Investigations of the optical and EPR spectra for (NiX6) (X=Cl, Br, I) clusters

In this paper, the formulae of optical spectral levels and electron paramagnetic resonance (EPR) spectra in trigonal symmetry of 3d⁸ ions are established on the basis of strong field mechanism and a two spin-coupling (SO) parameters model. Unlike the classical crystal-field approach which has only taken the SO coupling of the central metal ions into account, the contribution of the SO coupling of the ligand ions to the optical and EPR spectra has been included in these formulae. When the optical and EPR spectra of the strong covalent crystals are calculated, the reasonable results can be obtained if the two SO parameters model has been put into action. As an application, the optical and EPR spectra of the (NiX₆)⁴⁻ clusters in CsMgX₃:Ni²⁺ (X=Cl, Br, I) crystals have been studied by the complete diagonalization (of energy matrix) method (CDM). The calculated results agree well with experimental findings. From the investigations, a more valid method to calculate the optical and EPR spectra for 3d⁸ ions clusters is provided.     

Polarization of photons emitted in the process of resonant coherent excitation of relativistic ions under planar channeling conditions

Owing to the recent observation of the strong anisotropy of characteristic X-ray radiation accompanying the resonant coherent excitation of relativistic Fe²⁴⁺ ions under planar channeling conditions in a silicon crystal, the resonant coherent excitation method can be considered as a candidate for a source of polarized X-ray radiation. The Stokes parameters of the radiation have been calculated using the density matrix approach. The behavior of the polarization characteristics of the radiation in various directions has been explained by considering the properties of the resonance part of the crystal field, which excites an ion and has the form of an elliptically polarized electric field.     

Phenylacetylene chemisorbed on Pt (111), reactivity and molecular orientation as probed by NEXAFS Comparison with condensed multilayer and polyphenylacetylene

Near-edge X-ray absorption spectroscopy (NEXAFS) has been used to investigate the reactivity of phenylacetylene (PA) towards a Pt(111) surface and its molecular orientation. Spectra have been taken in the photon energy region 280–320 eV at both grazing and normal X-ray beam incidence angles with respect to the sample surface. NEXAFS K-edge spectra have been recorded for PA adsorbed in the monolayer as well as in the multilayer regime and, for comparison, the spectrum of the PA related polymer, polyphenylacetylene (PPA), has also been measured. The assignment of the π* and σ* resonances has been made on the basis of the chemical structure and interpreted with the help of ab initio calculations. Both the ionisation potentials for the C 1s electrons and the theoretical C K-edge absorption spectra have been calculated for all the six inequivalent carbon atoms of PA and of phenylethylene (PE) which is a model molecule for PPA. From the analysis of the data we have indication for an interaction involving mainly the CC triple bond of the acetylenic functional group of PA that undergoes opening and leads to the formation of a di-sigma bond with Pt, while the benzene CC bonds seem not affected. A polarization dependence has been evidenced for the π* resonances of PA adsorbed in monolayer condition indicating a molecular orientation of the phenyl ring π* orbitals between 37° and 34° relative to the normal to the crystal surface.     

Multiple scattering and neutralization aspects of low energy noble gas ions incident on a Cu (110) crystal

Experiments have been done with 1 keV Ne⁺ ions bombarding a Cu (110) single crystal in a (111) plane. From the measured energy spectra of the scattered ions the minimum and maximum scattering angles for multiple scattering on the surface are determined. These minimum and maximum scattering angles were also calculated using a computer model. The parameters for an interatomic potential function between ion and metal atom are determined by comparison of the experimental and calculated scattering angles. A neutralization model for an energetic ion at an atomic surface is given. The reliability of this model is tested by comparison of the results of computer calculations and the measured peak intensity distributions as a function of the scattering angle.     

