Formation of SF5 in electron attachment to SF6; swarm and beam results reconciled

Based on the results of recent swarm experiments, it has been proposed that the increase in the cross section for SF₅⁻ formation observed at an electron energy, E_e of about 0.3 eV in electron beam studies of electron attachment to SF₆ is due to the combined (opposing) effects of the vibrational heating of the molecule by the attached electron, which enhances the dissociation of the nascent (SF₆⁻)^* ion, and the reduction of the cross section for capture (s-wave) of the electron by SF₆ with increasing E_e. Further, it has been shown that the dissociation reaction is endothermic by 0.12 eV, and that, contrary to previous suggestions, there is no potential barrier to this dissociation reaction. Now we have carried out electron beam studies of the SF₆ attachment reaction in Berlin at gas temperatures, T_g, over the range 300 to 920 K and in Innsbruck at T_g below 300 K. These studies have provided support for the above proposals concerning the appearance of the SF₅⁻ peak and for a reaction endothermicity of 0.12 eV. Thus these studies have clarified the doubts about the products of the SF₆ attachment reaction at low electron energy.     

Comparative study of isotope and chemical effects on the exciton states in LiH crystals

Scientific interest, technological promise and increased availability of highly enriched isotope have led to a sharp rise in the number of experimental and theoretical studies with isotopically controlled crystals. Isotope pure compounds are really the material of future mankind. LiH has a giant isotope effect. Therefore, this review in the first step is devoted to some peculiarities of exciton states in isotope pure and mixed crystals of LiH. Excitons are the energetically lowest excitations of the electronic system in an ideal, crystallized insulator (semiconductor) at zero temperature. It is a collective excitation which has the full translational symmetry of the crystal lattice. For the first time a systematic analysis of experimental results is presented of isotopic and chemical effects on the exciton states observed in LiH_xD_1−x crystals of various isotopic (and chemical) composition (0≤x≤1) using low temperature optical and luminescence spectroscopy. LiH (LiD) is an direct band-gap material with an energy gap 4.992 (5.095) eV at low temperature. Substituting a light isotope with a heavy one (or H→F) increases the interband transition energy (E_g) and the binding energy (E_b) of the Wannier–Mott exciton as well as the magnitude of the longitudinal–transverse splitting. The nonlinear variation of E_g, E_b with the isotope (or F) concentration is due to the compositional disordering of the crystal lattice and is consist with the concentration dependence of line half-width in exciton reflection and luminescence spectra. The free exciton luminescence spectrum of the LiH (LiH_xD_1−x, LiH_xF_1−x; 0≤x≤1) crystals under optical (X-ray) excitations consists of a narrow zero-phonon line and its more wider 5LO replicas. At 100 % substitution of hydrogen by deuterium the energy shift of the maximum of zero-phonon line is the following: ΔE=E_n=1s(LiD)−E_n=1s(LiH)=95 meV. The shift of the emission line maximum of 2LO replica overlaps the energetical interval of ≤200meV. The nonlinear dependence of the free exciton luminescence (especially LiH_xF_1−x (LiD_xF_1−x)) intensity on the excitation density allows to consider these crystals as potential solid state lasers in the UV part of spectrum. It is shown that potential fluctuation due to compositional disorder of alloy have a strong effect on both the exciton broadening and the band-gap energy shift. The review closes with a brief discussion of the present and future applications of these crystals.     

Observation Of Mode Selective Vibrational Excitation Of SF6 By Collisions With 4–10 eV Li And H Ions

Structured time of flight spectra of both Li⁺ and H⁺ ions scattered from ground state SF₆, have been measured at angles (5.0° v_c.m. 16.6°) less than the rainbow angle at E_c.m., = 4.3 eV and 9.7 eV, respectively. The structure can be arttributed to vibrational excitation of the v₃ mode by H⁺ and excitation of the v₄ mode by Li⁺. The relative transition probabilities obey a Poisson distribution and can be explained by a simple forced oscillator model.     

Br2 in an ar matrix: an example of complete damping of the resonance raman scattering amplitude in the discrete resonance limit

Raman scattering and relaxed fluorescence is observed upon cw laser excitation resonant with the lower vibrational manifold of the X(¹Σ_o+u) → B(³Π_o+u) transition of matrix isolated Br₂ at liquid He temperatures. The excitation profile of the relaxed fluorescence maps out the resonances, but neither detectable enhancement of Raman scattering nor resonance fluorescence is observed.     

