Anion Photoelectron Spectra Research Articles

Spin State Energetics of VGe n -/0 (n = 5-7) Clusters and New Assignments of the Anion Photoelectron Spectra.

The geometrical structures, electron leading configurations, and relative energies of the low-lying states of VGe n -/0 (n = 5-7) clusters have been investigated with density functional theory and CASSCF/CASPT2 method. The pure GGA BP86 functional gave almost the same relative energy order for the low-lying states as the CASPT2 method. At the BP86 and CASPT2 levels, the ground states of VGe n -/0 (n = 5-7) clusters were proposed to be the 1 A1 and 2 A1 of A-VGe5 -/0 , 3 A″ and 2 A″, 2 A' (2 E2 ) of A-VGe6 -/0 , and 1 A' and 2 A' of A-VGe7 -/0 isomers. The adiabatic and vertical detachment energies (ADEs and VDEs) of the detachments of one electron from several orbitals of the anionic ground states were reported at the CASPT2 level. The calculated ADEs and VDEs corresponded well with the experimental values as observed in the 266 nm anion photoelectron spectra. © 2018 Wiley Periodicals, Inc.

Photoelectron spectra of copper oxide cluster anions from first principles methods.

We present results and analyses for the photoelectron spectra of small copper oxide cluster anions (CuO-, Cu , Cu , and Cu2O-). The spectra are computed using various techniques, including density functional theory (DFT) with semi-local, global hybrid, and optimally tuned range-separated hybrid functionals, as well as many-body perturbation theory within the GW approximation based on various DFT starting points. The results are compared with each other and with the available experimental data. We conclude that as in many metal-organic systems, self-interaction errors are a major issue that is mitigated by hybrid functionals. However, these need to be balanced against a strong role of non-dynamical correlation-especially in smaller, more symmetric systems-where errors are alleviated by semi-local functionals. The relative importance of the two phenomena, including practical ways of balancing the two constraints, is discussed in detail.

Exotic electronic structures of SmxCe3-xOy (x = 0-3; y = 2-4) clusters and the effect of high neutral density of low-lying states on photodetachment transition intensities.

Lanthanide (Ln) oxide clusters have complex electronic structures arising from the partially occupied Ln 4f subshell. New anion photoelectron (PE) spectra of SmxCe3-xOy- (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 Sm3Oy-/Sm3Oy 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 Sm2O- and Sm2O2- [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 SmxCe3-xOy- (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.

Another Look at Photoelectron Spectra of the Anion Cr2O2-: Multireference Character and Energetic Degeneracy.

Geometrical and electronic structures of the chromium oxide dimer in both charge states Cr2O2-/0 were carefully and thoroughly studied using the restricted active space RASSCF/RASPT2 and density matrix renormalization group followed by second-order perturbation theory DMRG-CASPT2 methods in conjunction with large basis sets. The anionic and neutral clusters Cr2O2-/0 are characterized by a D2 h structure and high-spin ground states (10 and 9, respectively). RASSCF wave functions point out strong multireference characters of their electronic ground state. The two lowest-lying states 10Ag and 10B2g of the anion Cr2O2- are nearly degenerate, and both were proven to be populated and involved in the anion photoelectron spectra of Cr2O2-. The neutral cluster exhibits a 9B2g ground state. Within the active space of 20 electrons in 18 orbitals used, 30 electronic transitions starting from both lower-lying anionic states to corresponding neutral states were predicted. Transitions from the anionic ground state 10Ag cause most of visible bands in the anion photoelectron spectra of Cr2O2-, while the first band with low intensity is determined to arise from a transition starting from the nearly degenerate state 10B2g. Furthermore, multidimensional Franck-Condon factor simulations were carried out to further support our band assignments.

The Electronic Structures of CoGe n-/0 ( n = 1-3) Clusters from Multiconfigurational CASSCF/CASPT2 and RASSCF/RASPT2 Calculations.

