ 4) fluorobenzene precursors. The spectra show a near-linear incremental increase in EA of 0.4 eV/ x . The spectra exhibit vibrationally unresolved and broad detachment transitions consistent with significant differences in the molecular structures of the anion and neutral radical species. The experimental EAs and broad spectra are consistent with density functional theory calculations on these species. While the anion detachment transitions all involve an electron in a non-bonding orbital, the differences in structure between the neutral and anion are in part due to repulsion between the lone pair on the C-center on which the excess charge is localized and neighboring F atoms. The C 6 H 5- x F x - (2 ≤ x ≤ 4) spectra show features at lower binding energy that appear to be due to constitutional isomers formed in the ion source.

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, Ce 2 OPt - , Ce 2 Pt - , and Ce 3 Pt - , 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 4 f 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 Ce 2 OPt - coordinates only with the Ce centers. The energy of the singly occupied Ce-local 4 f orbitals relative to the Pt-local orbitals changes with cluster composition. Discussion of the results includes potential implications for Ce-rich intermetallic materials.

Bimolecular reaction and collision complexes that drive atmospheric chemistry and contribute to the absorption of solar radiation are fleeting and therefore inherently challenging to study experimentally. Furthermore, primary anions in the troposphere are short lived because of a complicated web of reactions and complex formation they undergo, making details of their early fate elusive. In this perspective, the experimental approach of photodetaching mass-selected anion-molecule complexes or complex anions, which prepares neutrals in various vibronic states, is surveyed. Specifically, the application of anion photoelectron spectroscopy along with photoelectron-photofragment coincidence spectroscopy toward the study of collision complexes, complex anions in which a partial covalent bond is formed, and radical bimolecular reaction complexes, with relevance in tropospheric chemistry, will be highlighted., Competing Interests: The author declares no competing financial interest., (© 2022 The Author. Published by American Chemical Society.)

We present a study on the oxidation of isoprene under several different conditions that may model both atmospheric and combustion chemistry. Anions, formed by passing isoprene/oxidant gas mixtures through a pulsed discharge generating a range of species, are separated via mass spectrometry and characterized by anion photoelectron (PE) spectroscopy supported by computations. Specifically, a UV-irradiated isoprene/O 2 mixture, which additionally produces O 3 , and an isoprene/O 2 /H 2 mixture, which generates • OH when passed through the discharge, were sampled. The mass spectra of ions generated under both conditions show the production of intact molecular ions, ion-molecule complexes (e.g., O 2 - , O 4 - , and O 2 - ·isoprene), and singly deprotonated species (e.g., deprotonated isoprene, C 5 H 7 - ). In addition, both smaller and oxidized fragments are observed using both gas mixtures, though relative abundances differ. From the UV-irradiated isoprene/O 2 gas mixture, additional intact molecular products of reactions initiated by ozonolysis of isoprene, methylglyoxal, and dimethylglyoxal were observed. Fragmentation and oxidation of isoprene observed in both gas mixtures included species with m / z 39, 53, 67, 69, and 83 that we attribute to a series of alkyl- and alkenoxide-based anions. The coexistence of intact molecules and complexes with fragments and reaction products demonstrates the versatility of this ion source as a simple and efficient anion formation method for studying species that may be relevant in atmospheric and combustion chemistry.

Evidence of strong photoelectron-valence electron (PEVE) interactions has been observed in the anion photoelectron (PE) spectra of several lanthanide suboxide clusters, which are exceptionally complex from an electronic structure standpoint and are strongly correlated systems. The PE spectrum of Gd 2 O - , which should have relatively simple electronic structure because of its half-filled 4f subshell, exhibits numerous electronic transitions. The electron affinity determined from the spectrum is 0.26 eV. The intensities of transitions to excited states increase relative to the lower-energy states with lower photon energy, which is consistent with shakeup transitions driven by time-dependent electron-neutral interactions. A group of intense spectral features that lie between electron binding energies of 0.7 and 2.3 eV are assigned to transitions involving detachment of an electron from outer-valence σ u and σ g orbitals that have large Gd 6s contributions. The spectra show parallel transition manifolds in general, which is consistent with detachment from these orbitals. However, several distinct perpendicular transitions are observed adjacent to several of the vertical transitions. A possible explanation invoking interaction between the ejected electron and the high-spin neutral is proposed. Specifically, the angular momentum of electrons ejected from σ u or σ g orbitals, which is l = 1, can switch to l = 0, 2 with an associated change in the M s of the remnant neutral, which is spin-orbit coupling between a free electron and the spin of a neutral.

Complexes of anion-neutral pairs are prevalent in chemical and physical processes in the interstellar medium, the atmosphere, and biological systems, among others. However, bimolecular anionic species that cannot be described as simple ion-molecule complexes due to their competitive electron affinities have received less attention. In this study, the [O 2 - M ] - ( M = glyoxal, methylglyoxal, or biacetyl) anion photoelectron spectra obtained with several different photon energies are reported and interpreted in the context of ab initio calculations. The spectra do not resemble the photoelectron spectra of M - or O 2 - "solvated" by a neutral partner. Rather, all spectra are dominated by near-threshold autodetachment from what are likely transient dipole bound states of the cis conformers of the complex anions. Very low Franck-Condon overlap between the neutral M ·O 2 van der Waals clusters and the partial covalently bound complex anions results in low-intensity, broad direct detachment observed in the spectra. The [O 2 -glyoxal] - spectra measured with 2.88 and 3.495 eV photon energies additionally exhibit features at ∼0.5 eV electron kinetic energy, which is more difficult to explain, though there are numerous quasibound states of the anion that may be involved. Overall, these features point to the inadequacy of describing the complex anions as simple ion-molecule complexes.

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 x O 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.

Small clusters have captured the imaginations of experimentalists and theorists alike for decades. In addition to providing insight into the evolution of properties between the atomic or molecular limits and the bulk, small clusters have revealed a myriad of fascinating properties that make them interesting in their own right. This perspective reviews how the application of anion photoelectron (PE) spectroscopy, typically coupled with supporting calculations, is particularly well-suited to probing the molecular and electronic structure of small clusters. Clusters provide a powerful platform for the study of the properties of local phenomena (e.g., dopants or defect sites in heterogeneous catalysts), the evolution of the band structure and the transition from semiconductor to metallic behavior in metal clusters, control of electronic structures of clusters through electron donating or withdrawing ligands, and the control of magnetic properties by interactions between the photoelectron and remnant neutral states, among other important topics of fundamental interest. This perspective revisits historical, groundbreaking anion PE spectroscopic finding and details more recent advances and insight gleaned from the PE spectra of small covalently or ionically bound clusters. The properties of the broad range of systems studied are uniquely small-cluster like in that incremental size differences are associated with striking changes in stability, electronic structures, and symmetry, but they can also be readily related to larger or bulk species in a broader range of materials and applications.

