A theoretical explanation of the surface diffusion mechanism in metal electrodes in contact with electrolytes

A theoretical explanation for the surface diffusion mechanism observed in columnar structured metal electrodes in contact with electrolytes is given. The potential energy of a surface metal atom on which ions forming part of the supporting electrolyte are adsorbed is described by means of an anharmonic oscillator curve whereas the energy of a surface metal atom liberated from any adsorption interaction is approximated by a harmonic oscillator energy fuction. Geometric arguments allow to define a symmetry factor δ for which experimental values were previously obtained. A qualitative interpretation of the value of δ has been made.     

Ion transport in porous electrodes with mixed conductivity

A method for studying dc ion transport in porous mixed-conductivity electrodes in the course of the electrochemical reaction taking place in them has been suggested. The dependences of the proton conductivity of porous electrodes used in direct electrochemical energy converters (electrolyzers, fuel cells) on their composition and structure have been presented. These data are of practical importance and can be used to analyze ohmic losses in the electrodes and membrane electrode assembly and also to devise novel electrode materials.     

Photoelectrochemical behaviour of ruthenium disulphide electrodes in contact with aqueous electrolytes

RuS₂, which is a semiconductor, has been prepared as a sintered material. As an electrode, it has a high catalytic activity for oxygen evolution from aqueous solution in the dark. On illumination, an oxidation reaction occurs at potentials more negatively by 1 V. This photoreaction with water was studied as a function of the photon energy, the pH of the electrolyte, and the electrochemical history of the electrode. The interpretation of the behaviour is complicated by the apparent dependence of the double layer potential on the total electrode potential and the pH which leads to a considerable shift of the bands of the semiconductor with respect to the redox levels of the electrolyte. Kinetic evidence also indicates the involvement of surface states in a reduction of the oxidation products of water. The remarkable photoelectrochemical stability of RuS₂ is discussed in terms of a high concentration of Ru d-states in the valence band and the presence of S₂^2? ions in the crystal lattice. The prospects for RuS₂ as a photocatalytic material for oxygen evolution seem promising, but a full evaluation is at present hampered by the occurrence of unfavourable side reactions, probably caused by the polycrystalline and porous nature of the available material. In this first investigation we study the microcrystalline sintered product which would be more likely to be a practical electrode. Later we will present a more fundamental study using single crystals of RuS₂.     

Photo-induced reorientation of molecules in dye micro crystals

Upon massive generation of anisotropic grains of azo dye methyl orange in a gelatin film exposed to active polarized light, a cluster of micro crystals with optical axes of similar orientations has been produced. The anisotropy observed has been found to disappear under exposure to active natural light.     

Sensitized fluorescence of dye molecules in polymeric matrices

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 48, No. 2, pp. 242–248, February, 1988.     

IR spectroscopy of the acetonitrile molecules in porous glasses

The results of investigation of the IR spectra of optical density under the condition of attenuated total internal reflection of acetonitrile in porous glasses with the pores of different radii are presented. It is established that interaction between the acetonitrile molecules and the porous glass matrix with the pores of small dimensions (1.3–4 nm) significantly effects the spectral characteristics of these molecules.     

Laser kinetic spectroscopy of solvated dye molecules

Journal of Russian Laser Research -     

Quantum description of a dye laser threshold region

The renormalized Fokker-Planck equation for a dye laser is derived from the Liouvillevon Neuman equation and its stationary solution, as well as the numerically calculated line shapes and the noise spectra for the quantum threshold region are given. The calculations were performed for values of the laser parameters which enable to compare the semiclassical bistable solutions with the threshold and those without the threshold. A difference in the behavior of the line shapes in the threshold region for both cases are discussed.     

Wavelength-dependent rotation of dye molecules in a polar solution

Investigation of rotation movement of 3-amino-N-methylphthalimide in glycerol was carried out, taking into consideration the fluctuation of solvate structure. It was shown theoretically and experimentally that structural relaxation of the solvate shell, which follows excitation of the dye molecule, causes not only shift of the fluorescence spectrum in time but also additional rotation of the dye molecule. This effect, which may be called wavelength-dependent rotation, depends on the light frequency of both excitation and fluorescence. In particular, at excitation near the maximum of the absorption band, when the relaxation process is followed with the red shift of the fluorescence maximum, the anisotropy of fluorescence decreases faster in the red part of the fluorescence band than in the blue part. On the contrary, in the case of far anti-Stokes excitation, when the temporal shift of fluorescence is going to the blue, the anisotropy in the red part of the spectrum drops more slowly than in the blue part. Finally, there is a special excitation frequency which causes neither change of the fluorescence maximum nor acceleration of the rotational movement of the dye molecule. It is also shown that the temporal evolution of the spectrum and anisotropy of fluorescence in a polar dye solution may be quantitatively described using the socalled inhomogeneous broadening function (IBF). This function gives the distribution of dye molecules in a solution over frequencies of pure electronic transition due to fluctuations of the surrounding shell structure. Measurements of IBF changes in time carried out for 3-amino-N-methylphthalimide showed that during first 3 ns after excitation, the half-width of the IBF grows, and at the same time its maximum quickly shifts to the red. At the later time period there are only small changes of IBF position but considerable exponential decrease in its half-width. The IBF during this period preserves the Gaussian shape.     

