UV Photoelectron Spectra Research Articles

Electronic structure of peroxide-based explosives

The vertical, valence ionization energies for three peroxide based explosives: triacetone triperoxide (TATP), hexamethylene triperoxide diamine (HMTD) and diacetone diperoxide (DADP) have been calculated (as free molecules) using high-level ab initio coupled-cluster method: IP-EOM-CCSD. The calculated vertical, valence ionization energies were used to simulate UV photoelectron spectra. Our results, especially 1st ionization energies are discussed together compared with previous results on adiabatic energies. The variations of vertical ionization energies were rationalized by the analysis of the electronic structure of PBEs. The valence electronic structures depend on the number, type and position of peroxide groups. HOMO-LUMO gaps and the influence of electronic structures on explosive properties were also discussed.

Photoelectron spectra of infinitenes and topoisomers

The vertical, valence ionization energies of some [12]circulenes (C48H24) topoisomers have been calculated as free molecules in the gas phase using high-level ab initio coupled-cluster method: EOM-DLPNO-CCSD. Their valence electronic structures depend on molecular topology. We have used calculated vertical, valence ionization energies to simulate their UV photoelectron spectra (UPS). HOMO-LUMO bandgaps and the influence of molecular topology on the spectra were also studied. Our results indicate the presence and importance of stabilizing intramolecular π-π stacking interactions and molecular distortion (via strain energy) on electronic structures and UPS spectra. For example, the HOMO ionization energy increases along the sequence: infinitene<Möbius<Kekulene. The results presented may help in identification and analysis of photoelectron spectra of circulenes which can not be obtained in the gas phase, but rather as adsorbed molecules on metal surface.

Photoionization of pentalene and derivatives

The vertical, valence ionization energies for pentalene (C8H6) and some of its derivatives have been calculated as free molecules in the gas phase using high-level ab initio coupled-cluster method: EOM-DLPNO-CCSD. Their valence electronic structures depend on the number, type and position of substituents. We have used the calculated vertical, valence ionization energies to simulate UV photoelectron spectra. HOMO-LUMO gaps and the influence of substituents on ionization energies and electronic structure were also studied. We discuss the antiaromaticity and bis-annulation effects in pentalenes.

Design and Study of Nanoceria Modified by 5-Fluorouracil for Gel and Polymer Dermal Film Preparation.

In this work we studied nanoceria (CeO2NPs) and nanoceria modified by 5-fluorouracil (5FU) as potential APIs. Nanoceria were synthesized by precipitation in a matrix of hydroxyethyl cellulose or hydroxypropylmethyl cellulose, using cerium (III) nitrate and meglumine. Nanoceria properties were estimated by UV, FTIR and X-ray photoelectron spectra; scanning electron and atomic force microscopy; powder X-ray diffraction patterns and energy dispersive X-ray microanalysis. The cytotoxicity of nanoceria and polymer-protected nanoparticles was evaluated using the established cell line NCTC clone 929 (C3H/An mouse, subcutaneous connective tissue, clone of L. line). The morphology and metabolic activity of nanoparticles at 10 μg∙mL-1 of cells was not significant. In addition, the cytotoxic effects of nanoceria were assessed on two human colorectal cancer cell lines (HT29 and HCT116), murine melanoma B16 cells and normal human skin fibroblasts. An inhibitory effect was shown for HCT116 human colorectal cancer cells. The IC50 values for pure CeO2NPs and CeO2NPs-5FU were 219.0 ± 45.6 μg∙mL-1 and 89.2 ± 14.0 μg∙mL-1, respectively. On the other hand, the IC50 of 5FU in the combination of CeO2NPs-5FU was 2-fold higher than that of pure 5FU, amounting to 5.0 nmol∙mL-1. New compositions of nanoceria modified by 5-fluorouracil in a polymer matrix were designed as a dermal polymer film and gel. The permeability of the components was studied using a Franz cell.

Structural, electronic, and spectral properties of AlnCuk (n = 1–16; k= 0, ±1) alloy clusters

The structural, electronic, and spectral properties of AlnCuk (n = 1–16; k = 0, ±1) clusters have been examined via ab initio calculations combined with the CALYPSO method. It is found that the Cu atom in the ground-state AlnCuk cluster is situated on the surface for n = 1–11 and prefers a central site for n = 12–16 (except for in Al15Cu, Al12Cu+, and Al16Cu+). The stability, chemical activity, ionization potential, and electron affinity of the AlnCuk clusters are closely linked to the cluster size, the number of valence electrons, the point group, and the spin state. Simultaneously, we simulated UV spectra, photoelectron spectra, and vibrational and Raman spectra of the AlnCuk clusters. The global minimum structure of the AlnCu− cluster is recognized by our comparing the simulated and measured photoelectron spectra.

