Showing papers on "Valence (chemistry) published in 1999"
TL;DR: In this article, the preparation, characterization, and X-ray structure for the single-molecule magnet (PPh4)[Mn12O12(O2CPh)16(H2O)4]·8(CH2Cl2) (2) was reported.
Abstract: The preparation, characterization, and X-ray structure are reported for the single-molecule magnet (PPh4)[Mn12O12(O2CPh)16(H2O)4]·8(CH2Cl2) (2). Complex 2 crystallizes in the triclinic space group P1, which at 213 K has a = 17.2329(2), b = 17.8347(2), c = 26.8052(2) A, α = 90.515(2), β = 94.242(2), γ = 101.437(2)°, and Z = 2. The salt consists of PPh4+ cations and [Mn12O12(O2CPh)16(H2O)4]- anions. The (Mn12O12)15+ core of the anion is formed by an external ring of eight Mn atoms bridged by μ3−O2- ions to an internal tetrahedron of four Mn atoms. Because of disorder in both phenyl rings and solvate molecules, it was difficult to use bond valence sum values to determine definitively the oxidation state of each Mn atom. There is a Mn4O4 cubane unit in the internal part of the molecule and these Mn atoms are all MnIV ions. For the eight “external” Mn atoms the bond valence sum values did not define well their oxidation states. For these eight Mn atoms, it was not possible to determine whether a trapped-valen...
534 citations
TL;DR: In this article, a comparative study on the electronic spectra of a number of unsaturated organic molecules, using on the one hand density functional linear response theory and on the other multiconfigurational second-order perturbation theory, was performed, in order to establish the accuracy that the density functional based methods can give for excitation energies and energy surfaces for excited states.
Abstract: A comparative study has been performed on the electronic spectra of a number of unsaturated organic molecules, using on the one hand density functional linear response theory and on the other multiconfigurational second-order perturbation theory, in order to establish the accuracy that the density functional based methods can give for excitation energies and energy surfaces for excited states. The following molecules are included in the study: tetrazine; the five-membered ring systems cyclopentadiene, furan, pyrrole, and thiophene; acetone; and a dipeptide. The results show that DFT valence excited states have errors that vary between 0 and 1 eV, while Rydberg states are accurate to about 0.2eV in most cases. The use of an asymptotically corrected exchange-correlation potential was essential for the latter result. However, transitions which involve a considerable charge transfer have much larger errors. The results show in some cases a surprisingly strong interaction between valence and Rydberg excited st...
531 citations
TL;DR: In this paper, a spin-unrestricted time-dependent density functional theory (TDDFT) is applied to calculate vertical excitation energies for the first several excited states of four diatomic radicals and methyl, nitromethyl, benzyl, anilino, and phenoxyl radicals.
423 citations
TL;DR: In this paper, the authors report first-principles electronic-structure calculations that enable them to determine both the valency and the lattice size as a function of atomic number, and hence understand the valence transitions.
Abstract: The rare-earth metals have high magnetic moments and a diverse range of magnetic structures1. Their magnetic properties are determined by the occupancy of the strongly localized 4f electronic shells, while the outer s–d electrons determine the bonding and other electronic properties2. Most of the rare-earth atoms are divalent, but generally become trivalent in the metallic state. In some materials, the energy difference between these valence states is small and, by changing some external parameter (such as pressure), a transition from one to the other occurs. But the mechanism underlying this transition and the reason for the differing valence states are not well understood. Here we report first-principles electronic-structure calculations that enable us to determine both the valency and the lattice size as a function of atomic number, and hence understand the valence transitions. We find that there are two types of f electrons: localized core-like f electrons that determine the valency, and delocalized band-like f electrons that are formed through hybridization with the s–d bands and which participate in bonding. The latter are found only in the trivalent systems; if their number exceeds a certain threshold, it becomes energetically favourable for these electrons to localize, causing a transition to a divalent ground state.