Fully relativistic analysis of the covalence effects for the isoelectronic 3d ions (Cr, Mn, Fe) in SrTiO3

Fully relativistic calculations of the energy levels, absorption spectra, molecular orbitals (MO) compositions, covalence effects and energies of the charge transfer (CT) transitions for three isoelectronic ions Cr³⁺, Mn⁴⁺, Fe⁵⁺ in the SrTiO₃ (STO) crystal have been performed. The recently developed first-principles approach to the analysis of the absorption spectra based on the first principles discrete variational multi-electron method (DV-ME) (K. Ogasawara et al., Phys. Rev. B 64 (2001) 115413) was used in the calculations. As a result, energy levels of the above ions, their absorption spectra and energies of the lowest CT transitions were all calculated. By performing analysis of the MO population, it was shown that the degree of covalence of the chemical bonds between 3d ions and oxygen ions in SrTiO₃ increases in the following order: Cr³⁺→Mn⁴⁺→Fe⁵⁺, whereas the CT energies monotonically decrease in the same order.     

Mechanisms of formation of near-edge fine structure of <Emphasis Type="Italic">K</Emphasis> X-ray absorption spectra of metallic Cu,Ni, Co (HCP and FCC phases), and Cr

The K X-ray absorption near-edge fine structure spectra of metallic Cu, Ni, Co (hexagonal close-packed and face-centered cubic phases), and Cr have been studied. The experimental spectrum of cobalt has been measured in the high-temperature face-centered cubic phase. The calculations have been performed by the full multiple scattering method in the cluster approximation using the semiempirical muffin-tin potential. The spectra have been calculated for clusters containing from 13 to 935 atoms. It has been demonstrated that the spectra cease to depend on the cluster size for clusters containing about 400 atoms. It has been established that the intensity ratio of two peaks located at the fundamental edge of all the spectra under investigation sharply changes with a variation in the atomic potential strength. The mechanisms responsible for the formation of the near-edge structure of the spectra have been revealed using the results of the model calculations. The studies resulted in good agreement of the calculated spectra with the experiment.     

Growth, spectroscopic characteristics and laser potential of Yb3+:Ca3La2(BO3)4 crystal

Crystal of Yb³⁺-doped Ca₃La₂(BO₃)₄ has been grown by the Czochralski technique. The room temperature absorption and fluorescence spectra of the crystal have been investigated. The result showed that this crystal exhibits broad absorption and emission with the FWHM of 11 nm at 978 nm and 66 nm FWHM at 1025 nm, respectively. The stimulated emission cross-section of Yb³⁺ ions were calculated using the reciprocity method and Fuchtbauer-Ladenburg method, respectively. The room temperature fluorescence decay curves of ² F _5/2 manifold of Yb³⁺ ions were recorded for both crystal and powder samples. The effect of radiation tapping on the spectroscopic properties is discussed. The result that the lifetime of the powder sample is shorter than that of the bulk sample demonstrates the existence of radiation trapping effect. The laser potentiality was also evaluated and the results show that this crystal is a good candidate for tunable and ultrashort pulse lasers.     

Observation of X-ray spectra from nitrogen clusters irradiated with high-intensity ultrashort laser pulses

X-ray spectra from nitrogen clusters irradiated with ultrashort laser pulses are reported. The line spectra of H-like and He-like nitrogen ions have been observed by irradiation with 100 fs, 800 nm pulses at 7×10¹⁷ W/cm² irradiance. The generation of highly charged ions of N⁶⁺ and N⁷⁺ is explained by the optical field-induced ionization and the subsequent collisional ionizations in the clusters. The He-δline has anomalously high brightness compared to other He-like lines. It is ascribed to charge exchange, which preferentially populates the n=5 level of N⁵⁺. Received: 7 October 1999 / Published online: 23 February 2000     

Synthesis,spectroscopic properties and DFT study of (E)-1-[(3-(trifluoromethyl)phenylimino)methyl]naphthalen-2-olate

The Schiff base (E)-1-[(3-(trifluoromethyl)phenylimino)methyl]naphthalen-2-olate was synthesized from the reaction of 2-hydroxy-1-naphthaldehyde with 3-trifluoromethylaniline. The title compound has been characterized by FT-IR, UV-vis and X-ray single-crystal techniques. The present X-ray investigation shows that the compound exists in the zwitterionic form. Molecular geometry of the compound in the ground state have been calculated using the density functional method (DFT) with 6-31G(d,p) basis set and compared with the experimental data. The calculated results show that the optimized geometry can well reproduce the crystal structural parameters. By using TD-DFT method electronic absorption spectra of the compound have been predicted and a good agreement with the TD-DFT method and experimental one is determined. In addition DFT calculations of the compound, molecular electrostatic potential (MEP), frontier molecular orbital analysis (HOMO-LUMO) and non-linear optical (NLO) properties were performed at B3LYP/6-31G(d,p).     