UPS, XPS, NEXAFS and Computational Investigation of Acrylamide Monomer

Acrylamide is a small conjugated organic compound widely used in industrial processes and agriculture, generally in the form of a polymer. It can also be formed from food and tobacco as a result of Maillard reaction from reducing sugars and asparagine during heat treatment. Due to its toxicity and possible carcinogenicity, there is a risk in its release into the environment or human intake. In order to provide molecular and energetic information, we use synchrotron radiation to record the UV and X-ray photoelectron and photoabsorption spectra of acrylamide. The data are rationalized with the support of density functional theory and ab initio calculations, providing precise assignment of the observed features.

6p valence relativistic effects in 5d photoemission spectrum of Pb atom and bonding properties of Pb-dimer using Dirac–Hartree–Fock formalism including many-body effects

There has been strong recent interest related to the large spin–orbit coupling in Pb monolayers on various properties of graphene and other 2D-materials. The underlying physical/chemical origin of the spin–orbit splitting has been discussed in terms of the valence 6p atomic level of the lead atom. Indeed, the photoelectron spectra of the Pb atom were the subject of investigations about 50 years ago in Dave Shirley’s laboratory at UC Berkeley. In a paper published in 1975, using He-I UV photoelectron spectroscopy, we reported an unexpected relative intensity ratio for the observed atomic Pb peaks (2P1/2 and 2P3/2) after removal of a 6p valence electron and attributed it to the large spin–orbit interaction in that level. In this contribution, we use the Dirac–Hartree–Fock formalism to reanalyze the complex spectral features reported five years later, for the 5d He-II UV photoelectron spectrum of atomic lead, to extract the 6p valence contribution, which turns out to be significant. Furthermore, we calculate the energy levels of the Pb-dimer at the experimental equilibrium geometry of the molecule to also find the significant contribution of the spin–orbit splitting of the atomic 6p levels in the composition of the valence molecular orbitals of the dimer. Such an approach can be extended to larger systems like monolayers containing lead or other heavy atoms, thus helping in designing 2D-materials with controlled and better targeted properties.

Photoionization of zwitterionic liquids

The electronic structures of three zwitterionic liquids (Zw-IL) have been studied using high-level ab initio method: equation-of-motion coupled-cluster (IP-EOM-CCSD) method. This method was found to match experimental UV photoelectron spectrum (UPS) of related N-methylimidazole better than Outer-valence Green’s Function (OVGF) method or Symmetry Adapted Cluster-Configuration Interaction (SAC-CI) method. Zw-IL gas phase photoelectron spectra have been simulated in the valence ionization regions based on the vertical ionization energies calculated. Valence ionizations have been attributed to specific molecular orbitals to specific functional groups The calculated electronic structures and valence ionization energies of Zw-ILs can be related to their polarizability which in turn influences the AC conductivity. AC conductivity is one of the principal properties of Zw-ILs which makes them very useful as electrolyte materials in lithium ion batteries or electric capacitors.

Photoelectron spectroscopy of arylboronic acids

We report the gas-phase UV photoelectron spectra (UPS) of some arylboronic acids for the first time. The electronic structures of 2-methyl-, 3-methyl- and 3-chloro-phenylboronic acids and 3-thienyl-boronic acid have been studied in the gas phase by HeI/HeII photoelectron spectroscopy and EOM-CCSD quantum chemical calculations. The comparison of the spectra of arylboronic acids with the spectra of toluene, chlorobenzene and thiophene shows remarkably small changes in the ionization energies of the highest occupied π-orbitals upon the attachment of the boronic acid group B(OH)2 to the aryl core. The appearance of additional spectral bands with ionization energies of 10.5–11 eV corresponds to ionization of CB σ-bonding orbitals.

Binding energy of solvated electrons and retrieval of true UV photoelectron spectra of liquids.

The electronic energy and dynamics of solvated electrons, the simplest yet elusive chemical species, is of interest in chemistry, physics, and biology. Here, we present the electron binding energy distributions of solvated electrons in liquid water, methanol, and ethanol accurately measured using extreme ultraviolet (EUV) photoelectron spectroscopy of liquids with a single-order high harmonic. The distributions are Gaussian in all cases. Using the EUV and UV photoelectron spectra of solvated electrons, we succeeded in retrieving sharp electron kinetic energy distributions from the spectra broadened and energy shifted by inelastic scattering in liquids, overcoming an obstacle in ultrafast UV photoelectron spectroscopy of liquids. The method is demonstrated for the benchmark systems of charge transfer to solvent reaction and ultrafast internal conversion of hydrated electron from the first excited state.