258 citations
TL;DR: In this paper, photo-electron spectra of the MX2− superhalogen anions have been obtained for the first time, and the first vertical detachment energies (VDEs) were measured to be 5.92±0.04 (LiCl2−), 5.86± 0.06 (NaCl2+ ), 5.42±0, 5.88±0., 4.84± 0, 4.57 (LiI2+), and 4.50 eV (NaI2−) in excellent agreement with the experimental values
Abstract: Photoelectron spectra of the MX2− (M=Li, Na; X=Cl, Br, I) superhalogen anions have been obtained for the first time. The first vertical detachment energies (VDEs) were measured to be 5.92±0.04 (LiCl2−), 5.86±0.06 (NaCl2−), 5.42±0.03 (LiBr2−), 5.36±0.06 (NaBr2−), 4.88±0.03 (LiI2−), and 4.84±0.06 eV (NaI2−), which are all well above the 3.61 eV electron detachment energy of Cl−, the highest among atomic anions. Experimental photoelectron spectra have been assigned on the basis of ab initio outer valence Green function (OVGF) calculations. The corresponding theoretical first VDEs were found to be 5.90 (LiCl2−), 5.81 (NaCl2−), 5.48 (LiBr2−), 5.43 (NaBr2−), 4.57 (LiI2−), and 4.50 eV (NaI2−), in excellent agreement with the experimental values. Photodetachment from the top four valence molecular orbitals (2σg22σu21πu41πg4) of MX2− was observed. Analysis of the polestrength showed that all electron detachment channels in this study can be described as primarily one-electron processes.
254 citations
TL;DR: In this article, a multi-state empirical valence bond model was proposed to study the microscopic nature of the hydrated proton and its transport mechanism, fitted to ab-initio results.
Abstract: In order to study the microscopic nature of the hydrated proton and its transport mechanism, we have introduced a multi-state empirical valence bond model, fitted to ab-initio results [J. Phys. Chem. B 102, 4261 (1998) and references therein]. The model makes it possible to take into account an arbitrary number N of valence states for the system proton+water and the electronic ground-state is obtained by diagonalization of a N×N interaction matrix. The resulting force field was applied to the study, at low computational cost, of the structure and dynamics of an excess proton in liquid water. The quantum character of the proton is included by means of an effective parametrization of the model using a preliminary path-integral calculation. In the light of the simulations, the mechanism of proton transfer is interpreted as the translocation of a privileged H5O2+ structure along the hydrogen bond network, with at any time a special O–H+–O bond, rather than a series of H3O++H2O→H2O+H3O+ reactions. The transloc...
251 citations
TL;DR: A study of the structural and electronic properties of selected transition metals and their carbides and nitrides is presented in this article, focusing on assessing trends of possible importance for understanding their hardness.
Abstract: A study of the structural and electronic properties of selected transition metals and their carbides and nitrides is presented We focus on assessing trends of possible importance for understanding their hardness Lattice constants, bulk moduli (B[sub o]), and charge densities are calculated using the local density approximation with a pseudopotential plane wave approach An fcc lattice is employed for the transition metal elements in order to make comparisons and study trends relateable to their carbides and nitrides Our results show that both increasing the number of valence d electrons and the presence of f electrons in the core lead to larger (B[sub o]) Charge density plots and histograms enable us to explain the nature of the charge distribution in the interstitial region for the different compounds considered In addition, we include the heavier elements seaborgium, bohrium, and hasnium in order to test further trends Surprisingly, the calculated B[sub o] for Hs is comparable to that of diamond [copyright] [ital 1999] [ital The American Physical Society]
231 citations
TL;DR: In this article, the intercluster charge transfer complex of the triruthenium clusters is defined and two discrete and reversible single electron reductions are evident by cyclic voltammetry in the redox chemistry of 1−5.
Abstract: Intramolecular electron transfers within the mixed valence states of the ligand bridged hexaruthenium clusters Ru3(μ3-O)(μ-CH3CO2)6(CO)(L)(μ-L‘)Ru3(μ3-O)(μ-CH3CO2)6(CO)(L) (L‘ = 1,4-pyrazine; L = 4-dimethylaminopyridine (1), pyridine (2), 4-cyanopyridine (3), or L‘ = 4,4‘-bipyridine; L = 4-dimethylaminopyridine (4), pyridine (5), 4-cyanopyridine (6)) were examined. Two discrete and reversible single electron reductions are evident by cyclic voltammetry in the redox chemistry of 1−5, and the intercluster charge-transfer complexes are well-defined. The splitting of the reduction waves, ΔE, is related to the electronic coupling HAB between the triruthenium clusters, and varies from 80 mV for 5 to 440 mV for 1. In the case of 6, the splitting of the reduction waves, ΔE, is <50 mV and the intercluster charge-transfer complex is not defined. The mixed valence states of 1−3 also exhibit intervalence charge transfer (ICT) bands in the region 12 100 (1) to 10 800 cm-1 (3) which provide spectroscopic estimates of H...