Raman spectroscopy on high-pressure fluids of molecular oxygen and nitrogen

High-pressure Raman scattering experiments of O₂ and N₂ were carried out at 296 K and at pressures up to 10 and 6 GPa, respectively, and the pressure dependence of vibrational spectra of O₂ was studied on the basis of molecular interactions in comparison with N₂. An appreciable contribution of the attractive interaction to the vibrational frequency was observed for fluid O₂. The linewidth of the spectra for fluid O₂ showed a sharp increase with increasing pressure, and the fluid–solid transition was observed at 5.9 GPa with a sudden drop of the linewidth.     

Theoretical studies on the topographical features and energetics of diacetylene-hydrogen fluoride complexes

Ab initio supermolecular SCF calculations have been carried out on diacetylene-HF complexes at the STO-4-31G level. The reverse σ (Rσ) complex has been found to have the lowest energy. Of the two π complexes. T and L, the symmetrical one T is found to be energetically less stable than the asymmetrical one L. Theoretical vibrational analysis tends to support this stability order. Electrostatic interaction energy calculations also lead to an almost identical sequence. Hydrogen bond energies corrected for basis set superposition error indicate that ΔE_H (Rσ) > ΔE_H (Tπ) ≈ ΔE_H (Lπ).     

Enthalpies of transfer of several ions from water to water-n-propanol mixtures at 298.15 K

Measurements were made of the dissolution heats of NaBPh₄ and Ph₄PCl in water-n-propanol mixtures over the whole range of compositions. Assuming the equality of ΔH_tr+(Ph₄P⁺) and ΔH_tr+(BPh⁻₄), the transfer enthalpies of several ions from water to water-n-propanol mixtures at 298.15 K were calculated.     

Neutral scattering and vibrational excitation in K+Br2 Collisions

Differential cross sections are presented for neutral scattering of K atoms in collisions with Br₂ molecules in the energy range from 20 to 150 eV. In addition energy-loss spectra for the scattered K atoms are shown. The differential cross sections show a large peak near the forward direction. The energy-loss spectra point to considerable vibrational excitation at small angles. The results are attributed to reneutralization from an ion-pair state formed during the collision. In some cases this process can involve three potential surface crossings. The experimental results can be reproduced in simple trajectory calculations on diabatic potential surfaces. The calculations show that the forward scattering is rainbow scattering, caused by the internal motion of the Br₂^? molecular ion during the collision. There is no analog to this rainbow in atom-atom scattering. The internal moti is also responsible for the observed vibrational excitation.     

Direct dynamics study of <Emphasis Type="Italic">N</Emphasis>-protonated diglycine surface-induced dissociation. Influence of collision energy

A quantum mechanical and molecular mechanical (QM + MM) direct dynamics classical trajectory simulation is used to study energy transfer and fragmentation in the surface-induced dissociation (SID) of N-protonated diglycine, (gly)₂H⁺. The peptide ion collides with the hydrogenated diamond {111} surface. The Austin Model 1 (AM1) semiempirical electronic structure theory is used for the (gly)₂H⁺ intramolecular potential and molecular mechanical functions are used for the diamond surface potential and peptide/surface intermolecular potential. The simulations are performed at collision energies E_i of 30, 50, 70, and 100 eV and collision angle of 0° (perpendicular to the surface). The percent energy transfer to the peptide ion is nearly independent of E_i, while energy transfer to the surface increases with increase in E_i. A smaller percent of the energy remains in peptide translation as E_i is increased. These trends in energy transfer are consistent with previous trajectory simulations of SID. At each E_i the most likely initial pathway leading to fragmentation is rupture of the ⁺H₃NCH₂---CONHCH₂COOH bond. Fragmentation occurs by two general mechanisms. One is the traditional Rice-Ramsperger-Kassel-Marcus (RRKM) model in which the peptide ion is activated by its collision with the surface, “bounces off”, and then dissociates after undergoing intramolecular vibrational energy redistribution (IVR). The other mechanism is shattering in which the ion fragments as it collides with the surface. Shattering is the origin of the large increase in number of product channels with increase in E_i, i.e., 6 at 30 eV, but 59 at 100 eV. Shattering becomes the dominant dissociation mechanism at high E_i.     

