Benzene adsorption on nickel (100) and (111) faces studied by leed and high resolution electron energy loss spectroscopy

Studies of benzene (C₆H₆ and C₆D₆) adsorption have been performed by high resolution electron energy loss spectroscopy (HRELS) and LEED experiments on nickel (100) and (111) single crystal faces at room temperature. Chemisorption induces ordered structures, c(4 × 4) on Ni(100) and (2√3 × 2√3)R30° on Ni(111), and typical energy loss spectra with 4 loss peaks accurately identified with the strongest infrared vibration bands of the gazeous molecules. Benzene chemisorption preserves the aromatic character of the molecule and involves respectively 8 nickel surface atoms on the (100) face and 12 on the (111) face by adsorbed molecule. The interaction takes place via the π electrons of the ring. Significant shifts of the CHτ bending and CH stretching vibrations show a weakening of the CH bonds due to the formation of the chemisorption bond and a coupling of H atoms with the nickel substrate.     

Ab initio study of phase stability,thermodynamic and elastic properties of C3N2 derived from cubic C20

In this work, we report a quite different conclusion from Tian et al. [Phys. Rev. B 78 (2008) 235431]. It is proved that β-C₃N₂ is the only phase under high pressure, and α-C₃N₂ does not exist. β-C₃N₂ is a covalent crystal composed of strong CC and CN covalent bonds. Band gap of β-C₃N₂ increases with pressure. The width of antibonding state, shown in partial density of states (PDOS), keeps about 5 eV with rising pressures, which brings stable CN or CC covalent bonds. At sufficiently low temperatures, heat capacity (C_v) is proportional to T³; and at intermediate temperatures, C_v is governed by the details of vibrations of the atoms; finally, C_v reaches to β-C₃N₂'s Dulong–Pettit limit (about 120 J/mol K). Though thermal expansion coefficient (α) increases with temperature, α is less than 1×10⁻⁵ K⁻¹. Elastic constants rise with pressure, but shear moduli is quite steady which increases just a little with pressures.     

Highlights of the beam energy scan from STAR

The first part of the beam energy scan (BES) program at RHIC was successfully completed in the years 2010 and 2011. First STAR results from particle yield measurements are in good agreement with previously published data from SPS and AGS experiments whereas other results like azimuthal HBT and K/?? event-by-event fluctuations differ at some energies. In addition, new observations like the centrality dependence of chemical freeze-out parameters (T _ch and ?? _B ) or the smoothly increasing difference with decreasing energy in the elliptic flow v ₂ between particles and corresponding anti-particles, are discussed.     

Photoelectron spectra of ethylene and of the six deuterated derivatives

The photoelectron spectra of C₂H₄ and of six deuterated molecules of ethylene — C₂D₄, C₂D₃H, C₂H₃D, cis-C₂H₂D₂, trans-C₂H₂D₂ and gem-C₂H₂D₂ — have been recorded with the 584-Å resonance line of He. The adiabatic ionization potentials of the X²B_3u and the ²B_3g states of the seven isotopic components have been determined with an accuracy of about 7 meV. The ionization potentials of the other excited electronic states have been measured with a lower accuracy owing to a less well defined onset. The measured ionization potentials of C₂H₄ are 10.514 eV, 12.431 eV, 14.4₃ eV, 15.7₄ eV and 18.7 eV. The vibrational structure of the first electronic band shows that the two normal modes ν₂ (symmetric CC stretching) and ν₃ (symmetric HCH bending) are excited simultaneously. The measured vibrational frequencies show no abnormal isotopic effect if the assignment given in the literature for the ν₂ and ν₃ modes in C₂H₄⁺ are reversed and if a stronger excitation of the ν₃ symmetric bending mode in the least deuterated compounds is assumed. The vibrational modes most strongly excited in the second and third electronic bands are ν₁ (symmetric CH stretching) and ν₃, and in the fourth band ν₃.     

Vibrational spectra of ammonia chemisorbed on platinum (111): II. The electron scattering mechanism

We have measured the angle and energy dependence of the inelastic cross sections for NH₃ chemisorbed on Pt(111). The relative contributions of dipole and nondipole processes in the on-specular intensities were determined. The angular dependence (off-specular scattering) was found to be more useful than the impact energy dependence for discrimination between the two scattering mechanisms. For both molecular states of NH₃/Pt(111), strong impact scattering contributions were found in the NH stretching mode intensities. One particular mode, the degenerate stretch [ν_d(NH)] was selectively excited by impact scattering. The energy dependence showed an enhancement of all of the inelastic intensities at low impact energies (1 eV). This enhancement was found to be a result of a surface reflectivity increase, coupled with an increase in the inelastic cross section. The latter increase is not predicted by dipole theory. Determination of the molecular orientation was not possible, due to the complex contributions of dipole and impact scattering.     