181 citations
IBM1
TL;DR: In this article, the authors evaluate the continuum limit of the valence (quenched) approximation to the mass of the lightest scalar quarkonium state for a range of different quark masses and to the mixing energy between these states and the LSL glueball.
Abstract: We evaluate the continuum limit of the valence (quenched) approximation to the mass of the lightest scalar quarkonium state for a range of different quark masses and to the mixing energy between these states and the lightest scalar glueball Our results support the interpretation of $f_0(1710)$ as composed mainly of the lightest scalar glueball
180 citations
TL;DR: In this article, a manybody Green's function method is used to calculate vertical valence ionization potentials to high accuracy for the atoms and molecules Ne, N2, F2, CO2, P2, H2O, and H2S.
Abstract: A many‐body Green’s function method is used to calculate vertical valence ionization potentials to high accuracy for the atoms and molecules Ne, N2, F2, CO2, P2, H2O, and H2S. Large basis sets including several sets of polarization functions are used in the calculations to reach the limit of the presently achievable accuracy for molecular systems. The maximum errors in the computed ionization potentials are 0.1 to 0.25 eV depending on the molecule and the basis set. The results are extremely stable, when large basis sets are used. Comparison with other methods is made.
170 citations
TL;DR: In this paper, the Density Functional Theory (DFT) framework was used to analyze the interaction between CO and O on metal surfaces, an important issue in CO oxidation, and also in promotion and poisoning effects of catalysis.
Abstract: Ab initio total energy calculations within the Density Functional Theory framework were carried out for Pt(111), Pt(111)-p(2×2)-CO, Pt(111)-p(2×2)-O, and Pt(111)-p(2×2)-(CO+O) to provide an insight into the interaction between CO and O on metal surfaces, an important issue in CO oxidation, and also in promotion and poisoning effects of catalysis. The geometrical structures of these systems were optimized with respect to the total energy, the results of which agree with existing experimental values very well. It is found that (i) the local structures of Pt(111)-p(2×2)-(CO+O), such as the bond lengths of C−O, C−Pt, and O−Pt (chemisorbed O atom with Pt), are almost the same as that in Pt(111)-p(2×2)-CO and Pt(111)-p(2×2)-O, respectively, (ii) the total valence charge density distributions in Pt(111)-p(2×2)-(CO+O) are very similar to that in Pt(111)-p(2×2)-CO, except in the region of the chemisorbed oxygen atom, and also nearly identical to that in Pt(111)-p(2×2)-O, apart from in the region of the chemisorbed...
TL;DR: The electrochemical intercalation of vanadyl cations in aqueous solution leads to a solid deposit on the working electrode, called electrolytic vanadium oxide (e-V2O5).
TL;DR: In this paper, the charge-ordering states with lattice distortions of a halogen-bridged binuclear-metal mixed-valence complex (called MMX chain), Pt2(dta)4I (dta = CH3CS2-), have been investigated by transport, magnetic and optical measurements.
Abstract: The charge-ordering states with lattice distortions of a halogen-bridged binuclear-metal mixed-valence complex (called MMX chain), Pt2(dta)4I (dta = CH3CS2-), have been investigated by transport, magnetic, and optical measurements. This complex is a binuclear unit-assembled conductor containing metal−metal bonds. It exhibits metallic conduction above room temperature, representing the first example of a metallic halogen-bridged one-dimensional transition-metal complex. Below 300 K it shows semiconducting behavior, which is considered to be of the Mott−Hubbard type due to electron correlation. The metal−semiconductor transition at 300 K (= TM-S) is derived from a valence transition of Pt from an averaged-valence state of 2.5+ to a trapped-valence state of 2+ and 3+. The charge-ordering modes are considered to be −IPt2+−Pt3+−IPt2+−Pt3+−IPt2+−Pt3+−IPt2+−Pt3+−I for the semiconducting phase below TM-S and −I−Pt2.5+−Pt2.5+−I−Pt2.5+−Pt2.5+−I−Pt2.5+−Pt2.5+−I−Pt2.5+−Pt2.5+−I− for the metallic phase above TM-S. 129...
