Electronic structure of Cr clusters on graphite

The electronic structure of small chromium clusters deposited by evaporation onto clean polycrystalline graphite has been studied by means of Auger, X-ray Photoemission (XPS) and Electron Energy Loss (EELS) spectroscopies. The XPS results show an increase in the binding energy of both core levels and valence band reducing the cluster size. The EELS measurements show a variation of the intensity ratio of L₃-to-L₂ ionization core edges. We interpret this change as due to different redistribution, within the clusterd-band, of the empty states above the Fermi level. As a consequence the XPS results may also be attributed to sizeable change of the electronic structure of the small clusters.     

Ab initio MRD-CI study of ethane: The 14–25 eV PES region and Rydberg states of positive ions

Ab initio calculations of the MRD-CI type are reported for various states of the C₂H₆⁺ ion in two different nuclear geometries and the results are compared with the experimentally observed ethane PES in the 14–25 eV region. The calculated vertical IP values for ionization out of the 1e_u, 2a_2u and 2a_1g MO's respectively agree well with the locations of the three ionization maxima in this spectral range. The analogous findings for excitation out of the relaxed ionic ground state find several relatively low-lying species which occupy a 2pσ* MO in addition to states resulting from simple ionization of the neutral molecule. A number of Rydberg states are also calculated at the relaxed-ion geometry, from which results it is determined that the quantum defects for such species are from 0.40–0.45 units smaller than for their counterparts in neutral systems; these findings are clearly consistent with a decrease in the core penetrability of the Rydberg electron as the effective charge is increased to Z = 2.     

Small binary iron-carbon clusters with persistent high magnetic moments. A theoretical characterization

Characterization of the structural and electronic properties of binary iron-carbon clusters composed by six iron atoms and with up to nine carbon atoms was carried out with density functional theory calculations. Neutral, cations (q = +1), and anions (q = −1), some of them experimentally detected, were studied. The formation of dimers and trimers of carbon atoms over the iron surface were preferred. Moreover, some large carbon chains, with up to five atoms, were determined. High spin states emerged for the ground states, with multiplicities above 16, for all clusters independently of the number of carbon atoms attached to the iron core. All neutral clusters were stable because fragmentation (into carbon chains), dissociation (of a single carbon atom), and detachment of all carbons need high amounts of energy. Reactive species were defined by small HOMO-LUMO gaps. Charge transfer, to the carbon atoms, increased as the carbon content increased, producing, for some cases, an even-odd behavior for the magnetic moment of the Fe₆C_n particles.     

Propagator quantum chemical study of <Emphasis Type="Italic">S</Emphasis>-<Emphasis Type="Italic">cis</Emphasis>-(<Emphasis Type="Italic">Z</Emphasis>)-2-(2-formylethenyl)pyrrole: electronic structure and aspects of intramolecular hydrogen bond manifestation in ionization spectra

For the purpose of investigation of the electronic structures of functionalized pyrroles with potential biological activity the electronic structures and ionization spectra of S-cis-(Z)-2-(2-formylethenyl)pyrrole (FP) were calculated by the propagator quantum chemical method. The calculations were performed using the third-order algebraic diagrammatic construction method (ADC(3)) for one-particle Green´s function (electronic propagator) and the 6–31G** basis set. Going from FP (possessing the intramolecular hydrogen bond H?O) to its conformation FPR (without H?O bonding), the O1s-ionization energy and the ionization energy of the σ-type lone electron pair orbital of the O atom decrease by ~0.2 eV, which is a consequence of stopping the electron density transfer from the O atom. A strong electron density transfer through the hydrogen bond from the O atom to the NH group occurs in the nitrogen core level ionization spectrum as evidenced by a lower N1s-ionization energy of FP (by ~0.7 eV ) compared to that of FPR. The valence shell ionization spectra of FP and FPR calculated using the ADC(3) method are characterized by a high density of the satellite states. The results obtained indicate that the electronic structures of the compounds of the considered class are characterized by pronounced effects of electron correlation.     