Structure and bonding of hydrocarbon plasma generated carbon films: An electron energy loss study

Electron energy loss spectroscopy has been used to study the structure and the bonding in hard amorphous carbon films (aC:H) prepared by plasma decomposition of benzene. EXAFS spectra indicate an amorphous and not a microcrystalline structure for aC:H films. The near-edge structure of the carbon K-amsorption edge reveals a considerable amount of π bonding in the aC:H films. The optical constants were derived by a Kramers-Kronig analysis of the loss functions for energies less than 40 eV. From the sum rule for ε₂, we derive that $\text{1}\text{3}$ of the carbon atoms are trigonally bonded and 273 of the carbon atoms are tetrahedrally bonded.     

Low energy excitations in KMnO4 with 1 eV<=ΔE<=10 eV from electron-energy loss spectroscopy

Electron-energy loss spectra of potassium permanganate (KMnO₄) with primary electron energies 25 eV<=E₀<=500 eV show 7 peaks in the energy-loss range 1 eV<=ΔE<=10eV and are successfully analysed with a superposition of 7 independent Gaussians. The intensity of these lines follow roughly a power-law dependence on the primary energy I∝E ₀ ^-α . There are two groups of lines, the first with an exponent α≈0.5, while the lines in the second group decay much stronger with increasingE ₀ corresponding to a value 0.9<=α<=1.3. The 4 lines in the first group are identified as dipole allowed transitions by comparison to recent first principle molecular-orbital calculations for the (MnO₄) molecule by H. Nakai et al. The dipole-allowed excitation spectrum obtained from this analysis agrees very well with these first principle calculations.     

PdCO分子结构与势能函数

用密度泛函理论的B3LYP方法,对钯原子采用LANL2DZ收缩价基函数,碳原子和氧原子采用AUG-cc-pVTZ基组,对PdC,PdO和PdCO体系的结构进行优化,计算表明：PdC分子基态为¹Σ⁺态,键长为R_e=0.17285nm,离解能为4.919eV.PdO分子基态的平衡核间距为0.18546nm,其电子态为³Π,离解能为2.455eV,并拟合得到Murrell-Sorbie势能函数;PdCO分子有两 关键词： PdCO 分子结构 势能函数     

Microwave spectrum of 2-propene-1-imine,CH2CHCHNH

The reactive species, 2-propene-1-imine, has been identified by its microwave spectrum as a pyrolysis product of diallylamine vapor (100 mTorr, 400°C). Two entirely planar forms are observed, both with an “s-trans” CCCN configuration. The lower energy rotamer has an “anti” CCNH configuration, with $\text{r}\text{CC = 1.453(3)}\text{A}\text{?}$, $\text{r}\text{CC = 1.336(4)}\text{A}\text{?}$, and ∠ CCC = 122.9(3)° and a dipole moment of 2.01(2) D with 1.13(1) D and 1.66(2) D “a” and “b” components. The higher energy rotamer has a “syn” CCNH configuration and a dipole moment of 2.51(2) D with 2.39(2) and 0.77(3) D the “a” and “b” components. From relative intensity measurements, the ground state energy difference is determined to be 0.9 ± 0.1 kcal mole^?1.     

Einfluß der Kristallorientierung auf die Plasmonenintensität in Ag und Si

In energy loss measurements of electrons on monocrystals of silver and silicon in transmission the intensity of the volume plasma loss (Ag 3.8 eV, Si 16.6 eV) is found to be dependent on the crystal orientation. By tilting the crystal relatively to the incident beam (in general some degrees) the intensities of the elastic peak (I _el), the volume plasma peak (I _v) and in Ag additionally that of the surface loss (I _s) at 3.1 eV (damped by a carboncoating) show different variations.I _s in Ag andI _v in thick (>150 nm) Si is as expected proportional toI _el, whereasI _v in Ag and thin Si varies much stronger thanI _el, so that the quotientI _v/I_el is not a constant. Presumably an explanation of this phenomenon lies in the dynamic theory of electron diffraction including inelastic processes.     

Variations of the local density of electron states and short-range order in amorphous hydrated silicon films

Ultrasoft x-ray spectroscopy methods have been used to observed a change in the energy distribution of the silicon valence states after annealing a-Si:H films at 500 °C. This change appears as three distinct maxima in the density of states 3.5, 7.2, and 10.2 eV above the top of the valence band, which indicates ordering of the a-Si:H structural network. The energy distance between the latter two maxima (E?E _v=7.2 and 10.2 eV) supports electron-diffraction data indicating a decrease in the silicon-silicon interatomic distance by 0.2 Å in comparison with the crystal. The presence of a third maximum (E?E _v=3.5 eV) is connected with the change in the hybridization of the s-p-functions of silicon with decrease of the coordination number.     