Electron energy loss studies of room-temperature co adsorption on Cu(100) induced by electron or ion irradiation

A custom-built multi-technique portable spectrometer was used to study the vibrational (and electronic) excitations associated with the “anomalous” CO adsorption recently observed on clean and oxidized Cu(100) surfaces at room temperature. Results from the electron energy loss (and thermal desorption) experiments have provided strong evidence for in-situ CO production induced by low-energy electron or ion irradiation of CO or C₂H₄ on Cu(100). In particular, the C-O vibrational stretch frequency for the room-temperature CO adsorption was found to be red-shifted by 9 meV from its nominal position (259 meV) and could be observed even at temperature as high as 420 K. Several plausible mechanisms involving coadsorbate interactions with CO on metal surfaces have been discussed. A direct interaction model involving partial bonding between CO, adsorbed on an atop site, and a coadsorbed O atom in a four-fold hollow site was found to be adequate in explaining the observed red shift and the apparent stabilization of CO on Cu(100) at room temperature.     

The optical properties of polymethylmethacrylate polymer dispersed liquid crystals

The effect of the addition of polymer liquid crystals as dispersed molecules to polymethylmethacrylate (PMMA) on the optical properties in the UV-visible and near infrared regions is investigated. From transmission, absorption and reflection spectra the absorption coefficient (ω) and refractive index (n) at angular frequency of radiation (ω) have been calculated at room temperature. The values of the optical band gap (E_opt) have been obtained from the direct allowed transitions in k-space. The width of the tails of localized states in the band gap (ΔE) was evaluated from Urbach edges. Both the parameters (E_opt) and (ΔE) vary with the mixing ratio of dispersed liquid crystals.     

Hot luminescence study of the intramolecular thermal bath of aromatics

We report a hot luminescence study of the vibrational relaxation in the S₁ state of anthracene and perylene in various solid matrices at 4.2 K. A set of relaxational data is evaluated and presented. The need of an external thermal bath at the initial stage of relaxation of these molecules is discussed. Some problems of distinction of resonant secondary emission components are illustrated by the mixed crystal spectra. A novel approach to the separation of scattering and hot luminescence in strongly inhomogeneous spectra at resonant excitation is introduced.     

A simple method for calculating enantiomer ratio and equilibrium constants in biocatalytic resolutions

A computer programme for determination of equilibrium constant (K) and enantiomer ratio (E) in biocatalytic resolutions has been developed. The programme utilises experimental data, ee_s and ee_p measured at more than one conversion, and determines both K and E no matter whether the reaction is irreversible (K=0) or reversible (K>0). An estimation of errors in the calculations indicates that errors in E does not show a Gaussian distribution, while errors in K does. The usefulness of the programme has been tested in a lipase-catalysed transesterification of 1-phenoxy-2-propanol at various concentrations of acyl donor, with different solvents and at different water activities.     

Angular distribution of different vibrational components of the X and B states reached after resonant Auger decay of core-excited H2O: experiment and theory

Vibrationally resolved spectra have been obtained for the lowest-lying cationic states X (2)B(1), A (2)A(1), and B (2)B(2) of the water molecule reached after participator resonant Auger decay of core-excited states. The angular distribution has been measured of the first four vibrational components of the X state in the photon energy regions including the O 1s-->4a(1) and the O 1s-->2b(2) core excitations, and for different portions of the vibrational envelope of the B state in the photon energy region including the O 1s-->2b(2) core excitation. For the X state, a large relative spread in beta values of the different vibrational components is observed across both resonances. For the B state, a very different trend is observed for the high binding energy side and the low binding energy side of the related spectral feature as a function of photon energy. A theoretical method based on the scattering K matrix has been used to calculate both the photoabsorption spectrum and the beta values, by taking both interference between direct and resonant photoemission and vibrational/lifetime interference into account. The numerical results show qualitative agreement with the trends detected in the experimental values and explain the conspicuous variations of the beta values primarily in terms of coupling between direct and resonant photoemission by interaction terms of different sign for different final vibrational states.     