One-Electron Pseudo-Potential Determination of Stable Isomers of the Li*Ar n Excited Clusters: Absorption Spectra

We present pseudo-potential calculations of geometrical structures of stable isomers of LiAr _n clusters with both an electronic ground state and excited states of the lithium atom. The Li atom is perturbed by argon atoms in LiAr _n clusters. Its electronic structure obtained as the eigenfunctions of a single-electron operator describing the electron in the field of a Li⁺Ar _n core, the Li⁺ and Ar atoms are replaced by pseudo-potentials. These pseudo-potentials include core-polarization operators to account for the polarization and correlation of the inert core with the valence Lithium electron [J Chem Phys 116, 1839 1]. The geometry optimization of the ground and excited states of LiAr _n (n = 1–12) clusters is carried out via the Basin-Hopping method of Wales et al. [J Phys Chem 101, 5111 2; J Chem Phys 285, 1368 3]. The geometries of the ground and ionic states of LiAr _n clusters were used to determine the energy of the high excited states of the neutral LiAr _n clusters. The variation of the excited state energies of LiAr _n clusters as a function of the number of argon atoms shows an approximate Rydberg character, corresponding to the picture of an excited electron surrounding an ionic cluster core, is already reached for the 3s state. The result of optical transitions calculations shows that the absorption spectral features are sensitive to isomer structure. It is clearly the case for transitions close to the 2p levels of Li which are distorted by the cluster environment.     

Energy Stabilities,Magnetic Properties,and Electronic Structures of Diluted Magnetic Semiconductor Zn1-xMnxS(001) Thin Films

We investigate the electronic and magnetic properties of the diluted magnetic semiconductors Zn_1-xMn_xS(001) thin films with different Mn doping concentrations using the total energy density functional theory. The energy stability and density of states of a single Mn atom and two Mn atoms at various doped configurations and different magnetic coupling state were calculated. Different doping configurations have different degrees of p-d hybridization, and because Mn atoms are located in different crystal-field environment, the 3d projected densities of states peak splitting of different Mn doping configurations are quite different. In the two Mn atoms doped, the calculated ground states of three kinds of stable configurations are anti-ferromagnetic state. We analyzed the 3d density of states diagram of three kinds of energy stability configurations with the two Mn atoms in different magnetic coupling state. When the two Mn atoms are ferromagnetic coupling, due to d-d electron interactions, density of states of anti-bonding state have significant broadening peaks. As the concentration of Mn atoms increases, there is a tendency for Mn atoms to form nearest neighbors and cluster around S. For such these configurations, the antiferromagnetic coupling between Mn atoms is energetically more favorable.     

Electronic structure and chemical bond in carbides crystallizing in the Fe-W-C system

The electronic energy structure of MC, M₆C, and M₁₂C carbide systems and iron martensite in the absence of spin polarization was studied by the local coherent potential method using the cluster version of the MT approximation in terms of multiple scattering theory. The local partial density of the electronic states of atoms in crystals was calculated and their electronic structures were compared. The peculiarities of chemical bonding in crystals are discussed.     

A study of the electronic structure of phenylsilanes by X-ray emission spectroscopy and quantum chemical calculation methods

The electronic structure of a series of phenylsilanes Ph_4?n SiH _n (n = 0?C3) is studied by X-ray emission spectroscopy and quantum chemical calculations by the density functional theory method. Based on the calculations theoretical X-ray emission SiK??₁ spectra of phenylsilanes Ph_4?n SiH _n (n = 0?C4) are constructed and their energy structure and shape turn out to be well consistent with experiment. The distribution of the electron density of states with different symmetry of Si, C, H atoms are also constructed. An analysis of the obtained X-ray fluorescent SiK??₁ spectra and the distribution of the electron density of states in Ph₄Si and Ph₃SiH compounds shows that their energy structure is mainly determined by a system of the energy levels of phenyl ligands weakly perturbed by interactions with valence AOs of silicon. In the energy structure of MOs of the PhSiH₃ compound, energy orbitals related to t ₂ and a ₁ levels of tetrahedral SiH₄ are mainly presented.     