Photoelectron spectra of Gd 2 O 2 - obtained with photon energies ranging from 2.033 to 3.495 eV exhibit numerous close-lying neutral states with photon-energy-dependent relative intensities. Transitions to these states, which fall within the electron binding energy window of 0.9 and 1.6 eV, are attributed to one- or two-electron transitions to the ground and low-lying excited neutral states. An additional, similar manifold of electronic states is observed in an electron binding energy window of 2.1-2.8 eV, which cannot be assigned to any simple one-electron transitions. This study expands on previous work on the Sm 2 O - triatomic, which has a more complex electronic structure because of the 4f 6 subshell occupancy of each Sm center. Because of the simpler electronic structure from the half-filled 4f 7 subshell occupancy in Gd 2 O 2 and Gd 2 O 2 - , the numerous close-lying transitions observed in the spectra are better resolved, allowing a more detailed view of the changes in relative intensities of individual transitions with photon energy. With supporting calculations on numerous possible close-lying electronic states, we suggest a potential description of the strong photoelectron-valence electron interactions that may result in the photon-energy-dependent changes in the observed spectra.

Metal oxide cluster models of catalyst materials offer a powerful platform for probing the molecular-scale features and interactions that govern catalysis. This perspective gives an overview of studies implementing the combination of anion photoelectron (PE) spectroscopy and density functional theory calculations toward exploring cluster models of metal oxides and metal-oxide supported Pt that catalytically drive the hydrogen evolution reaction (HER) or the water-gas shift reaction. The utility in the combination of these experimental and computational techniques lies in our ability to unambiguously determine electronic and molecular structures, which can then connect to results of reactivity studies. In particular, we focus on the activity of oxygen vacancies modeled by suboxide clusters, the critical mechanistic step of forming proximal metal hydride and hydroxide groups as a prerequisite for H2 production, and the structural features that lead to trapped dihydroxide groups. The pronounced asymmetric oxidation found in heterometallic group 6 oxides and near-neighbor group 5/group 6 results in higher activity toward water, while group 7/group 6 oxides form very specific stoichiometries that suggest facile regeneration. Studies on the trans-periodic combination of cerium oxide and platinum as a model for ceria supported Pt atoms and nanoparticles reveal striking negative charge accumulation by Pt, which, combined with the ionic conductivity of ceria, suggests a mechanism for the exceptionally high activity of this system towards the water-gas shift reaction.

The broadband photoelectron source realized by detaching O 2 - ·X (X = neutral unsaturated molecule) complexes offers a unique opportunity to probe temporary anion states of the unsaturated species. Detachment of the ion molecule complex typically accesses a dissociative portion of the neutral potential, creating a continuum electron source that can undergo scattering with X. We present the application of this new approach to electron-neutral scattering toward a study of the series of fluorinated benzenes via photoelectron spectroscopy of O 2 - ·C 6 H 6-x F x (x = 0-6) measured with several photon energies. We compare these spectra to the reference O 2 - ·hexane spectrum and observe evidence of temporary anion states of C 6 H 6-x F x for species with x = 0-5 in the form of enhanced signal intensity at electron kinetic energies coinciding with the energies of the temporary anions. Furthermore, we observe autodetachment features in the x = 3, 5 spectra. Results of calculations on the isolated symmetric isomer of C 6 H 3 F 3 suggest that the molecule cannot support a weakly-bound non-valence state that could be associated with the observed autodetachment. However, C 6 HF 5 - is predicted to support a valence bound state, which, if produced by charge transfer from O 2 - with sufficient vibrational energy, may undergo autodetachment. Finally, the [O 2 ·C 6 F 6 ] - spectrum is unique insofar as the spectrum is substantially higher in binding energy and qualitatively different from the x = 0-5 spectra. This result suggests much stronger interactions and charge delocalization between O 2 - and C 6 F 6 .

The anion photoelectron imaging spectra of an ion with m / z 85, generated under ion source conditions that optimize • OH production in a coexpansion with isoprene, are presented and analyzed with supporting calculations. A spectroscopic feature observed at a vertical electron detachment energy of 2.45 eV, which dominates the photoelectron spectrum measured at 3.495 eV photon energy, is consistent with the OH - ·isoprene ion-molecule complex, while additional signal observed at lower electron binding energy can be attributed to other constitutional isomers. However, spectra measured over a 2.2-2.6 eV photon energy range, i.e., from near threshold of the predominant OH - ·isoprene detachment feature through the vertical detachment energy, exhibit sharp features with common electron kinetic energies, suggesting autodetachment from a temporary anion prepared by photoexcitation. The photon energy independence of the electron kinetic energy of these features along with the low dipole moment predicted for the neutral • OH·isoprene van der Waals complex, suggest a complex photon-driven process. We present calculations supporting a hypothesis that near-threshold production of the • OH···isoprene reactive complex results in hydrogen abstraction of the isoprene molecule. The newly formed activated complex anion supports a dipole bound state that temporarily traps the near zero-kinetic energy electron and then autodetaches, encoding the low-frequency modes of the dehydrogenated neutral isoprene radical in the electron kinetic energies.

Mixed-metal oxides have proven to be effective catalysts for the hydrogen evolution reaction, often outperforming either of the binary metal oxides. The reactivity of Mn x MoO y - (x = 1, 2; y = 3, 4) clusters toward H 2 O was investigated via time-of-flight mass spectrometry with clear evidence of cluster oxidation and corresponding H 2 production, specifically for Mn x MoO 3 - (x = 1, 2) clusters. Unlike previously studied Mo x O y - clusters, which assumed a broad distribution of stoichiometries (typically x ≤ y ≤ 3x), both MnMoO y - and Mn 2 MoO y - preferentially formed y = 3 and 4 compositions in significant quantities under our source conditions. The electronic and molecular structures of the Mn x MoO y (x = 1, 2; y = 3, 4) anion and neutral clusters were probed with anion photoelectron spectroscopy and analyzed with supporting density functional theory calculations. Our studies suggest that both metal centers are involved in initial cluster-water complex formation, while Mo is the center that undergoes oxidation; hence, reactivity terminates when Mo is saturated in its highest oxidation state of +6. Across these four clusters, Mn remains relatively reduced and is stable in a high-spin electronic configuration. The preferential reactivity of water molecules toward the Mo center rather than Mn is rationalized by the much lower relative oxophilicity of Mn.