Investigation of relaxations in polystyrene–polyoxyethylene copolymer by thermally stimulated current

The diblock copolymer containing 43.5 wt.% of polystyrene (PS) (M_w=15 000) and polyoxyethylene (POE) (M_w=19 500) was investigated by integral and by partial thermally stimulated depolarization current (TSDC) measurements in the temperature range of 173–293 K. The first peak P₁, widely distributed from 183 to 213 K, was a dipolar peak attributed to the initial stage of glass transition in POE. The second peak P₂, with a maximum at 273 K, was due to glass transition. The third peak P₃, with a maximum at about 263 K, was caused by the space charge relaxation localized at the interfaces between the ordered and nonordered phases in POE, Maxwell–Wagner relaxation, representing a precursor in melting of POE crystals. The results were supported by determination of activation energy E_a of the partial peaks, which covered the whole range of investigation, and by differential scanning calorimetry (DSC) measurements. The E_a(T) distribution showed a maximum in entropy change at 219±4 K. Some runs with samples containing different amounts of the PS component were considered. All the relaxations were coming from the POE block. The current due to PS slightly overlapped the total spectra without interaction. The influence of the repeated runs was observed and discussed.     

Rainbow scattering for Ar + Ar and Xe + Xe

Elastic differential scattering measurements have been performed on Ar⁺ + Ar and Xe⁺ + Xe. The rainbow scattering angle is found at τ = Eθ ≈ 115 eV deg for Ar⁺₂ and τ ≈ 93 eV deg for Xe⁺₂. These data are consistent with a potential well depth of 1.25 eV for Ar⁺₂ and 0.97 eV for Xe⁺₂.     

计时电量法求NiCl2(bpy)3在DMF中的扩散系数和速率常数

讨论了NiCl₂(bpy)₃(bpy：2,2-联吡啶)在DMF中的电化学行为. 控制电位使电极过程处于扩散控制下, 采用计时电量法求得了29 ℃时NiCl₂(bpy)₃在DMF中的扩散系数为5.99×10^－6 cm²•s^-1, 不同温度下的扩散系数随温度升高而增大. 选择合适的电极电位, 使电极过程处于扩散和电化学混合控制下, 采用计时电量法求得了不同电极电位下的反应速率常数k_f, 以及不同温度下的标准速率常数k⁰, 求得了表观活化能为14.4 kJ•mol^-1.     

Theoretical and experimental study of the biamperometry for irreversible redox couple in flow system

The biamperometry for the direct determination of irreversible redox analytes in flow system has been proposed based on coupling two independent and irreversible couples to form the biamperometric detection scheme. In this work, the method is studied both theoretically and experimentally. Equations describing the current–voltage characteristics and the current–concentration relationship are presented. The influence of the applied potential difference (ΔE) and the half-wave potential difference (ΔE_1/2) between two irreversible couples on the method are discussed. It shows that small ΔE_1/2 is favorable to construct the biamperometric detection system and to achieve high sensitivity and selectivity. Increasing ΔE leads to an increase in sensitivity. This is, however, accompanied by a decrease in selectivity and signal-to-noise ratio. To construct the biamperometric scheme for the irreversible systems with large ΔE_1/2, two approaches, adjusting acidity of supporting electroyte or adding new irreversible couple, are proposed by taking uric acid/platinum oxide and phenol/permanganate systems as examples. Uric acid and phenol are, respectively, detected in a flow injection system with a biamperometric detector.     

Polymer surface and thin film vibrational dynamics of poly(methyl methacrylate), polybutadiene, and polystyrene

Inelastic helium atom scattering has been used to investigate the vibrational dynamics at the polymer vacuum interface of poly(methyl methacrylate), polystyrene, and polybutadiene thin films on SiO(x)Si(100). Experiments were performed for a large range of surface temperatures below and above the glass transition of these three polymers. The broad multiphonon feature that arises in the inelastic scattering spectra at surface temperatures between 175 and 500 K is indicative of the excitation of a continuum of surface vibrational modes. Similarities exist in the line shapes of the scattering spectra, indicating that helium atoms scatter from groups of similar mass on the surface of these polymer thin films. The line shapes obtained were further analyzed using a semiclassical scattering model. This study has shown that quite different polymer thin films can have similar interfacial dynamics at the topmost molecular layer.