Density functional theory and multiconfigurational CASPT2 and RASPT2 methods are employed to investigate the low-lying states of CoGe n-/0 ( n = 1-3) clusters. With the RASPT2 approach, the active space is extended to 14 orbitals for CoGe-/0, 17 orbitals for CoGe2-/0, and 20 orbitals for CoGe3-/0. These active spaces include the 3d, 4s, and 4d of Co and 4p of Ge. The 4d of Co is incorporated into these active spaces in order to account for the important double-shell effect of Co. The structural parameters, vibrational frequencies, and relative energies of the low-lying states of CoGe n-/0 ( n = 1-3) are reported. The ground states of CoGe n- ( n = 1-3) are computed to be 3Φ of linear CoGe-, 3B1 of cyclic CoGe2-, and 3B1 of cyclic CoGe3- isomer. The ground states of the neutral clusters are calculated to be 2Δ of linear CoGe, 4B1 of cyclic CoGe2, and 4A″ of tetrahedral CoGe3 isomer. The calculated adiabatic and vertical detachment energies of the anionic ground states are in agreement with the experimental values as observed in the 266 nm anion photoelectron spectra.

Spin-Orbit Splittings and Low-Lying Electronic States of AuSi and AuGe: Anion Photoelectron Spectroscopy and ab Initio Calculations.

We measured the photoelectron spectra of diatomic AuSi- and AuGe- and conducted calculations on the structures and electronic properties of AuSi-/0 and AuGe-/0. The calculations at the CASSCF/CASPT2 level confirmed that experimentally observed spectra features of AuSi- and AuGe- can be attributed to the transitions from the 3Σ- anionic ground state to the 2Π (2Π1/2 and 2Π3/2), 4Σ-, 32Σ+, and 42Σ+ electronic states of their neutral counterparts. The electron affinities (EAs) of AuSi and AuGe are determined by the experiments to be 1.54 ± 0.05 and 1.51 ± 0.05 eV, respectively. The spin-orbit splittings (2Π1/2-2Π3/2) of AuSi and AuGe measured in this work are in agreement with the literature values. The energy difference between the 4Σ- (A) and 2Π1/2 states of AuSi obtained in this work is in reasonable agreement with the literature value, while that of AuGe obtained in this work by anion photoelectron spectroscopy is slightly larger than the literature value by neutral emission spectroscopy. The term energies of the 32Σ+ (B) and 42Σ+ (C) states of AuSi and AuGe were also determined based on the photoelectron spectra. Because of the different bond lengths between the anionic and neutral states, the electronic state terms energies of AuSi and AuGe estimated from the anion photoelectron spectra might be slightly different from those obtained from the neutral emission spectra.

Anion photoelectron spectroscopy of deprotonated indole and indoline

Anion photoelectron spectra of deprotonated indole have been obtained utilizing several photon energies. The slow electron velocity-map imaging spectrum of indolide allows for the determination of the electron affinity (EA) of indolyl, 2.4315 ± 0.0017 eV. The equilibrium geometry of indolide was shown to minimally distort upon photodetachment with only ring distortion vibrational modes of A' symmetry becoming significantly excited. Photoelectron spectra of indolide accessing the electronic ground state of indolyl displayed a photon energy dependence due to electron autodetachment. Combining the EA of indolyl with the previous work studying the dissociation energy of H-indolyl allows for a new independent measure of ΔacidH0KoN-Hindole ≤ 348.7 kcal/mol, which improves the previous measurement of the gas phase acidity. The anion photoelectron spectrum of deprotonated indoline consisted of a featureless broad band extending from ∼1.3 eV to 1.7 eV electron binding energy. The congested nature of the spectrum is likely due to the presence of multiple isomers of deprotonated indoline, including ring-opened structures.

Insight into ethylene interactions with molybdenum suboxide cluster anions from photoelectron spectra of chemifragments.

Recent studies on reactions between MoxOy- cluster anions and H2O/C2H4 mixtures revealed a complex web of addition, hydrogen evolution, and chemifragmentation reactions, with chemifragments unambiguously connected to cluster reactions with C2H4. To gain insight into the molecular-scale interactions along the chemifragmentation pathways, the anion photoelectron (PE) spectra of MoC2H2-, MoC4H4-, MoOC2H2-, and MoO2C2H2- formed directly in MoxOy- + C2H4 (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 η2-acetylene complexes, though for all but MoC4H4-, the possibility that vinylidene complexes are also present cannot be definitively ruled out. Structures that are consistent with the PE spectrum of MoC2H2- differ from the lowest energy structure, suggesting that the fragment formation is under kinetic control. The PE spectrum of MoO2C2H2- additionally exhibits evidence that photodissociation to MoO2- + C2H2 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.