Lanthanide ( Ln ) oxide clusters and molecular systems provide a bottom-up look at the electronic structures of the bulk materials because of close parallels in the patterns of Ln 4f N subshell occupancy between the molecular and bulk Ln 2 O 3 size limits. At the same time, these clusters and molecules offer a challenge to the theory community to find appropriate and robust treatments for the 4f N patterns across the Ln series. Anion photoelectron (PE) spectroscopy provides a powerful experimental tool for studying these systems, mapping the energies of the ground and low-lying excited states of the neutral relative to the initial anion state, providing spectroscopic patterns that reflect the Ln 4f N occupancy. In this Account, we review our anion PE spectroscopic and computational studies on a range of small lanthanide molecules and cluster species. The PE spectra of Ln O - ( Ln = Ce, Pr, Sm, Eu) diatomic molecules show spectroscopic signatures associated with detachment of an electron from what can be described as a diffuse Ln 6s-like orbital. While the spectra of all four diatomics share this common transition, the fine structure in the transition becomes more complex with increasing 4f occupancy. This effect reflects increased coupling between the electrons occupying the corelike 4f and diffuse 6s orbitals with increasing N . Understanding the PE spectra of these diatomics sets the stage for interpreting the spectra of polyatomic molecular and cluster species. In general, the results confirm that the partial 4f N subshell occupancy is largely preserved between molecular and bulk oxides and borides. However, they also suggest that surfaces and edges of bulk materials may support a low-energy, diffuse Ln 6s band, in contrast to bulk interiors, in which the 6s band is destabilized relative to the 5d band. We also identify cases in which the molecular Ln centers have 4f N +1 occupancy rather than bulklike 4f N , which results in weaker Ln -O bonding. Specifically, Sm centers in mixed Ce-Sm oxides or in Sm x O y - ( y ≤ x ) clusters have this higher 4f N +1 occupancy. The PE spectra of these particular species exhibit a striking increase in the relative intensities of excited-state transitions with decreasing photon energy (resulting in lower photoelectron kinetic energy). This is opposite of what is expected on the basis of the threshold laws that govern photodetachment. We relate this phenomenon to strong electron-neutral interactions unique to these complex electronic structures. The time scale of the interaction, which shakes up the electronic configuration of the neutral, increases with decreasing electron momentum. From a computational standpoint, we point out that special care must be taken when considering Ln cluster and molecular systems toward the center of the Ln series (e.g., Sm, Eu), where treatment of electrons explicitly or using an effective core potential can yield conflicting results on competing subshell occupancies. However, despite the complex electronic structures associated with partially filled 4f N subshells, we demonstrate that inexpensive and tractable calculations yield useful qualitative insight into the general electronic structural features.

A combined experimental and computational study of H 2 reactions with small 98 Mo x S y - clusters ranging from subsulfide ( x ∼ y ) to hypersulfide ( y > 2 x ) is presented. Results suggest that the subsulfides react with H 2 primarily by insertion of a more reduced Mo center into the H-H bond, forming a dihydride product. We find that this reaction occurs up to Mo oxidation states of +4. For the subsulfides containing a second metal in a sufficiently low oxidation state, a second insertion of H 2 occurs, leading to a tetrahydride product. The reaction mechanisms of the sulfides are found to be very similar, albeit slightly higher energetically to those of the analogous oxosulfides that are also observed at low abundances in the experiments. In addition, the experimental results show an overall reduction of hypersulfides in the presence of H 2 , suggesting loss of H 2 S neutral molecules.

The electronic and molecular structure of the CeB 6 molecular unit has been probed by anion PE spectroscopy and DFT calculations to gain insight into structural and electronic relaxation on edge and corner sites of this ionic material. While boron in bulk lanthanide hexaboride materials assumes octahedral B 6 3- units, the monomer assumes a less compact structure to delocalize the charge. Two competitive molecular structures were identified for the anion and neutral species, which include a boat-like structure and a planar or near-planar teardrop structure. Ce adopts different orbital occupancies in the two isomers; the boat-like structure has a 4f superconfiguration while the teardrop favors a 4f 6s occupancy. The B 6 ligand in these structures carries a charge of -4 and -3, respectively. The teardrop structure, which was calculated to be isoenergetic with the boat structure, was most consistent with the experimental spectrum. B 6 -local orbitals crowd the energy window between the Ce 4f and 6s (HOMO) orbitals. A low-lying transition from the B-based orbitals is observed slightly less than 1 eV above the ground state. The results suggest that edge and corner conductivity involves stabilized, highly diffuse 6s orbitals or bands rather than the bulk-favored 5d band. High-spin and open-shell low-spin states were calculated to be very close in energy for both the anion and neutral, a characteristic that reflects how decoupled the 4f electron is from the B 6 2p- and Ce 6s-based molecular orbitals.

Photoelectron imaging spectra of three alkenoxide radical anions (3-buten-1-oxide, 3-buten-2-oxide, and 2-propenoxide) are presented and analyzed with supporting results of density functional theory calculations. In all spectra, intense detachment features are observed at approximately 2 eV electron binding energy, which is similar to the electron affinities of saturated neutral alkoxy radicals [Ramond et al., J. Chem. Phys. 112, 1158 (2000)]. Photoelectron angular distributions suggest the presence of several overlapping transitions which are assigned to the X̃ and à states of multiple energetically competitive conformers. The term energy of the à state of the 2-propenoxy radical, 0.17 eV, is higher than that of 3-buten-2-oxy (0.13 eV) and 3-buten-1-oxy (0.05 eV) radicals. Comparing the butenoxy radicals, we infer that stronger interactions between the non-bonding O 2p orbitals and the π bond increase the splitting between the ground and the first excited state in the 3-buten-2-oxy radical relative to the 3-buten-1-oxy radical.

The photoelectron spectra of Sm 2 O - obtained over a range of photon energies exhibit anomalous changes in relative excited-state band intensities. Specifically, the excited-state transition intensities increase relative to the transition to the neutral ground state with decreasing photon energy, the opposite of what is expected from threshold effects. This phenomenon was previously observed in studies on several Sm-rich homo- and heterolanthanide oxides collected with two different harmonic outputs of a Nd:YAG (2.330 and 3.495 eV) [ J. Chem. Phys. 2017, 146, 194310]. We relate these anomalous intensities to populations of ground and excited anionic and neutrals states through the inspection of time-dependent perturbation theory within the adiabatic and sudden limits and for the first time show that transition intensities in photoelectron spectroscopy have a deep significance in gauging participation from excited states. We believe our results will have significance in the study of other electron-rich systems that have especially high density of accessible spin states.