Ab-initio calculation of structural, electronic, and optical characterizations of the intermetallic trialuminides ScAl3 compound

We present first-principles study of the electronic and the optical properties for the intermetallic trialuminides ScAl₃ compound using the full-potential linear augmented plane wave method within density-functional theory. We have employed the generalized gradient approximation (GGA), which is based on exchange-correlation energy optimization to calculate the total energy. Also we have used the Engel-Vosko GGA formalism, which optimizes the corresponding potential for calculating the electronic band structure and optical properties. The electronic specific heat coefficient (γ), which is a function of density of states, can be calculated from the density of states at Fermi energy N(E_F). The N(E_F) of the phase L1₂ is found to be lower than that of D0₂₂ structure which confirms the stability of L1₂ structure. We found that the dispersion of the band structure of D0₂₂ is denser than L1₂ phase. The linear optical properties were calculated. The evaluations are based on calculations of the energy band structure.     

Experimental and theoretical study of X-ray <Emphasis Type="Italic">K</Emphasis> edges absorption spectra of carbon and nitrogen in the phthalocyanine H<Subscript>2</Subscript>Pc molecule

The study of the electronic structure of H₂Pc was carried out to examine the structure of the lowest unoccupied molecular orbitals (LUMO) of molecule phthalocyanine by X-ray absorption spectroscopy using quantum-chemical calculations. The theoretical calculations were performed on the stationary theory (frozen orbital approximation, Z+1 model) and time-dependent density functional theory (TDDFT). A consideration of K edges absorption spectra of carbon and nitrogen in the common scale of binding energies allows estimating the contributions of AO of all phthalocyanine atoms to the LUMO, defining the sequence of levels, the binding energies of the corresponding levels, and also the character of electronic interactions between individual atoms. It was shown that the best agreement between the experimental and theoretical pre-edge structures of the absorption spectra of nitrogen and carbon for H₂Pc is observed in the case of the application of stationary density functional theory in Z+1 model to account for an X-ray hole. In this case the 2p _π AO of the Nα_(1,2) and Сα atoms make a predominant contribution to the LUMO. The 2p _π AO of the Nα_(1,2) atoms mainly contribute to the boundary LUMO with the energy ~–2.3 eV.     

Molecular orbital analysis of the XPS spectrum of a fluorine containing polyimide: PMDA–BDAF

The results of XPS measurements and molecular orbital calculations performed on the fluorine containing polyimide, PMDA–BDAF, are presented. The calculated carbon 1s (C1s) core energy level positions are compared with the level positions inferred from the XPS measurements. Within Koopman's approximation, the observed shape of the main XPS peak is consistent with the calculated distribution of C1s levels under this peak. Comparison of the magnitude of the carbonyl XPS peak intensity with the main peak intensity indicates a carbonyl C1s signal deficiency compared with that expected for “ideal bulk stoichiometry” i.e., for a polymer with no crosslinks or chain terminations. Comparison of data obtained from a grazing emission (surface sensitive) geometry with that obtained from a normal emission geometry, which probes more deeply into the bulk, indicates a signal enhancement of the C1s levels associated with carbon atoms of the CF₃ groups as one nears the polymer surface. Such enhancement might be due to either actual differences in chemical composition, or to preferential structural ordering near the polymer surface.     