TL;DR: In this paper, the effects of surface structure on electronic properties of CdSe clusters were investigated using a tight-binding model, taking into account experimental information about structure and shape of the nanocrystals, as well as the nature and distribution of capping ligands.
Abstract: We utilize a tight-binding model to study the effects of surface structure on electronic properties of CdSe clusters. The model takes into account experimental information about structure and shape of the nanocrystals, as well as the nature and distribution of capping ligands. The effects of both organic capping ligands and inorganic capping shells on the densities of states (DOS) and on the single-particle absorption spectra of the clusters are calculated for various cluster shapes and sizes, and are compared to results for clusters with truncated surfaces. For organic capping ligands, the effect of ligand hybridization is investigated and a simple model of surface reconstruction is developed. Both ligand hybridization and surface reconstruction are seen to have a major influence on the band edge electronic and optical properties. Inorganic capping shells give rise to differential localization of valence and conduction band edge states, with the hole primarily confined to the core region and the electron more evenly distributed over both core and shell.
01 Mar 1999
TL;DR: In this paper, a reinvestigation and extension of the observed VUV and EEL spectrum of pyrrole has been carried out, and the spectra assigned by means of high level multi-reference multi-root CI studies.
Abstract: A reinvestigation and extension of the observed VUV and EEL spectrum of pyrrole has been carried out, and the spectra assigned by means of high level multi-reference multi-root CI studies. A similar reasssessment of the UV-photoelectron spectrum leads to the conclusion that our previous assignment of IP3 to ionisation of the inner π-level (1b1) is correct and that most other assignments to ionisation of a σ-electron are incorrect. This is supported by the calculation of a wide range of Rydberg states derived from the five least bound orbitals, in the IP order: 1a2−1 1 ππ ∗ states of symmetry 1A1 and 1B2. The range of calculated valence states of ππ ∗ -type has been extended to include those from 1b1.
TL;DR: In this paper, a simple and accurate experimental method was developed to determine the macroscopic proton-ion adsorption stoichiometry for vanadate, phosphate, arsenate, chromate, molybdate, tungstate, selenate and sulfate.
TL;DR: In this paper, the colossal magnetoresistance effect (CMR) can be tuned in polycrystalline mixed valence manganite La 0.67Ca 0.33MnO3 via changing grain size by means of a sol-gel method.
Abstract: In this article, we show how colossal magnetoresistance effect (CMR) can be tuned in polycrystalline mixed valence manganite La0.67Ca0.33MnO3 via changing grain size by means of a sol-gel method. Below a critical diameter (150 nm), CMR disappears, but large intergrain MR remains even well above Tc (1.2Tc for ≈95 nm particles). Possible explanation for this effect involves single magnetic domain behavior in samples annealed at low temperature.
TL;DR: Long-range ferromagnetic ordering at 3 K is observed for the title compound, which may be considered as a fully localized mixed-valence species as well as a mixed-spin species (low-spin and high-spin Mn(2+) ions).
Abstract: Long-range ferromagnetic ordering at 3 K is observed for the title compound, which may be considered as a fully localized mixed-valence species (Mo(3+) and Mo(4+)) as well as a mixed-spin species (low-spin and high-spin Mn(2+) ions). Its two-dimensional structure consists of heart-shaped 48-membered rings, and each ring contains 16 metal centers (see picture).
TL;DR: In this paper, high resolution valence and core-level photoemission spectra of undoped and 3% Sb-doped images were presented, where the dominant screening response involves excitation of conduction electron plasmons.