Boronyl (B≡O) ligands have garnered much attention as isoelectronic and isolobal analogues of CO and CN - , yet successful efforts in synthesizing metal boronyl complexes remain scarce. Anion photoelectron (PE) spectroscopy and density functional theory calculations were employed to investigate two small CeO 2 B x - ( x = 2, 3) complexes generated from laser ablation of a mixed Ce/B pressed powder target. The spectra reveal markedly different bonding upon incorporation of an additional B atom. Most interestingly, CeO 2 B 2 - was found to have a Ce(I) center coordinated to two monoanionic boronyl ligands in a bent geometry. This result was unexpected as previous studies suggest electron-rich metals are most suitable for stabilizing such ligands; furthermore, it is one of the first examples of an experimental metal-polyboronyl complex. Introducing another boron atom, however, favors a much different geometry in which Ce(II) coordinates an O 2 B 3 3- unit through both the O and B atoms, which was evident in the markedly different PE spectra.

A mass spectrometric analysis of the anionic and cationic species generated by laser ablation of graphite fluoride (GF) and graphite targets performed under identical sets of conditions is presented. Under conditions that produce typical C n - cluster mass distributions from ablation of graphite, the mass spectra of anionic species generated by ablation of GF are congested with overlapping stoichiometric patterns such as C n F 2 n and C n F (2 n-2) . Some of the molecular formulas for these clusters, such as C 6 F 6 , C 6 F 12 , and C 7 F 8 , are evocative of stable neutral fluorocarbons. Additionally, the GF-ablation generated mass peaks broaden at higher masses more than the graphite-based counterparts, which may indicate cluster fragmentation. Furthermore, a pattern of fragmentation via loss of CF 2 is observed and is reminiscent of previous studies which determined CF 2 loss during thermal decomposition. No species were seen in the mass spectra of the cationic species generated from laser ablation of GF, while, under the same conditions, typical C n + cluster distributions were observed.

Lanthanide (Ln) oxide clusters have complex electronic structures arising from the partially occupied Ln 4f subshell. New anion photoelectron (PE) spectra of Sm x Ce 3-x O y - (x = 0-3; y = 2-4) along with supporting results of density functional theory (DFT) calculations suggest interesting x and y-dependent Sm 4f subshell occupancy with implications for Sm-doped ionic conductivity of ceria, as well as the overall electronic structure of the heterometallic oxides. Specifically, the Sm centers in the heterometallic species have higher 4f subshell occupancy than the homonuclear Sm 3 O y - /Sm 3 O y clusters. The higher 4f subshell occupancy both weakens Sm-O bonds and destabilizes the 4f subshell relative to the predominantly O 2p bonding orbitals in the clusters. Parallels between the electronic structures of these small cluster systems with bulk oxides are explored. In addition, unusual changes in the excited state transition intensities, similar to those observed previously in the PE spectra of Sm 2 O - and Sm 2 O 2 - [J. O. Kafader et al., J. Chem. Phys. 146, 194310 (2017)], are also observed in the relative intensities of electronic transitions to excited neutral state bands in the PE spectra of Sm x Ce 3-x O y - (x = 1-3; y = 2, 4). The new spectra suggest that the effect is enhanced with lower oxidation states and with an increasing number of Sm atoms, implying that the prevalence of electrons in the diffuse Sm 6s-based molecular orbitals and a more populated 4f subshell both contribute to this phenomenon. Finally, this work identifies challenges associated with affordable DFT calculations in treating the complex electronic structures exhibited by these systems, including the need for a more explicit treatment of strong coupling between the neutral and PE.

We report a first-principles quantum chemical study of the mechanistic pathways for the hydrogen evolution reaction (HER) by the molecular electrocatalyst [(PY5Me 2 )Mo(S 2 )] 2+ . By determining the relative thermodynamics of many possible species, we propose a mechanism fully consistent with all experimental observations. We also show the presence of two close-lying spin surfaces with the high spin state having a slightly less favorable reactivity profile than the low spin state. The energy of the high spin state is related to the ease of reduction of the S 2 moiety and can be disrupted by interaction between S 2 and a Lewis base. From this understanding, an explanation for the nearly 400 000-fold increase in turnover frequency on Hg drop electrode compared to glassy carbon is demonstrated. A next-generation catalyst based on the same motif has been designed to stabilize the more reactive low spin state and improve catalytic function without the need of Hg. Calculations indicate that this new species would have greatly improved HER reactivity and operate at a similar overpotential as the original system.

Recent studies on reactions between Mo x O y - cluster anions and H 2 O/C 2 H 4 mixtures revealed a complex web of addition, hydrogen evolution, and chemifragmentation reactions, with chemifragments unambiguously connected to cluster reactions with C 2 H 4 . To gain insight into the molecular-scale interactions along the chemifragmentation pathways, the anion photoelectron (PE) spectra of MoC 2 H 2 - , MoC 4 H 4 - , MoOC 2 H 2 - , and MoO 2 C 2 H 2 - formed directly in Mo x O y - + C 2 H 4 (x > 1; y ≥ x) reactions, along with supporting CCSD(T) and density functional theory calculations, are presented and analyzed. The complexes have spectra that are all consistent with η 2 -acetylene complexes, though for all but MoC 4 H 4 - , the possibility that vinylidene complexes are also present cannot be definitively ruled out. Structures that are consistent with the PE spectrum of MoC 2 H 2 - differ from the lowest energy structure, suggesting that the fragment formation is under kinetic control. The PE spectrum of MoO 2 C 2 H 2 - additionally exhibits evidence that photodissociation to MoO 2 - + C 2 H 2 may be occurring. The results suggest that oxidative dehydrogenation of ethylene is initiated by Lewis acid/base interactions between the Mo centers in larger clusters and the π orbitals in ethylene.

To probe the mechanism of sacrificial reagents in catalytic processes, product distributions from Mo x O y - clusters reacting individually with C 2 H 4 and H 2 O are compared with those from reactions with a C 2 H 4 + H 2 O mixture, with the thermodynamics explored computationally. These molecules were chosen to model production of H 2 from H 2 O via H 2 O + C 2 H 4 → H 2 + CH 3 CHO, mediated by Mo x O y - clusters. H 2 O is known to sequentially oxidize Mo x O y - suboxide clusters while producing H 2 , resulting in less reactive clusters. Mo x O y - (y ∼ x) clusters undergo chemi-fragmentation reactions with C 2 H 4 , with Mo x O y C 2 H z - complexes forming as the cluster oxidation state increases. Unique species observed in reactions with the C 2 H 4 + H 2 O mixture, Mo 2 O 5 C 2 H 2 - and MoO 3 C 2 H 4 - , suggest that the internal energy gained in new Mo-O bond formation from oxidation by H 2 O opens additional reaction channels. C 2 H 3 O - is observed uniquely in reactions with the C 2 H 4 + H 2 O mixture, giving indirect evidence of CH 3 CHO formation via the cluster mediated H 2 O + C 2 H 4 → H 2 + CH 3 CHO reaction; C 2 H 3 O - can form via dissociative electron attachment to CH 3 CHO. Calculations support mechanisms that invoke participation of two ethylene molecules on thermodynamically favorable pathways leading to experimentally observed products.