Nephelauxetic effect for the isoelectronic 3d-ions (Cr, Mn, Fe) in SrTiO3

The exchange charge model of crystal field theory has been used to analyze systematically the ground state absorption spectra of isoelectronic Cr³⁺, Mn⁴⁺, and Fe⁵⁺ ions in an octahedral coordination in the SrTiO₃ crystal. The parameters of the crystal field acting on the valence electrons of impurity ions are calculated from the available crystal structure data. A special attention is paid to the analysis of dependencies of the crystal field invariants and covalence effects on the impurity ion. It is shown numerically that the covalence effects between the above impurity ions and ligands increase with an increase of the 3d-ion oxidation state.     

Surface termination of BaTiO3 (001) single crystals: A combined electron spectroscopic and theoretical study

The surface termination of oxide surfaces is of crucial importance for the growth of a second material like metals or other oxides. In this study we have investigated the surface of a BaTiO₃ (001) single crystal during sample preparation by various electron spectroscopic methods. It is shown by X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and metastable induced electron spectroscopy (MIES) that during sputtering a Ba rich overlayer is formed, in which the Ba²⁺ ions are under coordinated. Below this layer, an oxygen deficient BaTiO₃ layer is found. During annealing, we observe the reformation of the crystalline structure. UP and MIE spectra provide clear evidence of a BaO terminated surface. X-ray photoelectron diffraction studies support this result, comparing recorded polar angle scans with calculated intensity modulations using multiple scattering cluster models.     

Ultraviolet photoelectron spectroscopy of nano In clusters Schottky barriers on sputtered InP

Plasmon peaks along with Auger P_LVV peak have been observed in the ultraviolet photoelectron spectra (UPSs) of InP after 5 min of sputtering with 0.5 kV Ar⁺ ions. Plasmon and Auger peaks are not observed in UPS of un-sputtered InP surface with native oxides of In and P. Filled electron energy levels are not observed near the Fermi level from 5 min sputtered InP surface due to increase of ionization potential of nano In clusters.     

Ionization of metastable He(1s2s) atoms by alkalated surfaces

We calculate the ionization probability for singlet and triplet metastable He(1s2s) atoms scattered from potassium covered surfaces, on the basis of the resonant tunnelling process. We do not assume the trajectory approximation, but we do assume classical motion on the potential energy curves, with crossing between neutral and ionic curves at a given point calculated from electronic tunnelling probabilities. Energy spectra of the He⁺ ions are also calculated. The results are found to be somewhat sensitive to tunnelling probability as a function of distance from the surface. Results are compared with experimental ionization probabilities measured by Roussel. The agreement is found to be fair if the tunneling rate is parameterized according to the theory of Janev et al. A simple interpretation is given.     

5,7¢-(亚甲胺基)-二-8-羟基喹啉及其衍生物的密度泛函理论计算

在B3LYP/6-31G(d)水平上对5, 7¢-(亚甲胺基)-二-8-羟基喹啉及其5种衍生物进行了几何构型全优化, 探讨了喹啉不同位H被吸电子基团CN及羟基O被S原子取代对分子电离势(IP)、电子亲和势(EA) 、电荷转移、前线轨道能量和电子光谱等性质的影响. 用含时密度泛函理论(TD-DFT)计算了分子在气相及液相的吸收光谱, 计算结果与实验值基本符合. 取代基对5, 7¢-(亚甲胺基)-二-8-羟基喹啉锌分子的性质有较大影响. 电子亲和势计算表明, 该类化合物的电子亲和势较大, 都是较好的电子传输材料.     

Investigation of the electronic structure of the BiSBr and BiSeBr clusters by density functional method

The energy levels of valence bands (VB) of the BiSBr and BiSeBr crystals have been calculated for investigation of the photoelectron emission spectra of BiSBr, BiSeBr and BiSI crystals. The molecular model of this crystal has been used for the calculation of VB by the Density Functional Theory (DFT) method. The molecular cluster, consisting of 20 molecules of BiSBr, BiSeBr, has been used for calculations of averaged total density of states, including atom vibrations. The spectra of the averaged total density of states from VB of BiSBr and BiSeBr clusters have been compared with the experimental photoelectron emission spectra from VB of BiSI crystals. The results clarify that the atomic vibrations in A⁵B⁶C⁷ type crystals with chain structure create a smoother appearance of the averaged total density of state spectrum and the experimental X-ray photoemission spectra (XPS).     

