No abstract

Aqueous solutions of the spin trap 5,5-dimethylpyrroline TV-oxide (DMPO) and of Br2,12, Mn04~, AuCl4", and Ag+ were irradiated with 1-MHz ultrasound under argon-hydrogen mixtures of various compositions. DMPO was found to trap only very small amounts of hydrogen atoms while substantially greater amounts could be scavenged by the other solutes. The yield of H atoms as measured by the ESR signal of the DMPO adduct decreased with increasing H2 concentration in the gas mixture. The yields of the reduction products of the other solutes passed through a maximum at about 20 vol % H2. The efficiency of scavenging did not correlate to the rate constant of reaction with H in homogeneous solution. It seems that the ability of a solute to be accumulated at the liquid-gas interface of cavitation bubbles strongly influences the scavenging efficiency. Thallium ions are reduced in solutions of high pH. This reduction is attributed to hydrated electrons which are formed in the reaction of the H atoms with OH" anions. It is concluded that the local concentration of H atoms in the interfacial region of the cavitation spots is about 10"1 23 M and that hydrated electrons are not primary products of sonolysis.

Extremely small colloidal ZnS particles (diameter ∼ 1.7 nm) were made by either photo‐degradation of 3 nm particles or rapid precipitation in phosphate containing solution at pH = 7‐−8. The absorption spectra of these particles are different from that of macrocrystalline ZnS, and the changes are regarded as an indication for the transition from semiconductor ZnS to polymolecular ZnS with decreasing particle size. – The dependence of the intensity of the fluorescence on particle size, temperature, photoanodic corrosion and the quenching of fluorescence were investigated for colloids on a silicon dioxide carrier, in phosphate solution, and without a stabilizer. Photo‐anodic corrosion strongly improves the fluorescence properties. – One adsorbed Cd2+ ion per colloidal particle is sufficient for efficient quenching of the fluorescence. However, a new fluorescence band appears which is explained by the formation of a layer of 1:1 co‐colloid at the surface of the ZnS particles. – Methylviologen was also found to be a very efficient quencher. The decay of the fluorescence is wavelength dependent, i.e. the fraction of long‐lived fluorescence is greater at longer wavelengths. – A mechanism is discussed, where the fluorescence centers are anion vacancies, and fluorescence is emitted when electrons trapped in states of different energies and exhibiting different life‐times tunnel to the localized positive holes.

No abstract

Precipitation of lead ions by hydrogen sulfide in the presence of poly(vinyl alcohol) in aqueous or methanolic solution yields Q‐PbS particles (quantized particles) which have a structured absorption spectrum. The particles have a rod‐like or spherical shape (size quantization mainly in two or three dimensions, respectively). PbS with a structured absorption can also be made in acetonitrile using poly(ethylene oxide) as stabilizer. PbS stabilized by sodium polyphosphate has a structureless absorption spectrum. — Depending on the method of preparation and surface modification after preparation PbS samples fluorescing in the red or/and infrared are obtained. Methyl viologen generally quenches but can in certain cases promote the fluorescence when it is present in low concentration. Oxygen quenches when the colloid is stabilized by polyphosphate. — The rate of the photoanodic dissolution in the presence of air is increased by methyl viologen. Half reduced methyl viologen does not transfer an electron to Q‐PbS but can do so in the case of larger particles. — Illumination under argon leads to increased Ostwald ripening of the particles. However, when sulfite is present in the deaerated solution, photoanodic dissolution takes place. — Attack by hydroxyl radicals generated radiolytically leads to Pb2+ + SO2−4. The fluorescence of a Q‐PbS particle is efficiently decreased after the attack by a single OH radical (i.e. after injection of one positive hole). — Single electron transfer from 1‐hydroxy methyl ethyl radicals, (CH3)2COH, to Q‐PbS also leads to a decrease in fluorescence. These effects are attributed to interactions of the charge carriers deposited by the radicals on the colloidal particles with the exitonic state produced by light absorption. — Accumulated electron transfer to PbS‐particles from organic radicals yields lead metal and organic sulfur compounds. In the beginning of this process, the fluorescence strongly increases (10% quantum yield) which is explained by destruction of surface defect states.