Anion Photoelectron Spectroscopy and High Level Ab Initio Calculations of the Halide–Nitric Oxide Dimer Complexes

Anion photoelectron spectra are presented for gas phase complexes formed between halide anions and nitric oxide, X−⋯NO where X− = Cl−, Br−, and I−. Electron binding energies are experimentally determined to be 3.82, 3.51, and 3.17 eV. Results from CCSD(T)/aug-cc-pVTZ calculations are presented for the anion species, whereby a single minimum of Cs symmetry is predicted. Binding energies (D0) of 15.3, 13.3, and 11.7 kJ mol−1 are predicted from complete basis set limit extrapolation, and are found to be in line with previous experimental studies.

Photoelectron spectroscopy and thermochemistry of o-, m-, and p-methylenephenoxide anions.

The anionic products following (H + H+) abstraction from o-, m-, and p-methylphenol (cresol) are investigated using flowing afterglow-selected ion flow tube (FA-SIFT) mass spectrometry and anion photoelectron spectroscopy (PES). The PES of the multiple anion isomers formed in this reaction are reported, including those for the most abundant isomers, o-, m- and p-methylenephenoxide distonic radical anions. The electron affinity (EA) of the ground triplet electronic state of neutral m-methylenephenoxyl diradical was measured to be 2.227 ± 0.008 eV. However, the ground singlet electronic states of o- and p-methylenephenoxyl were found to be significantly stabilized by their resonance forms as a substituted cyclohexadienone, resulting in measured EAs of 1.217 ± 0.012 and 1.096 ± 0.007 eV, respectively. Upon electron photodetachment, the resulting neutral molecules were shown to have Franck-Condon active ring distortion vibrational modes with measured frequencies of 570 ± 180 and 450 ± 80 cm-1 for the ortho and para isomers, respectively. Photodetachment to excited electronic states was also investigated for all isomers, where similar vibrational modes were found to be Franck-Condon active, and singlet-triplet splittings are reported. The thermochemistry of these molecules was investigated using FA-SIFT combined with the acid bracketing technique to yield values of 341.4 ± 4.3, 349.1 ± 3.0, and 341.4 ± 4.3 kcal mol-1 for the o-, m-, and p-methylenephenol radicals, respectively. Construction of a thermodynamic cycle allowed for an experimental determination of the bond dissociation energy of the O-H bond of m-methylenephenol radical to be 86 ± 4 kcal mol-1, while this bond is significantly weaker for the ortho and para isomers at 55 ± 5 and 52 ± 5 kcal mol-1, respectively. Additional EAs and vibrational frequencies are reported for several methylphenyloxyl diradical isomers, the negative ions of which are also formed by the reaction of cresol with O-.

Predicting the structural evolution of Gen−(3 ≤n≤ 20) clusters: an anion photoelectron spectroscopy simulation

Spectroscopic evidence for the structural evolution of Gen- (3 ≤ n ≤ 20) clusters is provided by means of simulated anion photoelectron spectra (PES). Both adiabatic detachment energies and HOMO-LUMO gaps of the selected low-lying candidates match well with those measured in experiment. This work demonstrates that PES is a highly sensitive technique for probing both electronic and geometric structures of atomic clusters, whose structural evolution is an elemental signature with characteristic features.

Encoding of vinylidene isomerization in its anion photoelectron spectrum.

Vinylidene-acetylene isomerization is the prototypical example of a 1,2-hydrogen shift, one of the most important classes of isomerization reactions in organic chemistry. This reaction was investigated with quantum state specificity by high-resolution photoelectron spectroscopy of the vinylidene anions H2CC- and D2CC- and quantum dynamics calculations. Peaks in the photoelectron spectra are considerably narrower than in previous work and reveal subtleties in the isomerization dynamics of neutral vinylidene, as well as vibronic coupling with an excited state of vinylidene. Comparison with theory permits assignment of most spectral features to eigenstates dominated by vinylidene character. However, excitation of the ν6 in-plane rocking mode in H2CC results in appreciable tunneling-facilitated mixing with highly vibrationally excited states of acetylene, leading to broadening and/or spectral fine structure that is largely suppressed for analogous vibrational levels of D2CC.

Ce in the +4 oxidation state: Anion photoelectron spectroscopy and photodissociation of small CexOyHz- molecules.

The anion photoelectron (PE) spectra of a range of small mono-cerium molecular species, along with the Ce2O4- and Ce3O6- 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 (CeO2), an unoccupied, narrow 4f band lies between the conventional valence (predominantly O 2p) and conduction (Ce 5d) bands. Within the CeO2-, CeO3H2-, 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 CeO2- but becomes destabilized as the Ce 4f-local orbital becomes stabilized with increasing coordination. CeO3-, a hyperoxide, undergoes photodissociation with 3.49 eV photon energy to form the stoichiometric neutral CeO2 and O-. In the CeO2-, Ce2O4- ,and Ce3O6- 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.

