Berichte der Bunsengesellschaft für physikalische Chemie最新文献

The electrochemical behaviour of the cathode/electrolyte interface which is formed between screen printed Lao_0.8Sr_0.2MNO₃-cathode and ZrO₂+8%Y₂O₃ electrolyte was investigated. The impedance of the cathode/electrolyte interface was calculated as a function of (I) the oxygen exchange velocity between cathode material and atmosphere, (II) interface microstructure and (III) the oxygen vacancies diffusion coefficient. A very good agreement between calculated and measured impedance spectra was achieved. The influence of the porosity of the cathode on the impedance is discussed.

Semiconductor nanocrystals prepared by methods of wet chemistry are similar to MBE grown quantum dots where the mobility of the charge carriers is reduced to zero dimensionality. In this paper we summarize the physics of a unique system in which the charge carriers are locally confined within a heterogeneous quantum dot. With high resolution electron microscopy we will show that epitaxial growth ot atomic layer precision is possible by methods of solution chemistry leading to CdS quantum dots with embedded HgS quantum wells (QDQWs). The photophysics of this system is investigated by time-correlated single photon counting, transient differential absorption and fluorescence line narrowing spectroscopies. The results reflect the very complex electronic structure of this new kind of matter which can be explained by an extended effective mass approach.

Statistical expressions for the Onsager phenomenological coefficients are now available for several models of matter transport in systems which are dilute in solutes, vacancies and interstitials. They describe transport by unpaired and paired defects, as in the familiar five-frequency model. The role of atomistic calculations of the parameters of such models in validating, simplifying or improving them is illustrated. In general it may be difficult to convincingly determine from experiment all the parameters of a given model or to distinguish between closely related models, such as between those retaining first or first and second neighbour binding between a solute and a vacancy. Nevertheless, there is particular utility in knowledge of all the Onsager coefficients for models suggested by atomistic calculations and other information, when one interprets phenomena involving coupled fluxes. This is illustrated by the example of coupled fluxes of a solute, vacancies and interstitials in a dilute alloy under steady irradiation by high energy particles. Here, essential contributions to modelling solute segregation to grain boundaries can be made; striking qualitative differences between closely related models can occur in this case. Finally, the limitations of the current statistical results are examined for systems where the solutes and defects carry effective charges. Limitations are suggested at low temperatures by the existence of a phase transition in the restricted primitive model of dilute aqueous electrolytes. The range of concentration and temperature within which the current procedures, stemming from the early work of Teltow and Lidiard, have been properly validated is limited. In the absence of recent theoretical advances Monte Carlo simulation is becoming the method of choice in investigating such limitations and in studying the transition to higher concentrations where there are larger clusters of defects.

The formation of NH₂ and NH radicals as well as H and N atoms during the thermal decomposition of hydrazine was investigated over a wide temperature range. For the first time frequency-modulated spectroscopy was used for the detection of NH₂ radicals behind shock waves. In comparison with the dual beam laser absorption technique, the detection limit has been improved by one and a half orders of magnitude.

For reaction (5a) NH₂ + M → NH + H + M the rate constants were obtained in the temperature range from 2200 to 4000 K

k_5a = (1.2 ± 0.5)10¹⁵ · exp[-(318±10) kJ mol⁻¹/RT] cm³/mol s

and for reaction (6) NH + M → N + H + M in the temperature range from 2500 to 3400 K

k₆ = (1.8 ± 0.8)10¹⁴ · exp[-(313 ± 15)kJ mol⁻¹/RT] cm³/mol s.

The experimental apparent activation energies were found to be in good agreement with theoretical calculations and recently recommended enthalpies of formation for NH₂ and NH. The competing NH₂ decomposition channel (5b) NH₂ + M → N + H₂ + M was shown to be of minor importance (⩽ 5%). The kinetic behavior of the unimolecular decomposition of NH₂ is compared with that of H₂0 and ³CH₂.

Some recent simulations of localized vibrational modes associated with substitutional impurities in crystals are discussed. Most of the examples are for impurities in III–V semiconductors and alkali halides, cases for which improvements in measurement and sample preparation techniques have generated new exacting tests for theoretical models. The displacements of atoms in the modes are revealed by the fine structure from isotopes of both the impurity and the host crystal atoms. Available schemes for modelling these dynamical properties of defects are reviewed.

The kinetics of the oxidation of Fe(bpy)²⁺₃ by S₂O²⁻₈ have been studied in water and different water-cosolvent mixtures. The cosolvents used were methanol t-butyl alcohol, ethane 1,2-diol and glycerol. The results are explained assuming an additional component of the reorganization free energy of the solvent in the mixtures, caused by dynamics of the changes in the composition of the (at least) innermost solvation shells of the reactants produced by the electron transfer. A quantitative estimation of this component is attempted.

Quasiclassical trajectory method (QCT) has been used to investigate the effect of vibrational excitation on the relative cross section for the reaction Cl+Na₂(v″). The calculated results are in good agreement with the experimental data, which show the increase of the cross section with increasing vibrational excitation. Furthermore, the spectroscopy of transition state in this reaction has been predicted out using QCT method based on LEPS potential energy surface.

Picosecond time-resolved fluorescence spectroscopy has been used to examine the dynamic solvent effect on the excited intramolecular charge-transfer (ICT)-state formation in 4-(9-anthrylmethyl)-N,N-dimethylaniline (AMDMA) in linear alcohol solvents as a function of pressure. The non-exponential population decay of the local excited (LE)-state emission is analyzed by fitting to a double exponential function. As solvent viscosity increases, the rate of the ICT-state formation of AMDMA becomes noticeably faster than the solvent relaxation time, which is gauged by the longitudinal relaxation time of the solvent (τ_L). The pressure effect on the ICT reaction of AMDMA in the excited state can be accounted for in terms of the pressure-tuning effect of the solvent viscosity.

In addition, the time-dependent Stokes shift (TDSS) of the ICT emission is measured at high pressures.

Molecular dynamics simulations of the solute migration by a vacancy have been made for a Lennard-Jones model of solid argon. The results have been used to analyse the heat of transport, Q*, for solutes with masses corresponding to those of the noble gases. The theory shows that there are three contributions to Q*, coming from the fluxes of the kinetic energy, potential energy and a virial of the forces. Trends in the kinetic and potential terms are found with increasing mass of the solute atoms which it seems would be shown by solute atoms more generally. The molecular dynamics calculation is in the centre of mass frame and we give the necessary transformations to the required lattice frame.