Abstract: High resolution valence- and core-level photoemission spectra of undoped and 3% Sb-doped ${\mathrm{SnO}}_{2}$ are presented. Conduction-band occupation due to Sb doping in ${\mathrm{SnO}}_{2}$ leads to a shift of valence-band features to high binding energy. However, the shift is less than the width of the occupied part of the conduction band. This is attributed to a shrinkage of the bulk band gap with doping, arising from an attractive dopant electron interaction and screening of the Coulomb repulsion between valence and conduction electrons. Core-level spectra provide evidence for strong screening by the conduction electron gas in 3% Sb-doped ${\mathrm{SnO}}_{2},$ giving rise to ``screened'' and ``unscreened'' final-state peaks in photoemission. The dominant screening response involves excitation of conduction electron plasmons.
TL;DR: In this article, the electronic structure of a class of half-Heusler compounds, where M is Y, La, Lu, Yb, and Pn is a pnicogen As, Sb, Bi, was studied.
Abstract: We have studied the electronic structure of a class of half-Heusler compounds $M\mathrm{NiPn},$ where M is Y, La, Lu, Yb, and Pn is a pnicogen As, Sb, Bi. All these systems excepting Yb are narrow-gap semiconductors and are potential candidates for high-performance thermoelectric materials. The Yb system shows heavy fermion characteristics. Calculations were carried out within density-functional theory (generalized gradient approximation) using self-consistent full-potential linearized augmented plane-wave method. Comparison of the electronic structures of isoelectronic systems YNiSb and ZrNiSn, another narrow-gap semiconductor, brings out the role of hybridization on the energy gap formation. We also find that in YNiPn systems, the gap narrows as we go from As to Bi, a result of relativistic lowering of the Pn valence s band and its influence on the lowest conduction band. Our band-structure results for YbNiSb differs drastically from a previous calculation using a different method, but agrees closely with a similar mixed valence system YbPtBi.
TL;DR: In this article, a spin-unrestricted approach to time-dependent density functional theory is applied to calculate vertical excitation energies of some small and medium-size organic radicals, including nitromethyl, benzyl, anilino and phenoxyl.
TL;DR: In this paper, it was shown that the concentrations of structural defects and electronic gap states are quantitatively correlated, and that this way the electron deficiencies theoretically calculated for the valence bands of corresponding idealized structures are compensated.
Abstract: From fine-structure investigations it is well known that in many icosahedral boron-rich solids the occupation densities of specific atomic sites are considerably reduced. Investigations of the electronic properties have proved that the electronic properties of these semiconductors are strongly influenced by high densities of intrinsic states in the band gaps. For -rhombohedral boron and boron carbide, the best investigated icosahedral boron-rich solids, it is shown that the concentrations of structural defects and electronic gap states are quantitatively correlated, and that this way the electron deficiencies theoretically calculated for the valence bands of corresponding idealized structures are compensated. Obviously, the structural defects in these crystals are the necessary consequence of the valence electron deficiency. It is suggested that this correlation holds for the icosahedral boron-rich solids in general.
TL;DR: In this paper, the structure of colloidal tetraalkylammonium (TAA) manganese oxides in sols and gels obtained by different preparation and heat treatment procedures was investigated.
Abstract: X-ray absorption spectroscopy at the Mn K edge was employed to elucidate the structure of colloidal tetraalkylammonium (TAA) manganese oxides in sols and gels obtained by different preparation and heat treatment procedures. Two series of colloidal TAA MnOx prepared with tetrapropylammonium (TPA) and tetraethylammonium (TEA) cations were studied. Several manganese oxides, birnessite, and feitknechtite were also measured and served as model compounds for structural refinements. Near edge structure (XANES) analysis revealed different average valences of the colloidal systems. As synthesized and heat-treated, TAA colloids exhibited an average valence of 3.6−3.7, whereas gelled TAA colloids showed a lower average valence of ∼3.5. Extended absorption fine structure (EXAFS) analysis was carried out to distances of ∼6.0 A around the central Mn atom using theoretical backscattering phases and amplitudes calculated from the ab initio FEFF code. All multiple-scattering (MS) paths with a weight of 2% and more with re...
TL;DR: The theoretical calculations predict that, with the exception of the p-nitro compound, all the compounds are planar molecules where the conjugation degree of the nitrogen lone pair with the quinone system depends on the position and magnitude of the electronic effect of the substituent in the aniline ring.