Conformational analysis, infrared/Raman spectral assignment, and electronic structural studies of 1,3-dimethyl-2-imidazolidinone using quantum chemical calculations

The conformational behavior of 1,3-Dimethyl-2-imidazolidinone (C5H10N2O; DMI) was investigated by quantum chemical calculations and vibrational (IR and Raman) spectral analysis. Ab initio (MP2) and DFT (B3LYP and ωB97XD) methods combined with the 6–311++G (d,p) and aug-cc-pVTZ basis sets were used. Aided by computational outcomes, the twist form (C2) was identified to be the most stable DMI conformer while the transition state planar assumption with C2v symmetry was higher than the twist conformer by 1.5–4.24 kcal/mol. In addition, the envelope form (Cs) was converged close to the planar form after allowing the structural parameters to relax with no constraints on the dihedral angles; therefore, it is not a minimum on the potential energy surface. The observed infrared and Raman spectral data are consistent with C2 molecular symmetry for DMI; therefore, confident vibrational spectral interpretations are reported herein supported by normal coordinate analysis and potential energy distributions (PEDs). The twist-to-planar energy barrier of DMI was predicted owing to the ring puckering using a two-variable scan of the potential energy surface at the B3LYP/6–311++G (d,p) level of theory. Finally, the OVGF and P3 calculations were performed for the twist conformer to predict the vertical ionization energies (IEs) and their corresponding outer-valence HOMOs. The reported gas-phase UV photoelectron spectrum was precisely interpreted. All results were analyzed herein and compared to similar molecules whenever appropriate.

Boron chelate complexes: X-ray and UV photoelectron spectra and electronic structure

Published data on the electronic structure of boron chelates are summarized for the first time. Ultraviolet photoelectron spectra of vapors, X-ray photoelectron spectra of molecular crystals, and results of modeling within the framework of the density functional theory are analyzed. Data on the effect of substituents on the electronic structure of complexes are systematized.

Band structure of CuMnAs probed by optical and photoemission spectroscopy

The tetragonal phase of CuMnAs progressively appears as one of the key materials for antiferromagnetic spintronics due to efficient current-induced spin-torques whose existence can be directly inferred from crystal symmetry. Theoretical understanding of spintronic phenomena in this material, however, relies on the detailed knowledge of electronic structure (band structure and corresponding wave functions) which has so far been tested only to a limited extent. We show that AC permittivity (obtained from ellipsometry) and UV photoelectron spectra agree with density functional calculations. Together with the x-ray diffraction and precession electron diffraction tomography, our analysis confirms recent theoretical claim [Phys. Rev. B 96, 094406 (2017)] that copper atoms occupy lattice positions in the basal plane of the tetragonal unit cell.

Electronic structure and optical properties of Eu(III) tris-β-diketonate adducts with 1,10-phenanthroline

Adducts of tris-β-diketonates of the rare earth metal Eu(III) with 1,10-phenanthroline are studied by photoelectron spectroscopy and quantum chemistry methods. The electronic structure, peculiarities of the nature of chemical bonds, and the geometric structure of the adducts are determined. The interpretation of UV photoelectron spectra of vapors and X-ray photoelectron spectra of solid is carried out with the chosen technique. DFT/TDDFT methods make it possible to study the 1,10-phenanthroline molecule influence on the adduct electronic structure and to analyze the electronic effects of substitution of methyl groups by trifluoromethyl groups in the ligands. At transition from the tris-β-diketonate complexes to the adducts, it is observed an increase of the absorption region and a decrease in the energy gap that contributes to the efficiency growth in electronic excitation energy transfer in the ligand-metal. Moreover, phenanthroline displaces water groups, that are luminescence quenchers, from the first coordination sphere, closes coordination in the adduct, and blocks their further attachment. Both factors contribute to an increase in the luminescence intensity.

Electronic structure of transition metal β-diketonates with an open d-shell

The electronic structure of β-diketonates of transition d5 and d7 metals - bis-acetylacetonates of manganese and cobalt (Mn(acac)2 and Co(acac)2) has been studied. In this paper, the effectiveness of the application of DFT methods for the description of the electronic structure of metal-open-shell complexes is shown. We assigned the bands of the UV photoelectron spectra of the high-spin bis-chelate complexes of Co (II) and Mn (II) to the spin-orbitals obtained from the calculation using the δ values determined earlier for nickel compounds. The possibility of application of the method of assigning spectra using DFA defects for similar objects is shown.