The anion photoelectron (PE) spectra of a range of small mono-cerium molecular species, along with the Ce 2 O 4 - and Ce 3 O 6 - stoichiometric clusters, are presented and analyzed with the support of density functional theory calculations. A common attribute of all of the neutral species is that the Ce centers in both the molecules and clusters are in the +4 oxidation state. In bulk ceria (CeO 2 ), an unoccupied, narrow 4f band lies between the conventional valence (predominantly O 2p) and conduction (Ce 5d) bands. Within the CeO 2 - , CeO 3 H 2 - , and Ce(OH) 4 - series, the PE spectra and computational results suggest that the Ce 6s-based molecular orbital is the singly occupied HOMO in CeO 2 - but becomes destabilized as the Ce 4f-local orbital becomes stabilized with increasing coordination. CeO 3 - , a hyperoxide, undergoes photodissociation with 3.49 eV photon energy to form the stoichiometric neutral CeO 2 and O - . In the CeO 2 - , Ce 2 O 4 - ,and Ce 3 O 6 - stoichiometric cluster series, the 6s destabilization with 4f stabilization is associated with increasing cluster size, suggesting that a bulk-like band structure may be realized with fairly small cluster sizes. The destabilization of the 6s-based molecular orbitals can be rationalized by their diffuse size relative to Ce-O bond lengths in a crystal structure, suggesting that 6s bands in the bulk may be relegated to the surface.

Anion photoelectron imaging was used to measure the photodetachment spectra of molecular complexes formed between O 2 - and a range of atmospherically relevant polar molecules, including species with a carbonyl group (acetone, formaldehyde) and alcohols (ethanol, propenol, butenol). Experimental spectra are analyzed using a combination of Franck-Condon simulations and electronic structure calculations. Strong charge-dipole interactions and H-bonding stabilize the complex anions relative to the neutrals, resulting in a ca. 1 eV increase in electron binding energy relative to bare O 2 - , an effect more pronounced in complexes with H-bonding. In addition, broken degeneracy of the O 2 -local π g orbitals in the complexes results in the stabilization of the low-lying excited O 2 (a 1 Δ g )·[polar VOC] state relative to the ground O 2 (X 3 Σ g - )·[polar VOC] state when compared to bare O 2 . The spectra of the O 2 - ·[polar VOC] complexes exhibit less pronounced laser photoelectron angular distribution (PADs). The spectrum of O 2 - ·formaldehyde is unique in terms of both spectral profile and PAD. On the basis of these experimental results in addition to computational results, the complex anion cannot be described as a distinct O 2 - anion partnered with an innocent solvent molecule; the molecules are more strongly coupled through charge delocalization. Overall, the results underscore how the symmetry of the O 2 π g orbitals is broken by different polar partners, which may have implications for atmospheric photochemistry and models of solar radiation absorption that include collision-induced absorption.

The anion photoelectron (PE) spectra along with supporting results of density functional theory (DFT) calculations on SmO - , SmCeO y - , and Sm 2 O y - (y = 1, 2) are reported and compared to previous results on CeO - [M. Ray et al., J. Chem. Phys. 142, 064305 (2015)] and Ce 2 O y - (y = 1, 2) [J. O. Kafader et al., J. Chem. Phys. 145, 154306 (2016)]. Similar to the results on Ce x O y - clusters, the PE spectra of SmO - , SmCeO y - , and Sm 2 O y - (y = 1, 2) all exhibit electronic transitions to the neutral ground state at approximately 1 eV e - BE. The Sm centers in SmO and Sm 2 O 2 neutrals can be described with the 4f 5 6s superconfiguration, which is analogous to CeO and Ce 2 O 2 neutrals in which the Ce centers can be described with the 4f 6s superconfiguration (Z Ce = Z Sm - 4). The Sm center in CeSmO 2 , in contrast, has a 4f 6 occupancy, while the Ce center maintains the 4f 6s superconfiguration. The less oxidized Sm centers in both Sm 2 O and SmCeO have 4f 6 6s occupancies. The 4f 6 subshell occupancy results in relatively weak Sm-O bond strengths. If this extra 4f occupancy also occurs in bulk Sm-doped ceria, it may play a role in the enhanced O 2- ionic conductivity in Sm-doped ceria. Based on the results of DFT calculations, the heteronuclear Ce-Sm oxides have molecular orbitals that are distinctly localized Sm 4f, Sm 6s, Ce 4f, and Ce 6s orbitals. The relative intensity of two electronic bands in the PE spectrum of Sm 2 O - exhibits an unusual photon energy-dependence, and the PE spectrum of Sm 2 O 2 - exhibits a photon energy-dependent continuum signal between two electronic transitions. Several explanations, including the high magnetic moment of these suboxide species and the presence of low-lying quasi-bound anion states, are considered.

Attempts to reconcile simulated photoelectron spectra of MoVO 4 - clusters are complicated by the presence of very low energy barriers in the potential energy surfaces (PESs) of the lowest energy spin states and isomers. Transition state structures associated with the inversion of terminal oxygen ligands are found to lie below, or close to, the zero point energy of associated modes, which themselves are found to be of low frequency and thus likely to be significantly populated in the experimental characterization. Our simulations make use of Boltzmann averaging over low-energy coordinates and full mapping of the PES to obtain simulations in good agreement with experimental spectra. Furthermore, molecular orbital analysis of accessible final spin states reveals the existence of low energy two-electron transitions in which the final state is obtained from a finite excitation of an electron along with the main photodetachment event. Two-electron transitions are then used to justify the large difference in intensity between different bands present in the photoelectron spectrum. Owing to the general presence of terminal ligands in metal oxide clusters, this study identifies and proposes a solution to issues that are generally encountered when attempting to simulate transition metal cluster photoelectron spectroscopy.