Insights into Geometric and Electronic Structures of VGe3-/0 Clusters from Anion Photoelectron Spectrum Assignment.

The global minima of both neutral and anionic clusters of VGe3-/0 were determined using different quantum chemical methods (DFT, RCCSD(T), CASSCF/CASPT2). On the basis of the ground states identified, most excited bands in the anion photoelectron spectrum of VGe3- were assigned. The tetrahedral isomers of both charged states are the most stable ones. A singlet state (Cs,1A') of the tetrahedral isomer has the globally lowest energy on the potential hypersurface of VGe3-. Two states 12A' and 12A″ of the neutral tetrahedral isomer are nearly degenerate and identified as the competing ground state of VGe3. From the anionic ground state, four of five bands in the anion photoelectron spectrum of VGe3- were determined to be the consequences of one-electron transitions starting from the anionic ground state 1A'. Both nearly degenerate neutral ground states are responsible for generation of the first band. Two different transitions from the anionic ground state 1A' to the first two nearly degenerate excited states (22A' and 22A″) of the neutral underlie the second lowest ionization band. Two higher levels of ionization recorded in the spectrum were assigned to the two higher excited states 42A' and 52A' of the neutral. Franck-Condon factor simulations of the first band were performed to obtain more insights into the experimental bands of the spectrum.

Photoelectron spectroscopy of the thiazate (NSO-) and thionitrite (SNO-) isomer anions.

Anion photoelectron spectra of the thiazate (NSO-) and thionitrite (SNO-) isomers are reported. The NSO- photoelectron spectrum showed several well-resolved vibronic transitions from the anion to the NSO radical neutral. The electron affinity of NSO was determined to be 3.113(1) eV. The fundamental vibrational frequencies of NSO were measured and unambiguously assigned to be 1202(6) cm-1 (ν1, asymmetric stretch), 1010(10) cm-1 (ν2, symmetric stretch), and 300(7) cm-1 (ν3, bend). From the presence of vibrational hot band transitions, the fundamental vibrational frequencies of the NSO- anion were also measured: 1280(30) cm-1 (ν1, asymmetric stretch), 990(20) cm-1 (ν2, symmetric stretch), and 480(10) cm-1 (ν3, bend). Combined with the previously measured ΔacidH298 Ko(HNSO), D0(H-NSO) was found to be 102(5) kcal/mol. Unlike the results from NSO-, the SNO- photoelectron spectrum was broad with little structure, indicative of a large geometry change between the anion and neutral radical. In addition to the spectrally congested spectrum, there was evidence of a competition between photodetachment from SNO- and SNO- photodissociation to form S- + NO. Quantum chemical calculations were used to aid in the interpretation of the experimental data and agree well with the observed photoelectron spectra, particularly for the NSO- isomer.

The electron shuffle: Cerium influences samarium 4f orbital occupancy in heteronuclear Ce-Sm oxide clusters.

The anion photoelectron (PE) spectra along with supporting results of density functional theory (DFT) calculations on SmO-, SmCeOy-, and Sm2Oy- (y = 1, 2) are reported and compared to previous results on CeO- [M. Ray et al., J. Chem. Phys. 142, 064305 (2015)] and Ce2Oy- (y = 1, 2) [J. O. Kafader et al., J. Chem. Phys. 145, 154306 (2016)]. Similar to the results on CexOy- clusters, the PE spectra of SmO-, SmCeOy-, and Sm2Oy- (y = 1, 2) all exhibit electronic transitions to the neutral ground state at approximately 1 eV e-BE. The Sm centers in SmO and Sm2O2 neutrals can be described with the 4f56s superconfiguration, which is analogous to CeO and Ce2O2 neutrals in which the Ce centers can be described with the 4f 6s superconfiguration (ZCe = ZSm - 4). The Sm center in CeSmO2, in contrast, has a 4f6 occupancy, while the Ce center maintains the 4f 6s superconfiguration. The less oxidized Sm centers in both Sm2O and SmCeO have 4f6 6s occupancies. The 4f6 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 O2- 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 Sm2O- exhibits an unusual photon energy-dependence, and the PE spectrum of Sm2O2- 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.