Investigation of surface self-diffusion of Ni atoms on the Ni(100) plane by means of work function measurements and Monte Carlo calculations

A novel method is presented to study migration of adsorbed metal atoms on a clean (hkl) surface of a metal substrate. The system used was Ni on Ni(100). We make use of the assumption that each single step of random walk motion of an individual adatom on the metal substrate refers to an activation energy ΔE_if (indices i and f giving the number of occupied nearest neighbour sites in the initial and final position respectively). Furthermore, we assume that the dipole moment p_v of an adatom is determined by the number v of occupied nearest neighbour sites. Via their dipole moments the adsorbed atoms induce a change of work function ΔΦ of the substrate metal, ΔΦ being related to the average dipole moment per unit area by the Helmholtz equation. To measure ΔΦ we use a pendulum device. At fixed low coverage θ the variation of ΔΦ with temperature T has been calculated numerically within the framework of a Monte Carlo simulation (MCS) model, using various activation energies ΔE_if and dipole moments p_v as input parameters. By fitting these data to the experimental curves we could derive the following dipole moments and activation energies: p₀ = 0.45 ± 0.05 D, p₁ = 0.36 ± 0.05 D, p₂ = 0.27 ± 0.05 D, p₃ = 0.18 ± 0.05 D, ΔE₀₀ 0.60 ± 0.02 eV, ΔE₁₀ = 0.70 ± 0.025 eV, ΔE₂₁ = 0.80 ± 0.05 eV, ΔE₂₀ = 0.90 ± 0.05 eV. We compare these results with those of other workers.     

Ion-electron coincidence study of the thermal He(23S) + H2 Penning reaction

A new Penning-electron-Penning-ion coincidence method is described. It is applied to the study of the thermal reaction of He(2³S) with H₂. The main results reported are separate electron energy spectra that are coincident with the three different ions formed: HeH₂⁺, HeH⁺ and H₂⁺. Based on these results it is shown that the Penning reaction of the He(2³S)/H ₂ system proceeds in two well-separated steps: (i) ionization at distances R (HeH₂) ? 6a₀ in which H₂⁺ (v) is formed in different vibrational states; and (ii) reactive collision of H₂⁺ (v) with He. For the second step the variation of the branching ratios with vibrational quantum numbers v = 0 to v = 10 is derived, and it is shown that these branching ratios may be regarded as relative vibrational-energy-dependent cross-sections for the collision of H₂⁺ (v) with He at an average relative kinetic energy of ～20 meV.     

Surface-enhanced Raman spectroscopy of electrochemically characterized interfaces; potential dependence of Raman spectra for thiocyanate at silver electrodes

Surface-Enhanced Raman Spectra have been obtained for thiocyanate anions at silver electrodes following an oxidation-reduction cycle (ORC) as a function of electrode potential and electrolyte composition and compared with the extent of thiocyanate adsorption determined under the same conditions from differential capacitance-potential data. A spectrograph equipped with an optical multichannel analyzer (OMA) detector and a scanning spectrometer were used to make the Raman measurements. Spectra were obtained over the frequency range 100–2200 cm^?1, where all three normal vibrations, CN stretching (v_CN, 2090–2120 cm^?1), CS stretching (v_CN, 735 cm^?1), NCS angle bending (δ_NCS, 450 cm^?1) occur, along with a low frequency vibration attributed to a metal-ligand stretching mode (v_ml, 200–215 cm^?1) arising from a silver-sulfur surface bond. Both v_CN and v_ML decreased in frequency as well as intensity as the potential was made more negative in the region ?100 to ?700 mV versus Ag/AgCl for bulk thiocyanate concentrations of one millimolar and above, even though the thiocyanate surface concentration remained close to a monolayer throughout. By means of rapid time-resolved spectral measurements following potential steps using the OMA, the decrease of the intensity and frequency of the v_CN mode with increasing negative electrode potential was separated into a rapid “reversible” component and a slower “irreversible” decay. The latter component is attributed to the decay of Raman-active sites associated with the dissipation of metastable silver clusters formed during the ORC, that are prevented from rearranging at more positive potentials due to the presence of surrounding anionic adsorbate.     