Effect of contact with air on the photoluminescence spectrum of porous silicon

A study has been made of the transformation of photoluminescence (PL) spectra of porous silicon (PS) induced by its ageing, including the early stages of contact with air. The sample was prepared under conditions that minimized this contact, and spectral measurements were carried out in a high vacuum or in liquid nitrogen. The PS PL spectra obtained under continuous measurement in high vacuum are always dominated by one emission band of PS nanoelements, which shifts toward shorter wavelengths with ageing by 150 nm. At 80 K, the band intensity is considerably higher than at 300 K, and this difference grows with ageing. Exposure of a sample to air for a few tens of seconds is long enough to strongly transform its time-resolved PL spectra, which is evidence of a change in the sample surface. The effect of immersion of PS samples in liquid nitrogen on PL spectra is associated not only with their cooling, but also with the field of adsorbed nitrogen molecules, whose influence becomes weaker with increasing thickness of the oxidized near-surface layer. The variation of the spectral properties and kinetics of the long-wavelength PS PL band with temperature, medium (liquid nitrogen or vacuum), and exposure time suggest that these factors affect carrier migration between silicon nanoelements.     

Dynamics of different molecules adsorbed in porous media

We present in this paper a comparative study on the dynamics of benzene, cyclohexane, and methanol molecules, confined in the pores of MCM-41 molecular sieve and HZSM-5 zeolite. The quasi-elastic neutron scattering (QENS) measurements revealed that the physical state of these adsorbed molecules depended not only on the structural characteristics of the host matrix but also on the chemical properties, such as dipole moment, of the guest molecules. Thus, while no motion was observed in the time-scale of 10⁻¹⁰−10⁻¹² s in the case of methanol, the larger size benzene and cyclohexane molecules are found to perform six-fold and three-fold jump rotation, respectively, when adsorbed inside the cages of HZSM-5 at room temperature. At the same time, all the three molecules are found to undergo a translational motion inside the pores of MCM-41 molecular sieves, the value of diffusion constant being the lowest in case of methanol because of its higher polarity. Translationl motion of the guest molecules inside the pores of MCM-41 can be satisfactorily described by Chudley-Eliott fixed jump length diffusion and accordingly the residence time, jump length and diffusion constant are estimated.     

Entrapment of dye molecules within submicron silver particles

We describe a method for the preparation of metal–organic composites submicron particles. Specifically, the preparation of silver particle-clusters 150–200 nm in size, doped with an organic dye Congo-red, is reported. The use of sodium citrate coupled with sodium hypophosphite facilitated the formation of these particle-clusters, which were fully characterized by TEM analysis, Zeta potential and size measurements, scanning electron microscopy, UV–Vis measurements, and thermogravimetric analysis. The latter reveals a catalytic action of the metal on the thermal oxidative decomposition of the entrapped dye. The use of these particles to obtain dense thin metallic films was demonstrated by the coating of ITO glass.     

Collective spontaneous emission of dye molecules and lasing

The threshold pump power density for lasing in dye solutions is found to depend on the photon energy of pumping radiation. An increase in the pumping photon energy can significantly lower the threshold pump power of dye lasers. For an ethanol solution of rhodamine 6G with a concentration of 4×10¹⁸ cm^?3, the threshold power density for pumping radiation with a wavelength of 532 nm is 20-fold higher than for pumping radiation with a wavelength of 347 nm. This phenomenon is associated with the competition of collective spontaneous emission, which can lead to the efficient deactivation of excited molecules in femtosecond times, and the dephasing of excited molecules due to the intramolecular nonradiative processes of absorbed-energy conversion. An increase in the dephasing rate with the increasing energy of exciting photons lowers the efficiency of collective spontaneous emission and increases the concentration of dephased excited molecules responsible for lasing.