Raman spectra of molecules considered to be surface enhanced

High resolution electron energy loss spectroscopy (HREELS) and low energy electron diffraction (LEED) have been used to study the structure of adsorbed benzene (C₆H₆ and C₆D₆) monolayers on the Rh(111) surface at 300K. A surface bonding geometry is proposed for benzene adsorbed to give a c(2?3×4) rectangular structure, which involves very little perturbation of the molecular structure with the ring plane parallel to the surface. Only one chemical environment for adsorbed benzene is indicated by a single frequency shift of the symmetric CH out-of-plane bending mode. The adsorption site is tentatively assigned to benzene centered over a single Rh atom.     

Accurate ab initio-based DMBE potential energy surface for HLi2(X 2A′) via scaling of the external correlation

A globally accurate potential energy surface is reported for the electronic ground-state HLi₂ by fitting ab initio energies to double many-body expansion formalism. The total 3726 ab initio energies used to map the HLi₂ potential energy surface are calculated using the multi-reference configuration interaction method, with their dynamical correlation being semiempirically corrected by the double many-body expansion-scaled external correlation method. The current potential energy surface generates an excellent fit of the ab initio energies, showing a small root-mean squared derivation of 0.636 ? kcal ? mol^-1. The topographical features of the HLi₂ potential energy surface are examined in detail, which concludes that the H + Li₂(X ? ¹ Σ _g ) → Li + LiH(X ? ¹ Σ) reaction is essentially barrierless and the exothermicity is calculated to be 33.668 ? kcal ? mol^-1, thus corroborates the available experimental and theoretical results.     

Polarization contributions to the vacancy binding energy in doped oxides

An effective medium theory due to Onsager is generalized to yield the macroscopic dielectric constant of a random assembly of associated vacancy-dopant bound pairs embedded in a dielectric host. The model is then further developed to give the vacancy-dopant binding energy, employing concepts long-established in electrolyte theory. The results of this analysis are used to compute the activation enthalpy for electrical conduction in the ionic conduction regime. The values so found are in reasonable accord with experimental data from various sources on Ce_1?xCa_xO_2?x, with x?0.02, assuming a vacancy migration enthalpy of 0.71 eV. The latter is the only adjustable parameter in the theory here developed. With this same value, the predicted variation of the low-temperature conduction activation enthalpy in Ce_1?2xY_2xO_2?x and Ce_1?2xGd_2xO_2?x (again at small x) are acceptably reproduced, although more experimental data would be desirable. The dielectric constant of the yttria-doped material is also described by the present model, again with no adjustable parameters. Several different features of the theory lead to its loss of validity in more concentrated mixtures. These are examined in detail.     

Oxidation of benzene on a vanadium-molybdenum catalyst in the presence of thiophene

The catalytic oxidation of benzene by air oxygen on a vanadium-molybdenum mixed oxide (1 ? x)V₂O₅ · xMoO₃ (x = 0.25) over a temperature range of 200–320°C is studied. It is shown that the introduction of small amounts of thiophene into benzene inhibits the oxidation to maleic anhydride in this temperature range. It is established that the operation of the catalyst is accompanied by significant changes in its phase composition and morphology, with a few first operation cycles being characterized by a high conversion of benzene. A possible mechanism of the process is proposed.     

Bestimmung von optischen Konstanten an Palladium und Silber aus Energieverlustmessungen im Energiebereich von 2 bis 90 eV

The energy loss functions Im (?1/?) of Silver and Palladium are determined from energy loss experiments with 60 keV electrons. The influence of surface effects and multiple losses is discussed and the values of Im (?1/?) are corrected for them. By means of Kramers-Kronig analysis the optical constants? ₁ and? ₂ are calculated from the energy loss function and found to be in good agreement with data derived from optical experiments by various authors.     

Energieverlustmessungen mit 60 keV Elektronen an amorphem und polykristallinem Selen und Tellur und Bestimmung optischer Konstanten

The energy loss functions Im—(1/?) of amorphous and polycrystalline Selenium and Tellurium are determined from energy loss spectra of 60 keV electrons in the energy range up to 30 eV. The optical constants ?₁ and ?₂ are calculated from the energy loss function by Kramers Kronig analysis. The energy difference in the position of the volume plasma loss of amorphous and polycrystalline foils is compared with the calculated change in free electron plasma energy resulting from density change. Characteristic structure dependence of the optical constants are found to be similar for Selenium and Tellurium. They are discussed in terms of the results of band structure calculations.     