The anion photoelectron (PE) spectra of Ce 2 O y - (y = 1, 2), Ce 3 O y - (y = 0-4), Ce 4 O y - (y = 0-2), and Ce 5 O y - (y = 1, 2) are reported and analyzed with supporting results from density functional theory calculations. The PE spectra all exhibit an intense electronic transition to the neutral ground state, all falling in the range of 0.7 to 1.1 eV electron binding energy, with polarization dependence consistent with detachment from diffuse Ce 6s-based molecular orbitals. There is no monotonic increase in electron affinity with increasing oxidation. A qualitative picture of how electronic structure evolves with an oxidation state emerges from comparison between the spectra and the computational results. The electronic structure of the smallest metallic cluster observed in this study, Ce 3 , is similar to the bulk structure in terms of atomic orbital occupancy (4f 5d 2 6s). Initial cerium cluster oxidation involves largely ionic bond formation via Ce 5d and O 2p orbital overlap (i.e., larger O 2p contribution), with Ce-O-Ce bridge bonding favored over Ce=O terminal bond formation. With subsequent oxidation, the Ce 5d-based molecular orbitals are depleted of electrons, with the highest occupied orbitals described as diffuse Ce 6s based molecular orbitals. In the y ≤ (x + 1) range of oxidation states, each Ce center has a singly occupied non-bonding 4f orbital. The PE spectrum of Ce 3 O 4 - is unique in that it exhibits a single nearly vertical transition. The highly symmetric structure predicted computationally is the same structure determined from Ce 3 O 4 + IR predissociation spectra [A. M. Burow et al., Phys. Chem. Chem. Phys. 13, 19393 (2011)], indicating that this structure is stable in -1, 0, and +1 charge states. Spectra of clusters with x ≥ 3 exhibit considerable continuum signal above the ground state transition; the intensity of the continuum signal decreases with increasing oxidation. This feature is likely the result of numerous quasi-bound anion states or two-electron transitions possible in molecules with abundant nearly degenerate partially occupied orbitals.

The anion photoelectron imaging spectra of O 2 - ·VOC and O 4 - ·VOC (VOC = hexane, isoprene, benzene, and benzene-d 6 ) complexes measured using 3.49 eV photon energy, along with the results of ab initio and density functional theory results are reported and analyzed. Photodetachment of these anionic complexes accesses neutrals that model collision complexes, offering a probe of the effects of symmetry-breaking collision events on the electronic structure of normally transparent neutral molecules. The energies of O 2 - ·VOC spectral features compared to the bare O 2 - indicate that photodetachment of the anion accesses a modestly repulsive region of the O 2 -VOC potential energy surface, with subtle VOC dependence on the relative energies of the O 2 (X 3 Σ g - )·VOC ground state and O 2 (a 1 Δ g )·VOC excited state. In contrast, a significantly higher intensity of the transition to the O 2 (a 1 Δ g )·VOC excited state relative to the O 2 (X 3 Σ g - )·VOC ground state is observed for VOC = benzene, with a less pronounced effect observed for VOC = isoprene. Similar spectral effects are observed in the O 4 - ·benzene and O 4 - ·isoprene PE spectra. Several explanations are considered, with involvement of a temporary anion state emerging as the most plausible.

The electronic structures of several small Ce-Pt oxide complexes were explored using a combination of anion photoelectron (PE) spectroscopy and density functional theory calculations. Pt and Pt2 both accept electron density from CeO diatomic molecules, in which the cerium atom is in a lower-than-bulk oxidation state (+2 versus bulk +4). Neutral [CeO]Pt and [CeO]Pt2 complexes are therefore ionic, with electronic structures described qualitatively as [CeO(+2)]Pt(-2) and [CeO(+)]Pt2 (-), respectively. The associated anions are described qualitatively as [CeO(+)]Pt(-2) and [CeO(+)]Pt2 (-2), respectively. In both neutrals and anions, the most stable molecular structures determined by calculations feature a distinct CeO moiety, with the positively charged Ce center pointing toward the electron rich Pt or Pt2 moiety. Spectral simulations based on calculated spectroscopic parameters are in fair agreement with the spectra, validating the computationally determined structures. In contrast, when Pt is coupled with CeO2, which has no Ce-localized electrons that can readily be donated to Pt, the anion is described as [CeO2]Pt(-). The molecular structure predicted computationally suggests that it is governed by charge-dipole interactions. The neutral [CeO2]Pt complex lacks charge-dipole stabilizing interactions, and is predicted to be structurally very different from the anion, featuring a single Pt-O-Ce bridge bond. The PE spectra of several of the complexes exhibit evidence of photodissociation with Pt(-) daughter ion formation. The electronic structures of these complexes are related to local interactions in Pt-ceria catalyst-support systems.

A systematic comparison of MxOy(-) + ROH (M = Mo vs W; R = Me vs Et) reaction rate coefficients and product distributions combined with results of calculations on weakly bound MxOy(-)·ROH complexes suggest that the overall reaction mechanism has three distinct steps, consistent with recently reported results on analogous MxOy(-) + H2O reactivity studies. MxOy(-) + ROH → MxOy+1(-) + RH oxidation reactions are observed for the least oxidized clusters, and MxOy(-) + ROH → MxOyROH(-) addition reactions are observed for clusters in intermediate oxidation states, as observed previously in MxOy(-) + H2O reactions. The first step is weakly bound complex formation, the rate of which is governed by the relative stability of the MxOy(-)·ROH charge-dipole complexes and the Lewis acid-base complexes. Calculations predict that MoxOy(-) clusters form more stable Lewis acid-base complexes than WxOy(-), and the stability of EtOH complexes is enhanced relative to MeOH. Consistent with this result, MoxOy(-) + ROH rate coefficients are higher than analogous WxOy(-) clusters. Rate coefficients range from 2.7 × 10(-13) cm(3) s(-1) for W3O8(-) + MeOH to 3.4 × 10(-11) cm(3) s(-1) for Mo2O4(-) + EtOH. Second, a covalently bound complex is formed, and anion photoelectron spectra of the several MxOyROH(-) addition products observed are consistent with hydroxyl-alkoxy structures that are formed readily from the Lewis acid-base complexes. Calculations indicate that addition products are trapped intermediates in the MxOy(-) + ROH → MxOy+1(-) + RH reaction, and the third step is rearrangement of the hydroxyl group to a metal hydride group to facilitate RH release. Trapped intermediates are more prevalent in MoxOy(-) reaction product distributions, indicating that the rate of this step is higher for WxOy+1RH(-) than for MoxOy+1RH(-). This result is consistent with previous computational studies on analogous MxOy(-) + H2O reactions predicting that barriers along the pathway in the rearrangement step are higher for MoxOy(-) reactions than for WxOy(-).

Results of a systematic comparison of the MoxOy (-) + H2O and WxOy (-) + H2O reaction rate coefficients are reported and compared to previous experimental and computational studies on these reactions. WxOy (-) clusters undergo more direct oxidation by water to yield WxOy+1 (-) + H2, while for MoxOy (-) clusters, production of MoxOyH2 (-) (trapped intermediates in the oxidation reaction) is comparatively more prevalent. However, MoxOy (-) clusters generally have higher rate coefficients than analogous WxOy (-) clusters if MoxOy+1H2 (-) formation is included. Results of calculations on the M2Oy (-) + H2O (M = Mo, W; y = 4, 5) reaction entrance channel are reported. They include charge-dipole complexes formed from long-range interactions, and the requisite conversion to a Lewis acid-base complex that leads to MxOy+1H2 (-) formation. The results predict that the Lewis acid-base complex is more strongly bound for MoxOy (-) clusters than for WxOy (-) clusters. The calculated free energies along this portion of the reaction path are also consistent with the modest anti-Arrhenius temperature dependence measured for most MoxOy (-) + H2O reactions, and the WxOy (-) + H2O reaction rate coefficients generally being constant over the temperature range sampled in this study. For clusters that exhibit evidence of both water addition and oxidation reactions, increasing the temperature increases the branching ratio toward oxidation for both species. A more direct reaction path to H2 production may therefore become accessible at modest temperatures for certain cluster stoichiometries and structures.