Abstract: We synthesized and analyzed 19 compounds of 3'- (meta-) and 4'- (para-) substituted 2-[(R-phenyl)amine]-1,4-naphthalenediones (PANs) R = p-MeO, p-Me, p-Bu, p-Hex, p-Et, m-Me, m-Et, H, p-Cl, p-Br, m-F, m-Cl, p-COCH(3), m-CN, m-NO(2), m-COOH, and p-COOH. Despite the fact that the nitrogen atom, which binds the quinone with the meta- and para-substituted ring, interferes with the direct conjugation between both rings, the UV-vis spectra of these compounds show the existence of an intramolecular electronic transfer from the respective aniline to the p-naphthoquinone moiety. In accordance with this donor-acceptor character, the cyclic voltammograms of these compounds exhibit two, one-electron reduction waves corresponding to the formation of radical-anion and dianion, where the half-wave potential values vary linearly with the Hammett constants (sigma(x)). The analysis of the different voltammetric parameters (e.g., voltammetric function, anodic/cathodic peak currents ratio, and the separation between the anodic and cathodic potential peaks) show that with the exception of the carboxylic PAN derivatives, all compounds present the same reduction pathway. We investigated the molecular and electronic structures of these compounds using the semiempirical PM3 method and, within the framework of the Density Functional Theory, using the Becke 3LYP hybrid functional with a double zeta split valence basis set. Our theoretical calculations predict that, with the exception of the p-nitro compound, all the compounds are planar molecules where the conjugation degree of the nitrogen lone pair with the quinone system depends on the position and magnitude of the electronic effect of the substituent in the aniline ring. The Laplacians of the critical points (nabla(2)rho), for the C-O bonds, show that the first reduction wave corresponds to the carbonyl group in alpha-position to the aniline, and that the second one-electron transfer is due to the C(4)-O(2) carbonyl reduction. Thus, the higher reaction constant value (rho) obtained for the second one-electron transfer is due to the fact that the displacement of the nonshared electrons of the amino nitrogen merely modifies the electron density of C(4)-O(2) bond. The positive correlation between the LUMO energy values calculated for these compounds and the E(1/2) potentials corresponding to the C(1)-O(1) carbonyl reduction show that the electron addition takes place at the lowest unoccupied molecular orbital, supporting the fact that this wave is also prone to the substituent effect.
TL;DR: In this article, the electron localization function (ELF) of theoretically calculated electron densities is used to determine basins that are either core or valence basins, which are characterized by the number of core basins with which they share a common boundary.
Abstract: Except for the case of van der Waals interactions, homopolar bonds are covalent and therefore a concentration of the electron density is expected at the bond midpoint. Many experimental and theoretical studies have reported standard deformation density maps and molecular density minus spherical atoms densities, which show a depletion of electron density between formally covalently bonded atoms. For example, electron deficits are found in the theoretical map of the FF bond in F2, in the experimental map of the NN bond in carbonohydrazide, and in the experimental and theoretical maps of the OO bond in 1,2,7,8-tetraaza-4,5,10,11-tetraoxatricyclo[6.4.1.1]tetradecane. Other partitioning schemes, such as subtraction of valence state atoms rather than spherical atoms from the total density, have been proposed to interpret these unexpected features. In the present work we examine these electronically depleted covalent bonds on the basis of the topological analysis of the electron localization function (ELF) of theoretically calculated electron densities. The attractors of ELF determine basins that are either core or valence basins. The valence basins are characterized by the number of core basins with which they share a common boundary, and this number is called the synaptic order. Disynaptic valence basins have been found for the FF bond in F2, for the NN bond in carbonohydrazide and for the OO bond in 1,2,7,8-tetraaza-4,5,10,11- tetraoxatricyclo[6.4.1.1]tetradecane. In the case of F2, polarization functions increase the V(F, F′) basin population, whereas accounting for the Coulomb correlation lowers this basin population. The results calculated for F2 are compared with those obtained for other diatomic molecules, such as N2 and O2, and the ELF picture of the bond compared with the molecular orbital analysis. In the case of carbonohydrazyde, the V(N, N′) basin population is the lowest among all the populations of the disynaptic valence basins present in the molecule, in good agreement with the experimental observations. Analogous results are obtained for the V(O, O′) basin population in 1,2,7,8-tetraaza-4,5,10,11- tetraoxatricyclo[6.4.1.1]tetradecane. Population fluctuation analysis indicates a strong delocalization of the electron density toward lone pairs and adjacent bonds. ©1999 John Wiley & Sons, Inc. J Comput Chem 20: 1517–1526, 1999
18 Oct 1999
TL;DR: In this article, a systematic study of the Mn 2p, 3s and 3p core-level photoemission and satellite structures for Mn model compounds was performed and the results were best understood in terms of the configuration-interaction (CI) model including intrashell electron correlation, charge-transfer and final-state screening.