Adsorption of hydrogen on a Pt(111) surface

The H₂/Pt(111) system has been studied with LEED, ELS, thermal desorption spectroscopy and contact potential measurements. At 150 K H₂ was found to adsorb with an initial sticking coefficient of about 0.1, yielding an atomic H:Pt ratio of about 0.8:1 at saturation. H₂/D₂ exchange experiments gave evidence that adsorption is completely dissociative. No exrea LEED spots due to adsorbed hydrogen were observed, but the adsorbate was found to strongly damp the secondary Bragg maxima in the I/V spectrum of the specular beam. The primary Bragg maxima were slightly increased in intensity and shifted to somewhat lower energy. A new characteristic electron energy loss at ?15.4 eV was recorded upon hydrogen adsorption. The thermal desorption spectra were characterized by a high temperature (β₂-) state desorbing with second order kinetics below 400 K and a low temperature (β₂-) state that fills up, in the main, after the first peak saturates. The β₂-state is associated with an activation energy for desorption E1 of 9.5 kcal/mole. The decrease E1 with increasing coverage and the formation of the β₁-state are interpreted in terms of a lateral interaction model. The anomalous structure in the thermal desorption spectra is attributed to domains of non-equilibrium configuration. The work function change Δ? was found to have a small positive maximum (～ 2 mV) at very low hydrogen doses (attributed to structural imperfections) and then to decrease continuously to a value of ?230 mV at saturation. The variation of Δ? with coverage is stronger than linear. The isosteric heats of adsorption as derived from adsorption isotherms recorded via Δ? compared well with the results of the analysis of the thermal desorption spectra.     

Reflection electron energy loss spectroscopy in Mn x Si1 − x composite structures

The dependences of the product of the electron inelastic mean free path λ and the differential cross section for inelastic electron scattering K on energy loss are determined from the experimental energy-loss spectra of the reflected electrons of Mn _x Si_{1 ? x} (0 ≤ x ≤ 1) composite structures. It is shown that the minimum values for the dependences of the product λK on the electron-energy loss can be used for the quantitative determination of element concentrations in this composite structure.     

Saturation recovery electron spin–lattice relaxation studies on benzene anion radical and its derivatives: application of Kivelson–Orbach mechanism of electron spin relaxation

The role of low-lying excited states on the spin–lattice relaxation times (T₁) of organic radicals has been investigated. To test the applicability of Kivelson's electric field fluctuation model (D. Kivelson, J. Chem. Phys. 45, 1324 (1966)), based on the Orbach mechanism of spin relaxation, the T₁s of the anion radicals of benzene, benzene-1-d, toluene, ethyl benzene, isopropyl benzene, t-butyl benzene, p-xylene, 1,2,4-trimethyl benzene and 1,3,5-trimethyl benzene in liquid solutions, with potassium cation as the counter ion, have been measured by the pulse saturation recovery technique. The energy gap between the ground and the first excited electronic states changed with the substitutions to different extent. The spin–lattice relaxation rates showed correlation with this energy gap. Anion radicals of benzene and benzene-1-d showed the shortest T₁ among the radicals studied here. A small but measurable energy splitting due to the deuterium substitution in benzene-1-d radical was obtained from the temperature dependence of T₁. Spin–lattice relaxation times of benzene anion measured here decreased monotonically in the range of ?60 to ?125 °C, in contrast to some reported claims of very unusual temperature dependence, based on the continuous wave microwave power saturation studies. Our results also showed that the ion pairing between benzene anion and potassium cation did not significantly influence the spin–lattice relaxation times.     

Cation sitting in aromatic cages: ab initio computational studies on tetramethylammonium–(benzene)n (n=3–4) complexes

Quantum chemistry study was performed on interaction between tetramethylammonium (TMA) and aromatic cages by means of the MP2 method to show how TMA sits in an aromatic cage that is composed of benzenes. The MP2 calculations on TMA–(benzene)_n complexes demonstrate that the more the benzene molecules in the aromatic cage, the stronger the binding strength between the cage and TMA. In details, the structure of TMA–(benzene)_n (n = 1–4) complexes can be easily constructed by superimposing n TMA‐benzene complexes via TMA, and the binding energies of the TMA–(benzene)_n complexes are the sum of the n corresponding TMA‐benzene systems. For instance, the distances between the N of TMA and the plane of the benzene ring are 4.238, 4.252, 4.264 ,and 4.276 Å, respectively, for TMA–(benzene)_n (n = 1–4) complexes, and the BSSE corrected binding energies at MP2/6‐311++G** level are ?8.8, ?17.3, ?25.8 and ?34.3 kcal/mol, respectively, for TMA– (benzene)_n (n = 1–4) complexes. Thus, this study provides us useful information on how a cation interacts with an aromatic cage in terms of complex geometry and binding strength. Copyright © 2007 John Wiley & Sons, Ltd.     