Theoretical studies of excited states of H3O and of core-excited NH3: Comparisons with equivalent core analysis of NH3 core-excitation spectra and implications for the radiation chemistry of aqueous systems

The H₃O radical has been studied within the ab initio LCAO SCF MO model. A flexible basis set including diffuse basis functions at both O and H has been used in order to represent the excited states adequately. Calculated excitation energies are 1.87, 2.87–3.16, and 3.36–3.47 eV; the calculated ionization energy is 4.75 eV. These represent well the experimental values (good to ±0.3 eV) of 1.6, 2.9, and 3.5 eV for excitation, and 5.0 eV for ionization, deduced by equivalent core analysis of high energy electron impact energy loss studies of NH₃. Similar explicit calculations on the N-1s core-excited states of NH₃ have also been made to examine directly the equivalent core concept. “Excited states” (relative to the lowest bound core-excited state) at 1.72, 2.85–3.09, and 3.32 eV, and the “ionization energy” of 4.68 eV, agree well with experiment and support the equivalent core concept. The possible significance of these H₃O results in the radiation chemistry of aqueous media is discussed in view of the fact that the maximum in the absorption spectrum of the hydrated electron lies near that of H₃O.     

Electronic structure and specroscopic properties of norfloxacin,enoxacin, and nalidixic acid

The X-ray photoelectron spectra (XPS) of the core electrons of the carbon, oxygen, nitrogen, and fluorine atoms of antibiotics from the class of quinolones (nalidixic acid, norfloxacin, and enoxacin), having different antibacterial activities, were measured and interpreted. The integrated intensities of the π-π* and n-π* transition bands in the absorption spectra of the compounds were analyzed. The maxima of the luminescence spectral bands are given along with the lifetimes of the excited states and quantum yields of the protolytic forms of the compounds in aqueous media. The XPS data were compared with the Mulliken charges on heteroatoms obtained by quantum-chemical calculations.     

Electronic structure of ZrCuSiAs and ZrCuSiP by X-ray photoelectron and absorption spectroscopy

The electronic structures of quaternary pnictides ZrCuSiPn (Pn=P, As) were analyzed by X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge spectroscopy (XANES). Shifts in the core-line XPS and the XANES spectra indicate that the Zr and Cu atoms are cationic, whereas the Si and Pn atoms are anionic, consistent with expectations from simple bonding models. The Cu 2p XPS and Cu L-edge XANES spectra support the presence of Cu¹⁺. The small magnitudes of the energy shifts in the XPS spectra suggest significant covalent character in the Zr-Si, Zr-Pn, and Cu-Pn bonds. On progressing from ZrCuSiP to ZrCuSiAs, the Si atoms remain largely unaffected, as indicated by the absence of shifts in the Si 2p_3/2 binding energy and the Si L-edge absorption energy, while the charge transfer from metal to Pn atoms becomes less pronounced, as indicated by shifts in the Cu K-edge and Zr K, L-edge absorption energies. The transition from two-dimensional character in LaNiAsO to three-dimensional character in ZrCuSiAs proceeds through the development of Si-Si bonds within the [ZrSi] layer and Zr-As bonds between the [ZrSi] and [CuAs] layers.     

Resonant photoemission and absorption spectroscopy study of the Cr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ti<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Se<Subscript>2</Subscript> electronic structure

Solid solutions of 1T-Cr _x Ti_1?x Se₂ (x = 0?0.83) were synthesized for the first time. To study the electronic structure of Cr _x Ti_1?x Se₂ monocrystals, photoemission spectra of core levels, resonance spectra of valence bands, and absorption spectra of Ti and Cr were obtained. Titanium and chromium atoms were found to have the oxidation state 4+ and 3+, which is supported by atomic multiplet calculations for Ti and Cr in the octahedral environment. According to calculation of the local density of chromium electronic states, the Cr3d electrons are spin-polarized, and the density of chromium states is of half-metal nature. The calculation agrees well with the experimental data.     

First-Principles Study of the Structures and Electronic Properties of Ni<Subscript><Emphasis Type="Italic">n</Emphasis>–1</Subscript>Al (<Emphasis Type="Italic">n</Emphasis> = 2-20) Clusters

The electronic properties, such as binding energy, magnetic property, charge transfer, ionization potential, and electron affinity, of Ni_n–1Al (n = 2-20) neutral and ionic clusters are studied using the density functional theory calculations with the PBE exchange-correlation energy functional. The calculated total magnetic moments and ionization potential can decrease and increase with the addition of the Al atom, respectively. The calculated electron affinity has occurred with no significant change, except the Ni₁₆Al cluster.     