Spectral properties of Pr3+ ions in Bi2(MoO4)3 crystal

A Bi₂(MoO₄)₃ single crystal doped with Pr³⁺ ions has been grown by the Czochralski technique. The polarized absorption and fluorescence spectra as well as the fluorescence decay curve of Pr³⁺ ions in the crystal were measured at room temperature. The spectroscopic parameters, including the Judd–Ofelt intensity parameters Ω_t (t=2, 4, and 6), spontaneous emission probabilities, fluorescence branching ratios, radiative lifetimes, stimulated emission cross sections, and fluorescence quantum efficiencies, were calculated. The spectral properties related to laser performance of this crystal were analyzed. The ¹ D ₂ multiplet of the crystal may be a good upper level for a solid-state laser.     

An X-ray diffraction study of anharmonic thermal vibrations in the ionic conductor,barium chloride fluoride (BaClF)

Integrated intensities of X-ray reflections from a barium chloride fluoride single crystal were measured at 24, 180, 289, 402, 467 and 610°C. The intensities were used for the determination of the anharmonic potential parameters up to the third order. Harmonic potential parameters for each ion were obtained from the temperature dependence of thermal parameters, and anharmonic potential parameters were determined by the least-squares procedures, utilizing the temperature factor formalism based on the cumulant expansion. As a result, it has been found that the Ba²⁺ and Cl^? ions show an anharmonicity, while no significant anharmonic effect was detected for the F^? ion. From the potential calculated around the Cl^? and F^? ions, it was found that both Cl^? and F^? ions diffuse more easily within the (001) plane than along the c axis.     

Fluid synthesis and structure of a new polymorphic modification of boron nitride

A new previously unknown phase of boron nitride with a hardness of 0.41–0.63 GPa has been pre-pared by the supercritical fluid synthesis. The presence of a new phase is confirmed by the X-ray spectra and IR absorption spectra, where new reflections and bands are distinguished. The fundamental reflection of the X-ray diffraction pattern is d = 0.286–0.291 nm, and the characteristic band in the infrared absorption spectrum is observed at 704 cm^?1. The X-ray diffraction pattern and the experimental and theoretical infrared absorption spectra show that a new synthesized boron nitride phase can be a cluster crystal (space group 211) with a simple cubic lattice. Cage clusters of a fullerene-like morphology B₂₄N₂₄ with point symmetry O are arranged in lattice sites.     

Cluster modelling of core electron photoabsorption of CO adsorbed on Cu(100)

Quantum chemical calculations of discrete and continuum near-edge X-ray absorption spectra of carbon monoxide adsorbed on Cu(100) are presented and discussed. The surface is modelled by clusters using one-electron effective core potentials for distant atoms, allowing cluster sizes up to 50 atoms to be treated. The employed theoretical approach, the direct, atomic orbital, static exchange technique, implementing the independent-particle approximation close to the basis set limit, is shown to provide good representations of the near edge X-ray absorption spectra both for free and adsorbed CO. The experimental observations of reduction of energy and intensity of the first π1 resonance and a compression of the discrete part of the spectra are reproduced by the calculations. The origin of these effects is discussed.     

Features of the local structure of rare-earth dodecaborides RB12 (R = Ho, Er, Tm, Yb, Lu)

The parameters of the local crystal structure of dodecaborides RB₁₂ (R = Ho, Er, Tm, Yb, Lu) have been determined by extended X-ray absorption fine structure (EXAFS) spectroscopy. It has been shown that the vibrations of the rare-earth ion with respect to the boron cage are well described in the harmonic approximation. At the same time, the displacement of rare-earth ions from equilibrium positions of the crystal structure should be taken into account to determine the length of the R-B bond. The analysis of EXAFS spectra has revealed the displacement of 1–6% of rare-earth ions by about 0.2–0.3 Å in all compounds under investigation; this displacement at low temperatures results in the formation of a cage glass phase.