Angle-resolved photoelectron spectroscopy of benzene and hexafluorobenzene as a function of photon energy

Angle-resolved photoelectron spectra of benzene have been recorded using synchrotron radiation as the photon source. From these results the angular dis the first three orbitals; 1e_1g(π), 3e_2g (σ) and 1a_2u(π). Calculations of β employing the MS Xα method have also b and theory for the first two orbitals is excellent. Poor agreement in the core of the third orbital is believed to be due to overlap of the second and has been verified by data obtained for perfluorobenzene. The universality of the energy dependence of β for π orbitals, noted previously for unsatu aromatic systems. The importance of the results for benzene for the future application of angle-resolved photoelectron spectroscopy to complex molecule     

Comparative studies on CH⋯F− hydrogen bond formation in benzene and exocyclically substituted pentafulvene derivatives

Optimization of CH?F^? complexes of exo‐substituted pentafulvene and meta‐substituted and para‐substituted benzene (substituents: NMe₂, NHMe, NH₂, NHOH, OH, OMe, Br, Cl, F, Me, CCH, CF₃, CONH₂, COMe, CHO, NO₂, NO, and CN) have been performed at the density functional theory level by using Becke hybrid B3LYP functional with 6‐311++G(d,p) basis set. The acidity of the ring CH bond in benzene and fulvene are of similar magnitude, whereas the acidity of the fulvene exocyclic CH₂ group is significantly higher. Various properties based on the H?F^? hydrogen bond (bond length, electron density at BCP, and bond dissociation energy), and the whole molecule (HOMA, sEDA, pEDA, substituent active region, and substituent effect stabilization energy) were analyzed and compared between the fulvene and benzene systems. Sensitivity of the ring CH?F^? hydrogen bond and other substituent dependent properties to substituent effect is substantially greater in fulvene than that of benzene derivatives. In fulvene, the 3‐position is more sensitive than the 4‐position. The sEDA and pEDA parameters used to measure sigma‐electron and pi‐electron excess/deficiency of the ring are mutually correlated for the studied systems. Copyright © 2013 John Wiley & Sons, Ltd.     

Characteristics electron energy loss studies of molecular halogens adsorbed on Fe(100)

The electron energy loss spectra for several molecular Br₂ monolayers adsorbed on a chemisorbed bromine overlayer on Fe(1 0 0) show a sharp loss at about 3.0 to 3.8 eV. For one or more molecular layers of I₂ adsorbed on a chemisorbed iodine overlayer on Fe (1 0 0), a sharp electron loss feature is observed at 4.4 ± 0.2 eV. It is suggested that the electron energy losses for condensed Br₂ at 3.0 eV and for condensed I₂ at 4.4 eV are a result of a 1π_g to 2σ_u electron excitation.     

Resonant polaron coupling of the n = 1 Landau level and the 2p+ donor state in GaAs

The n = 0 → > n = 1 Landau level and 1s ?2p⁺ impurity transitions in GaAs were investigated up to energies above the optical phonon energy ?Ω_LO and d.c. magnetic fields up to 25 T. Pinning of both transitions to an energy slightly above and below ?Ω_LO was observed. At an energy very close to ?Ω_LO two additional impurity transitions are found. These features are attributed to the resonant polaron effect which leads to hybridization and dipole selection rule breakdown. Also the spin doublet splitting of both transitions were resolved showing a strong magnetic field dependence which can not be explained by nonparabolicity of the conduction band alone.     

K-shell excitation spectra of CO,N2 and O2

Electron energy loss spectra of CO, N₂ and O₂ have been recorded in the regions of carbon, nitrogen and oxygen K-shell excitation and ionisation. These results are compared to previous energy loss, photoabsorption and theoretical studies of the same spectral regions. Several inconsistencies in the published spectra are clarified in the present work. Comparisons with recent calculations of the K-shell continua of these molecules are presented. Vibrational structure in the K → π * transitions of CO (C 1s) and N₂ (N 1s) has been resolved in high-resolution studies (< 0.1 eV FWHM) of these species.     

Dielectric relaxation in vanadium phosphate glasses

An asymmetric distribution of relaxation times has been inferred from an increase in the Cole-Cole distribution parameter α with increasing values of ωτ in 62% v₂O₅–38% P₂O₂ glass. The conventional Debye type relaxation loss peaks in the frequency range 10²–10⁵ Hz are observed in this sample above 85°K. The extrapolated values of dielectric constant and relaxation time below 100°K seem unexpectedly large while the high temperature extrapolated values of ?' are close to ?_∞ as expected. Probably the conventional dielectric loss peaks are observed only above a critical temperature at which the carriers gain sufficient energy to be excited to the conduction band edge. Below this temperature hopping of carriers within kT of the Fermi level may dominate and conventional Debye type dielectric loss peaks may lose their significance as envisaged in the models of frequency dependent ac conductivity.     