An ab initio SCF and CI study of ketene imine

The electronic structures of the ground and excited states of ketene imine (_H^HCCNH) have been studied by ab initio SCF and CI calculations. The nucleophilic nature of the β carbon with respect to nitrogen has been discussed using calculated electrostatic potentials and by calculated energy differences between the parent and protonated species. The electronically excited ¹A″ and ³A″ states are found to be almost degenerate.     

X-ray photoelectron spectroscopy study of intercalated compounds of fluorinated graphite C2FxBr0.01·yCH3CN

The energy level structure of fluorinated graphite intercalation compounds C₂F_xBr_0.01·yCH₃CN (x = 0.49–0.87, y = 0.084–0.136) has been studied by X-ray photoelectron spectroscopy providing the information on the electronic structure of compounds in question. The analysis of variations of the binding energy of core levels C1s, F1s, and O1s opens the possibility to explore the nature of the chemical bond C-F in the fluorographite matrix with a varying degree of fluorination, as well as to model the structure of these compounds. The examination of decomposition results of spectra into components has revealed the occurrence of C-F fragments, carbon atoms not bonded directly to fluorine atoms, and “graphite-like” areas, whose contribution to the overall structure increases with the degree of matrix fluorination decreasing. The presence of oxygen was considered from the viewpoint of surface phenomena characteristic of low-temperature carbon materials.     

Structural, thermodynamic and optical properties of MgF2 studied from first-principles theory

A detailed theoretical study of structural, electronic, elastic, thermodynamic and optical properties of rutile type MgF₂ has been carried out by means of first-principles Density Functional Theory (DFT) calculations using plane wave pseudo-potentials within the local density approximation and generalized-gradient approximation for the exchange and correlation functionals. The calculated ground state properties and elastic constants agree quite well with experimental values. From the calculated elastic constants we conclude that MgF₂ is relatively hard when compared to other alkaline-earth fluorides and ductile in nature. The thermodynamic properties such as heat capacity, entropy, free energy, phonon density of states and Debye temperatures are calculated at various temperatures from the lattice dynamical data obtained through the quasi-harmonic Debye model. From free energy and entropy it is found that the system is thermodynamically stable up to 1200 K. The imaginary part of the calculated dielectric function ε₂(ω) could reproduce the six prominent peaks which are observed in experiment. From the calculated ε(ω), other optical properties such as refractive index, reflectivity and electron energy-loss spectrum are obtained up to the photon energy range of 30 eV.     

Structural,Relative Stable,and Electronic Properties of Pb<Subscript>n</Subscript>Sn<Subscript>n</Subscript> (n = 2–12) Clusters were Investigated Using Density Functional Theory

The structural, relative stable and electronic properties of Pb_nSn_n (n = 2–12) alloy clusters were systematically studied using density functional theory. The isomers of Pb_nSn_n alloy clusters were generated and determined by ab initio molecular dynamics. By comparing the calculated parameters of Pb₂ dimer and Sn₂ dimers with the parameters from experiments, our calculations are reasonable. With the lowest-energy structures for Pb_nSn_n clusters, the average binding energies, fragmentation energies, second- order energy differences, vertical ionization potentials, vertical electron affinities, HOMO–LUMO gaps, and density of states were calculated and analyzed. The results indicate that the Sn atoms have a tendency to bond together, the average binding energies tend to be stable up to n = 8, Pb₈Sn₈ cluster is a good candidate to calculate the molecular interaction energy parameter in Wilson equation, the clusters become less chemical stable and show an insulator-to-metallic transition, 3, 6, 8 and 11 are magic numbers of Pb_nSn_n (n = 2–12) clusters, the charges always transfer from Sn atoms to Pb atoms in Pb_nSn_n clusters except for Pb₁₀Sn₁₀ cluster, and density of states of Pb_nSn_n clusters becoming continuous and shifting toward negative with the increasing size n.