Theoretical study on the anion photoelectron spectra of Ln(COT)2− including the spin-orbit effects

The multiplet level splittings for both anion and neutral sandwich complexes Ln(COT)2 (Ln=Ce-Yb, COT=1,3,5,7-cyclooctatetraene) were calculated with spin-orbit interactions to analyze their anion photoelectron spectra. The theoretically simulated spectra obtained with these energies and the pole strengths are generally consistent with the experimental spectra for the X peak. The magnitudes of the energy splittings, relative peak intensities, and their Ln dependence are reproduced. In comparison to our previous calculations, the inclusion of spin-orbit interactions with the SO-MCQDPT2 method makes the simulated spectra more consistent with the results of the experiment.

Benchmarking the GW Approximation and Bethe-Salpeter Equation for Groups IB and IIB Atoms and Monoxides.

Energies from the GW approximation and the Bethe-Salpeter equation (BSE) are benchmarked against the excitation energies of transition-metal (Cu, Zn, Ag, and Cd) single atoms and monoxide anions. We demonstrate that best estimates of GW quasiparticle energies at the complete basis set limit should be obtained via extrapolation or closure relations, while numerically converged GW-BSE eigenvalues can be obtained on a finite basis set. Calculations using real-space wave functions and pseudopotentials are shown to give best-estimate GW energies that agree (up to the extrapolation error) with calculations using all-electron Gaussian basis sets. We benchmark the effects of a vertex approximation (ΓLDA) and the mean-field starting point in GW and the BSE, performing computations using a real-space, transition-space basis and scalar-relativistic pseudopotentials. While no variant of GW improves on perturbative G0W0 at predicting ionization energies, G0W0ΓLDA-BSE computations give excellent agreement with experimental absorption spectra as long as off-diagonal self-energy terms are included. We also present G0W0 quasiparticle energies for the CuO-, ZnO-, AgO-, and CdO- anions, in comparison to available anion photoelectron spectra.

Elucidating cerium + H2O reactivity through electronic structure: A combined PES and DFT study

The anion photoelectron (PE) spectra of CexOyHz− products formed in sequential reactions between cerium oxide and hydroxide reactions with water are presented and interpreted with supporting density functional theory calculations. The PE spectra of CexOyHz− (x=2, 3) complexes formed in the sequence of reactions initiated with CexOx−+H2O, exhibit a prominent photodetachment transition indicating that the highest occupied molecular orbital in these species can be described as combinations of Ce 6s atomic orbitals. The electrons in these diffuse orbitals reliably have low binding energies in the range of 0.6–1eV. The combination of poor orbital overlap and low binding energy is amenable to ionic CeOH bond formation in reactions with water, rather than CeO bond formation, which is observed in more reduced clusters that have occupied Ce 5d-based molecular orbitals. Spectral simulations based on computational results, which predict numerous close-lying spin states arising from the singly occupied 4f orbital on each Ce center, support specific structural assignments for the Ce2OyHz− cluster series. A range of Ce3OyHz− products also exhibit the characteristic Ce 6s-based MO photodetachment transition. Based on comparison with calculations that predict stable cerium hydride structures, we infer that the sequence of Ce3OyHz−+H2O reactions proceeds along a path of metastable structures.

Probing Structural, Electronic, and Magnetic Properties of Iron-Doped Semiconductor Clusters Fe2Gen–/0 (n = 3–12) via Joint Photoelectron Spectroscopy and Density Functional Study

We present a joint experimental and theoretical study on double iron atom doped germanium clusters, Fe2Gen–/0 (n = 3–12). The experimental photoelectron spectra of cluster anions are reasonably reproduced by theoretical simulations. The low-lying structures of the iron-doped semiconductor clusters are obtained by using an ab initio computation-based genetic-algorithm global optimization method. We find that the smaller-sized Fe2Gen– (n = 3–8) clusters adopt bipyramid-based geometries, while the larger ones (n ≥ 9) adopt polyhedral cagelike structures with one interior Fe atom. Interestingly, starting from n = 8, the most stable anionic clusters Fe2Gen– exhibit structures that are different from that of their neutral counterparts Fe2Gen. Robust ferromagnetic interaction is found between the two doped iron atoms in the neutral clusters Fe2Gen, while the total spin moment always remains at 4 μB for all the neutral double iron atom doped germanium clusters up to n = 12. This behavior is in stark contrast to t...