The Crystallographic and Electronic Phase Diagrams of Yttria-Stabilized Zirconia Model Electrolytes

Yttria-stabilized zirconia model electrolyte systems with four different compositions are analyzed regarding their crystallographic and electronic structure. By investigating the unit cell height, obtained from electron diffraction patterns, it is shown that a phase transformation between the tetragonal and cubic polymorphs takes place between 8 and 9.3 mol%. Furthermore, the direct band gaps are shown to exhibit the same behavior as the lattice parameter, featuring a discontinuity at the phase transition. By measuring the emitted Čerenkov radiation, an electronic transition that is smaller than the band gaps is found, suggesting that localized defect states are present within the band gap, which is in agreement with UV photoelectron spectra.

Photoelectron spectra of 2-thiouracil, 4-thiouracil, and 2,4-dithiouracil.

Ground- and excited-state UV photoelectron spectra of thiouracils (2-thiouracil, 4-thiouracil, and 2,4-dithiouracil) have been simulated using multireference configuration interaction calculations and Dyson norms as a measure for the photoionization intensity. Except for a constant shift, the calculated spectrum of 2-thiouracil agrees very well with experiment, while no experimental spectra are available for the two other compounds. For all three molecules, the photoelectron spectra show distinct bands due to ionization of the sulphur and oxygen lone pairs and the pyrimidine π system. The excited-state photoelectron spectra of 2-thiouracil show bands at much lower energies than in the ground state spectrum, allowing to monitor the excited-state population in time-resolved UV photoelectron spectroscopy experiments. However, the results also reveal that single-photon ionization probe schemes alone will not allow monitoring all photodynamic processes existing in 2-thiouracil. Especially, due to overlapping bands of singlet and triplet states the clear observation of intersystem crossing will be hampered.

The study of the electronic structure of some N-heterocyclic carbenes (NHCs) by variable energy photoelectron spectroscopy.

The photoionization of three N-heterocyclic carbenes (NHCs) has been studied in the valence and core regions using synchrotron radiation. We observed different variations in the relative band intensities with photon energy for the NHCs in the valence ionization region. This is due to the intra-ring interactions between the C=C bond, nitrogen and carbene lone pairs in the heterocyclic ring of NHCs. In the core ionization region we observed chemical shifts which are consistent with the relative electron affinities of atoms and intramolecular electron density shifts. The core electron binding energies calculated via the unrestricted ΔDFT (B3LYP and M06-2X) approach are in very good agreement with the experiment. The shake-up portion of the core photoionization spectra is adequately described by the time-dependent DFT calculations relying on the CAM-B3LYP functional.

Photoelectron spectra and electronic structure of some spiroborate complexes

The electronic structure of the valence and core levels of three spiroborate complexes – boron 1,2-dioxyphenylene β-diketonates – has been investigated by methods of UV and X-ray photoelectron spectroscopy and quantum chemical density functional theory. The ionization energy of π- and n-orbitals of the dioxyphenylene fragment and β-diketonate ligand were measured from UV photoelectron spectra. This made it possible to determine the effect of substitution of one or two methyl groups by the phenyl in diketone on the electronic structure of complexes. The binding energy of nonequivalent carbon and oxygen atoms were measured from X-ray photoelectron spectra. The results of calculations of the energy of the valence orbitals of complexes allowed us to refer bands observed in the spectra of the valence electrons to the 2s-type levels of carbon and oxygen.

The electronic states of pyridine-N-oxide studied by VUV photoabsorption andab initioconfiguration interaction computations

The first vacuum-ultraviolet absorption spectrum of pyridine-N-oxide has been obtained, and has led to the identification of nearly 30 Rydberg states. These states were identified by use of the vibrational envelope ("footprint") of the UV-photoelectron spectrum, and are based on the first to the third ionization energies (IE). The adiabatic IE order, central to the Rydberg state symmetry identification, is confirmed by multi-configuration SCF calculations as: 1(2)B1 < 1(2)B2 < 1(2)A2 < 2(2)B1. Several excited valence state equilibrium structures were determined by multi-configuration SCF and coupled cluster procedures. Multi-reference multi-root CI was used to calculate both Rydberg and valence state vertical excitation energies and oscillator strengths, which were correlated with the experimental measurements.