The photoelectron (PE) spectrum of PrO(-) exhibits a short 835 ± 20 cm(-1) vibrational progression of doublets (210 ± 30 cm(-1) splitting) assigned to transitions from the 4f(2) [(3)H4] σ6s (2) Ω = 4 anion ground state to the 4f(2) [(3)H4] σ6s Ω = 3.5 and 4.5 neutral states. This assignment is analogous to that of the recently reported PE spectrum of CeO(-), though the 82 cm(-1) splitting between the 4f [(2)F2.5] σ6s Ω = 2 and Ω = 3 CeO neutral states could not be resolved [Ray et al., J. Chem. Phys. 142, 064305 (2015)]. The origin of the transition to the Ω = 3.5 neutral ground state is 0.96 ± 0.01 eV, which is the adiabatic electron affinity of PrO. Density functional theory calculations on the anion and neutral molecules support the assignment. The appearance of multiple, irregularly spaced and low-intensity features observed ca. 1 eV above the ground state cannot be reconciled with low-lying electronic states of PrO that are accessible via one-electron detachment. However, neutral states correlated with the 4f(2) [(3)H4] 5d superconfiguration are predicted to be approximately 1 eV above the 4f(2) [(3)H4] σ6s Ω = 3.5 neutral ground state, leading to the assignment of these features to shake-up transitions to the excited neutral states. Based on tentative hot band transition assignments, the term energy of the previously unobserved 4f(2) [(3)H4] σ6s Ω = 2.5 neutral state is determined to be 1840 ± 110 cm(-1).

The anion photoelectron (PE) spectra of EuH(-) and the PE spectrum of overlapping EuOH(-) and EuO(-) anions are presented and analyzed with supporting results from density functional theory calculations on the various anions and neutrals. Results point to ionically bound, high-spin species. EuH and EuOH anions and neutrals exhibit analogous electronic structures: Transitions from (8)Σ(-) anion ground states arising from the 4f(7)σ(6s)(2) superconfiguration to the close-lying neutral (9)Σ(-) and (7)Σ(-) states arising from the 4f(7)σ(6s) superconfiguration are observed spaced by an energy interval similar to the free Eu(+) [4f(7)6s] (9)S - (7)S splitting. The electron affinities (EAs) of EuH and EuOH are determined to be 0.771 ± 0.009 eV and 0.700 ± 0.011 eV, respectively. Analysis of spectroscopic features attributed to EuO(-) photodetachment is complicated by the likely presence of two energetically competitive electronic states of EuO(-) populating the ion beam. However, based on the calculated relative energies of the close-lying anion states arising from the 4f(7)σ(6s) and 4f(6)σ(6s)(2) configurations and the relative energies of the one-electron accessible 4f(7) and 4f(6)σ(6s) neutral states based on ligand-field theory [M. Dulick, E. Murad, and R. F. Barrow, J. Chem. Phys. 85, 385 (1986)], the remaining features are consistent with the 4f(6)σ(6s)(2) (7)Σ(-) and 4f(7)σ(6s) (7)Σ(-) anion states lying very close in energy (the former was calculated to be 0.15 eV lower in energy than the latter), though the true anion ground state and neutral EA could not be established unambiguously. Calculations on the various EuO anion and neutral states suggest 4f-orbital overlap with 2p orbitals in species with 4f(6) occupancy.

The photoelectron images of O3 (-) and O3 (-) ⋅ (O2)n (n = 1-4) have been measured using 3.49 eV photon energy. The spectra exhibit several processes, including direct photodetachment and photodissociation with photodetachment of O(-) photofragments. Several spectra also exhibit autodetachment of vibrationally excited O2 (-) photofragments. Comparison of the bare O3 (-) photoelectron spectra to that of the complexes shows that the O3 (-) core is preserved upon clustering with several O2 molecules, though subtle changes in the Franck-Condon profile of the ground state photodetachment transition suggest some charge transfer from O3 (-) to the O2 molecules. The electron affinities of the complexes increase by less than 0.1 eV with each additional O2 molecule, which is comparable to the corresponding binding energy [K. Hiraoka, Chem. Phys. 125, 439-444 (1988)]. The relative intensity of the photofragment O(-) detachment signal to the O3 (-) ⋅ (O2)n direct detachment signal increases with cluster size. O2 (-) autodetachment signal is only observed in the O3 (-), O3 (-) ⋅ (O2)3, and O3 (-) ⋅ (O2)4 spectra, suggesting that the energy of the dissociative state also varies with the number of O2 molecules present in the cluster.

The photoelectron spectrum of CeO(-) exhibits what appears to be a single predominant electronic transition over an energy range in which numerous close-lying electronic states of CeO neutral are well known. The photoelectron spectrum of Ce(OH)2 (-), a molecule in which the Ce atom shares the same formal oxidation state as the Ce atom in CeO(-), also exhibits what appears to be a single transition. From the spectra, the adiabatic electron affinities of CeO and Ce(OH)2 are determined to be 0.936 ± 0.007 eV and 0.69 ± 0.03 eV, respectively. From the electron affinity of CeO, the CeO(-) bond dissociation energy was determined to be 7.7 eV, 0.5 eV lower than the neutral bond dissociation energy. The ground state orbital occupancies of both CeO(-) and Ce(OH)2 (-) are calculated to have 4f 6s(2) Ce(+) superconfigurations, with open-shell states having 4f5d6s superconfiguration predicted to be over 1 eV higher in energy. Low-intensity transitions observed at higher electron binding energies in the spectrum of CeO(-) are tentatively assigned to the (1)Σ(+) (Ω = 0) state of CeO with the Ce+26s2 superconfiguration.