Piezoelectric properties of new ternary Bi1/2(Na, Li)1/2TiO3–(Bi1/2K1/2)TiO3–Ba0.85Ca0.15Ti0.90Zr0.10O3 ceramics

0.852[Bi_1/2(Na_1?x Li _x )_1/2]TiO₃?C0.110(Bi_1/2K_1/2)TiO₃?C0.038Ba_0.85Ca_0.15Ti_0.90Zr_0.10O₃ (BNLT?CBKT?CBCTZ-x) new ternary piezoelectric ceramics were fabricated by the conventional solid-state method, and their piezoelectric properties as a function of the Li content were mainly investigated. A?stable solid solution with a single perovskite structure has been formed, and the depolarization temperature (T _d) of these ceramics was identified by using the temperature dependence of the dielectric loss. The T _d value of these ceramics gradually decreases, while the T _m value increases with increasing the Li content. The dielectric constant increases and the dielectric loss decreases with increasing the Li content, and an enhanced piezoelectric behavior of d ₃₃??223 pC/N and k _p??35.2?% has been demonstrated in these ceramics with x=0.06.     

Stability of the long-period stacking order in Mg3ln AND Mg3(In1−yCdy) alloys

The stability of the stacking ordered structures of close-packed layers in Mg-In alloys near Mg₃In and pseudo-binary alloys Mg₃(In_1?yCd_y) for 0 ? y ? 1 is studied from a standpoint based on the pseudopotential theory. An expression of the structure-dependent energy for an arbitrary type of layer stacking is given by the method described in the previous papers [1,2]. The numerical results explain well the observed trends in the composition and pressure dependences of stacking sequence; the orders of appearance of stacking variants are 3R → 12R → 2H with an increasing Mg-content for Mg-In alloys near Mg₃In, 12R → 18R → 2H with an increasing Cd-content for Mg₃(In_1?yCd_y) and 12R → 18R → 24R with an increasing pressure for Mg₃ln. Speaking in detail, however, the present calculation fails to reproduce a 12R-structure (β₁-phase) at 25% In in the Mg-In system, an 18R-structure of Mg₃(In_0.65Cd_0.35) and an 18R-structure of Mg₃In found under pressure of 20–55 kb, although the energy differences are small in all the cases. From the analysis of components of the structure-dependent energy, it is concluded that the favorable type of layer stacking is determined predominantly by a contribution from the band-structure energy term of a state with the disordered arrangement of constituent atoms and also that an energy required to the alternation of stacking of layers is apparently smaller than the ordering energy. If a specific layer sequence is characterized by a hexagonality, the alloys vary their sequences in such a way that the hexagonality increases from zero (3R) to unity (2H) as the electron-to-atom ratio of the alloys decreases from 2·3 to 2·0.The Ewald and Madelung constants are calculated for various types of layer structure. Both constants change linearly with hexagonality, without regard to period, symmetry and layer sequence.     

Intensities and satellites of 4ƒ-photoelectrons in thorium and uranium dioxides

In a comparison between photoelectron-spectroscopic intensities and theoretical orbital ionization cross-sections, it is found that whereas the intensities and cross-sections agree for the ratio of Th(4?) to O(1s) in ThO₂, the ratio for U(4?) to O(1s) in UO₂ differs from the theoretical ratio by 36%. It is shown that the deviation is attributable to the loss of U(4?) photoelectrons through the degradation of their kinetic energies into the tail on the high binding energy side of the spectrum. The results for ThO₂ imply a linear dependence of the inelastic scattering mean free path on kinetic energy; the results for UO₂ imply extensive configuration interaction between the 4? core-hole and unoccupied states of the cation, and they demonstrate the use of theoretical cross-sections to determine the extent.     

VSe2−xSx materials as battery cathode

The properties of the solid solution VSe_2?xS_x 0 ? x_nom ? 2 have been investigated for secondary battery application. The phase VSe₂ is observed for 0 ? x_nom ? 1.2 and the phase V₅S₈ is found using RX analysis for x_nom >1.2. The amount of lithium chemically incorporated in this structure by reaction with n-butyllithium is 2 Li/vanadium for 0 ? x_nom ? 0.8 and 1.4 Li/vanadium for V₅S₈. An electrochemical technique (galvanostatic) indicates that the amount of lithium incorporated depends on the x_nom values, the grain size and the discharge rate. The best results are obtained for 0.2 ? x_nom ? 0.6 (capacity = 164?172 Ah kg^?1 and energy density = 385?465 Wh kg^?1).