Low energy excitations in KMnO4 with 1 eV<=ΔE<=10 eV from electron-energy loss spectroscopy

Electron-energy loss spectra of potassium permanganate (KMnO₄) with primary electron energies 25 eV<=E₀<=500 eV show 7 peaks in the energy-loss range 1 eV<=ΔE<=10eV and are successfully analysed with a superposition of 7 independent Gaussians. The intensity of these lines follow roughly a power-law dependence on the primary energy I∝E ₀ ^-α . There are two groups of lines, the first with an exponent α≈0.5, while the lines in the second group decay much stronger with increasingE ₀ corresponding to a value 0.9<=α<=1.3. The 4 lines in the first group are identified as dipole allowed transitions by comparison to recent first principle molecular-orbital calculations for the (MnO₄) molecule by H. Nakai et al. The dipole-allowed excitation spectrum obtained from this analysis agrees very well with these first principle calculations.     

New simple pulse sequences for distinguishing between CH3 and CH signals in 1H-noise-decoupled 13C NMR spectra

Combined pulse sequences are developed for partial suppressing of the ¹J(¹³C¹H) dependence of the APT measured intensities of ¹³C signals. The sequences contain one or several time periods with different lengths τ without irradiation by the decoupler field. Consequently the relative intensities of CH₃ and CH signals have unequivocally separated fields of values for the range of expected coupling constants.     

Low energy resonances in21Ne(p,γ)22Na examined with a high energy resolution ion beam

TheE _R=126, 272 and 291 keV resonances in the²¹Ne(p, γ)²²Na reaction have been investigated with a high-energy-resolution ion beam. TheE _R=272 keV resonance was found to consist of two states separated by (888+5) eV, where the lower (higher) energy member is a high-spin (low-spin) state. All four resonances have widths less than a few eV, which is an improvement of nearly two orders of magnitude below previously reported limits. The influence of atomic effects on the determination of the correct value for the resonance energy is examined.     

Internal hydrogen bond,torsional motion,and molecular properties of 2-methoxyethylamine by microwave spectroscopy: Methyl barrier to internal rotation for 2-methoxyethanol

The microwave spectra of four substituted isotopic species of 2-methoxyethylamine (NH₂, NHD, NDH, ND₂) have been assigned. The molecule is found to exist in a gauche form with an intramolecular hydrogen bond of the NH?O type. The four possible sets of the amino hydrogen r_s corrdinates give different H?H distances, probably because the -NH₂ group is involved in large amplitude vibrations and because of changes in the heavy atom positions arising from the deuteration of the hydrogen bond. For the most abundant species many vibrational states have been analyzed and assigned to the two possible CO torsions in the molecule. A value V₃ = 3150 ± 50 cal/mol was found for the methyl torsional barrier and V₁ = 9 ± 3 kcal/mol for the other CO torsional barrier. A third set of observed vibrational satellites is probably assignable to the CC torsion. The determination of the dipole moment and of the quadrupole coupling constants gave values which were not in good agreement with those predicted from nonhydrogen bonded molecules. In addition a value V₃ = 3100 ± 100 cal/mol was calculated for the CH₃ torsional barrier in the related 2-methoxyethanol, using previous experimental data (Canad. J. Chem.50, 1149–1156 (1972)).     

The microwave spectrum,substitution structure and dipole moment of cyanobutadiyne,HCCCCCN

Cyanobutadiyne (cyanodiacetylene), HCCCCCN, is sufficiently stable at low pressures to permit its rotational spectrum to be studied by microwave spectroscopy. The spectrum consists of a series of R-branch transitions typical of a linear molecule. The transitions with J = 9 to 14 which lie between 26.5 and 40.0 GHz have been measured for the vibrational ground state. Transitions have also been detected in natural abundance for all possible singly substituted ¹³C and ¹⁵N isotopic species. Deuteriated cyanobutadiyne, DCCCCCN, has also been synthesized and its ground state spectrum recorded. These measurements have enabled a complete substitution structure to be derived for the first time for a polyacetylene: r₈(HC_a) = 1.0569 ± 0.001, r₈(C_aC_b) = 1.2087 ± 0.001, r₈(C_bC_c) = 1.3623 ± 0.003, r₈(C_cC_d) = 1.2223 ± 0.004, r₈(C_dC_e) = 1.3636 ± 0.003, $\text{r}{}_8\text{(}\text{C}{}_{\mathrm{e}}\text{}\text{N}\text{) = 1.1606 ± 0.001}\text{A}\text{?}\text{(10}{}^{\mathrm{?10}}\text{m}\text{)}$. The spectroscopic parameters for the ground state are B₀ = 1331.3313 ± 0.001 MHz and D₀ = 0.0257 ± 0.002 KHz. The dipole moment, determined from the Stark effects of the J = 9 and 10 lines, is 4.33 ± 0.03 Debye.