Reactions between small cerium oxide cluster anions and deuterated water were monitored as a function of both water concentration and temperature in order to determine the temperature dependence of the rate constants. Sequential oxidation reactions of the Ce(x)O(y)⁻ (x = 2, 3) suboxide cluster anions were found to exhibit anti-Arrhenius behavior, with activation energies ranging from 0 to -18 kJ mol⁻¹. Direct oxidation of species up to y = x was observed, after which, -OD abstraction and D₂O addition reactions were observed. However, the stoichiometric Ce₂O₄⁻ and Ce₃O₆⁻ cluster anions also emerge in reactions between D₂O and the respective precursors, Ce₂O₃D⁻ and Ce₃O₅D₂⁻. Ce₂O₄⁻ and Ce₃O₆⁻ product intensities diminish relative to deuteroxide complex intensities with increasing temperature. The kinetics of these reactions are compared to the kinetics of the previously studied Mo(x)O(y)⁻ and W(x)O(y)⁻ reactions with water, and the possible implications for the reaction mechanisms are discussed.

To test recent computational studies on the mechanism of metal oxide cluster anion reactions with water [Ramabhadran, R. O.; et al. J. Phys. Chem. Lett. 2010, 1, 3066; Ramabhadran, R. O.; et al. J. Am. Chem. Soc. 2013, 135, 17039], the reactivity of molybdenum oxo–cluster anions, Mo(x)O(y)(–) (x = 1 – 4; y ≤ 3x) toward both methanol (MeOH) and ethanol (EtOH) has been studied using mass spectrometric analysis of products formed in a high-pressure, fast-flow reactor. The size-dependent product distributions are compared to previous Mo(x)O(y)(–) + H2O/D2O reactivity studies, with particular emphasis on the Mo2O(y)(–) and Mo3O(y)(–) series. In general, sequential oxidation, Mo(x)O(y)(–) + ROH → Mo(x)O(y+1)(–) + RH, and addition reactions, Mo(x)O(y)(–) + ROH → Mo(x)O(y+1)RH(–), largely corresponded with previously studied Mo(x)O(y)(–) + H2O/D2O reactions [Rothgeb, D. W., Mann, J. E., and Jarrold, C. C. J. Chem. Phys. 2010, 133, 054305], though with much lower rate constants than those determined for Mo(x)O(y)(–) + H2O/D2O reactions. This finding is consistent with the computational studies that suggested that −H mobility on the cluster–water complex was an important feature in the overall reactivity. There were several notable differences between cluster–ROH and cluster–water reactions associated with lower R–OH bond dissociation energies relative to the HO–H dissociation energy.

A computational investigation of the Mo2O(y)(-) + H2O (y = 4, 5) reactions as well as a photoelectron spectroscopic probe of the deuterated Mo2O6D2(-) product have been carried out to understand a puzzling question from a previous study: Why is the rate constant determined for the Mo2O5(-) + H2O/D2O reaction, the terminal reaction in the sequential oxidation of Mo2O(y)(-) by water, higher than the W2O5(-) + H2O/D2O reaction? This disparity was intriguing because W3O(y)(-) clusters were found to be more reactive toward water than their Mo3O(y)(-) analogs. A comparison of molecular structures reveals that the lowest energy structure of Mo2O5(-) provides a less hindered water addition site than the W2O5(-) ground state structure. Several modes of water addition to the most stable molecular and electronic structures of Mo2O4(-) and Mo2O5(-) were explored computationally. The various modes are discussed and compared with previous computational studies on W2O(y)(-) + H2O reactions. Calculated free energy reaction profiles show lower barriers for the initial Mo2O(y)(-) + H2O addition, consistent with the higher observed rate constant. The terminal Mo2O(y)(-) sequential oxidation product predicted computationally was verified by the anion photoelectron spectrum of Mo2O6D2(-). Based on the computational results, this anion is a trapped dihydroxide intermediate in the Mo2O5(-) + H2O/D2O → Mo2O6(-) + H2/D2 reaction.

The photoelectron spectra of WO3H(-) and WO2F(-) are presented and analyzed in the context of a series of previous similar measurements on MO(y)(-) (M = Mo, W; y = 0-3), MO4H(-) and AlMOy(-) (y ≤ 4) complexes. The electronic structures of the WO3H and WO2F anion and neutral complexes were investigated using the B3LYP hybrid density functional method. The spectra of WO3H(-), WO2F(-), and previously measured AlWO3(-) photoelectron spectra show that the corresponding neutrals, in which the transition metal centers are all in a +5 oxidation state, have comparable electron affinities. In addition, the electron affinities fit the general trend of monotonically increasing electron affinity with oxidation state, in spite of the WO3H(-), WO2F(-), and AlWO3(-) having closed shell ground states, suggesting that the oxidation state of the metal atom has more influence than shell closing on the electron affinity of these transition metal-oxo complexes. Results of DFT calculations suggest that the neutrals are pyramidal and the anions are planar. However, the barriers for inversion on the neutral surface are low, and attempts to generate simple Franck-Condon simulations based on simple normal coordinate displacement, ignoring the effects of inversion, are inadequate.

The electronic and molecular structures of Al2MoO(y) (y = 2-4) anion and neutral complexes were studied using anion photoelectron spectroscopy and density functional theory calculations. The spectra are broad, reflecting significant structural changes in the transition from anion to neutral, and the neutral electron affinities determined from the spectra are similar for all three species. The calculations suggest that the lowest energy isomers of the neutral clusters can be described as predominantly (Al(+))2[MoO(y)(-2)] ionic complexes, in which the Al(+) cations bond with O(2-) anions in a way that minimizes repulsion with the positively charged Mo center. The anion structures for all three complexes favor closer Mo-Al and Al-Al internuclear distances, with the extra negative charge distributed more evenly among all three metal centers. Energetically, the fully occupied 3s orbitals on the Al centers are lower than the Mo-local 4d-like orbitals and above the O-local 2p-like orbitals. In the case of Al2MoO2(-), there is direct Al-Al covalent bonding. The calculated spectroscopic parameters for these species are consistent with the observed spectra, though definitive assignments are not possible due to the broad, unresolved spectra observed and predicted.

Molecular hydrogen (H2) is an excellent alternative fuel. It can be produced from the abundantly present water on earth. Transition-metal oxides are widely used in the environmentally benign photocatalytic generation of H2 from water, thus actively driving scientific research on the mechanisms for this process. In this study, we investigate the chemical reactions of W3O5(-) and Mo3O5(-) clusters with water that shed light on a variety of key factors central to H2 generation. Our computational results explain why experimentally Mo3O5(-) forms a unique kinetic trap in its reaction while W3O5(-) undergoes a facile oxidation to form the lowest-energy isomer of W3O6(-) and liberates H2. Mechanistic insights on the reaction pathways that occur, as well as the reaction pathways that do not occur, are found to be of immense assistance to comprehend the hitherto poorly understood pivotal roles of (a) differing metal-oxygen and metal-hydrogen bond strengths, (b) the initial electrostatic complex formed, (c) the loss of entropy when these TMO clusters react with water, and (d) the geometric factors involved in the liberation of H2.