Abstract: We report a systematic study of the Mn 2p, 3s and 3p core-level photoemission and satellite structures for Mn model compounds. Charge-transfer from the ligand state to the 3d metal state is observed and is distinguished by prominent shake-up satellites. We also observe that the Mn 3s multiplet splitting becomes smaller as the Mn oxidation state increases, and that 3s-3d electron correlation reduces the branching ratio of the 7S:5S states in the Mn 3s spectra. In addition, as the ligand electronegativity decreases, the spin state purity is lost in the 3s spectra as evidenced by peak broadening. Our results are best understood in terms of the configuration-interaction (CI) model including intrashell electron correlation, charge-transfer and final-state screening.
TL;DR: In this article, the electronic structure of, and the effects of cesium (Cs) and oxygen (O) adsorption on, the p-GaN(001) surface was investigated via photoemission spectroscopy.
Abstract: The electronic structure of, and the effects of cesium (Cs) and oxygen (O) adsorption on, the p-GaN(001) surface are investigated via photoemission spectroscopy. Bulk- and surface-sensitive photoemission measurements, and oxygen exposure of clean surfaces, demonstrate the existence of filled surface states which extend ∼0.6 eV above the valence band maximum. The valence band maximum measured after the removal of the surface states gives a downward band bending and electron affinity equal to 1.2±0.2 and 3.3±0.2 eV, respectively. The surface dipole layer induced by exposure to oxygen followed by Cs deposition lowers the vacuum level by 2.8±0.3 eV with respect to the valence and conduction band edges. Under these conditions, the vacuum level is approximately 0.7 eV below the conduction band minimum of the bulk, corresponding to the effective negative electron affinity at this surface.
TL;DR: In this article, femtosecond transient absorption (population and anisotropy) spectroscopy was combined with ab initio electronic structure methods to study excited-state deactivation pathways of the pyridine molecule in liquid solutions.
Abstract: In this work, we combined femtosecond transient absorption (population and anisotropy) spectroscopy with ab initio electronic structure methods to study excited-state deactivation pathways of the pyridine molecule in liquid solutions. Studies of the effects of excitation energy, deuteration and substitution, solvent polarity and viscosity, and protonation of pyridine were performed. The experiments reveal the dynamics of S_1(nπ^*) and S_2(ππ^*) excited states of pyridine. The photoexcitation of the S_2(ππ^*) state leads to formation of the prefulvenic form of pyridine, a valence isomer, in ∼2.2 ps, while nonradiative deactivation of the S_1(nπ^*) state occurs in 9−23 ps and is to a large extent due to intersystem crossing. Using ab initio methods at the CASSCF and time-dependent DFT levels, we calculated the potential energy surfaces of the ground and S_2(ππ^*) states. A conical intersection was found responsible for the ultrafast deactivation of the pyridine molecule.
TL;DR: In this article, core level x-ray absorption and photoemission spectra of free water clusters are presented and compared to similar spectra for the isolated molecule and solid ice, and discussed in connection to calculated structures.
Abstract: Core level x-ray absorption and photoemission spectra of free water clusters are presented. Core and valence photoemission spectra show a weak but gradual change with cluster size. Comparisons to spectra of the isolated molecule and solid ice indicate that water molecules have a lower average coordination in clusters than in the bulk solid. X-ray absorption spectra reveal spectral characteristics between the free molecule and the bulk. These are compared to similar spectra of the isolated molecule and solid ice, and discussed in connection to calculated structures.