The thickness and contact angle of sodium dodecyl sulfate foam films depending on the concentration of LiCl

 In the present study a detailed investigation of the thickness and contact angles of foam films prepared from sodium dodecyl sulfate as a function of LiCl concentration was carried out. Concentration of LiCl was varied in a wide range from 0.05 to 2.0 M. It was found that in the investigated system Newton black foam films are formed at LiCl concentrations higher than 1.4 M. This observation is proved by contact angle measurements and calculated values for the change of interaction free energy between common and Newton black films. Received: 6 January 1998 Accepted: 11 March 1998     

New high-energy luminescence bands from Co2+ in ZnSe

Three sharp absorption features in the energy range 2.36–2.55 eV have been detected in the transmission spectrum of Co-diffused ZnSe, and a number of luminescence transitions originating from the lowest of these states at 2.361 eV have been observed. Photoluminescence excitation spectra prove that these are high energy excited states of the Co²⁺_Zn impurity, a conclusion confirmed by comparison of measured and predicted luminescence energies. This represents the first identification of luminescence branching from a higher excited state of a transition metal ion in any semiconductor. The sharp, weakly phonon-coupled transitions involve either intra-impurity excitation or transitions from the impurity to localised states split off from a minimum in the conduction band. The implications of these observations for the mechanism of host-impurity energy transfer and for the nature of the excited state wavefunctions are discussed.     

Hydrogenic systems,frequency standards and fundamental constants

Hydrogenic (two-body) systems are the only atomic systems for which uncertainties in calculations of the energy levels approach the current state of the art in frequency measurement. This article discusses progress in the theory and measurement of transition frequencies in hydrogenic systems. These studies have relevance to the determination of fundamental constants and the testing of physical theories, especially quantum electrodynamics. A set of high accuracy calculable frequency standards could also be realized by using hydrogenic systems.     

Direct sample fraction deposition using electrospray in narrow-bore size-exclusion chromatography/matrix-assisted laser desorption/ionization time-of-flight mass spectrometry for polymer characterization

A direct sample fraction deposition method was developed for off-line size-exclusion chromatography (SEC)/matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass spectrometry. By using electrospray, the SEC eluent, together with a suitable matrix solution added coaxially, was directly deposited on the MALDI plate. Owing to the formation of very small droplets in electrospray, solvent evaporation is much faster. The fractionation volume in narrow-bore SEC, which can directly be collected in one MALDI spot, can easily be optimized in the range of a few microlitres. In addition, fairly homogeneous sample spots were obtained. The possible influence of composition variation of the SEC effluent on the analytical results using direct fraction deposition was investigated; no substantial effects were observed. The applicability of the method was demonstrated by characterizing a broad poly(methyl methacrylate) sample. Copyright 2000 John Wiley & Sons, Ltd.     

Phospholipid foam films studied by contact angle measurements and fluorescence microscopy

 Foam films drawn from suspensions of the phospholipid 1,2-dimyristoyl-sn-glycero-3-phosphorylcholine (DMPC) in water/ethanol mixtures were used for the investigation of the relation between the properties of the monolayers and the interaction between the film surfaces. The film thickness and the contact angle between the film and the meniscus were measured as a function of the temperature in a range around the temperature of the main phase transition for the lipid. Additionally, fluorescence microscopy was applied to investigate the distribution of a fluorescent lipidlike dye in the surface of the film and the meniscus. From the contact angle the free energy of film formation was calculated. At the temperature of the chain-melting phase transition the film thickness decreases by 0.7 nm. This can be related to a decrease in the thickness of the hydrocarbon layers of the lipid monolayers at this temperature. The decrease in the film thickness leads to a reduction in the free energy by increasing the van der Waals attraction between the film surfaces. No structures were observed in the monolayers of the film in the fluorescence investigation. However, on formation of the very thin equilibrium film the dye was expelled from the film area, indicating an increase in the packing density of the lipid, if the monolayers are in adhesive contact in the film. Received: 31 January 2000 Accepted: 25 February 2000     

Time domain investigation on vibrational dephasing and spectral diffusion in CO-doped solid N2

Infrared picosecond accumulated photon echo experiments have been performed for the first time, using the Orsay Free Electron Laser, on the v = 0-->v = 1 transition of CO in solid nitrogen. The vibrational dephasing time is found to be exceptionally long ( T2>/=120 ns) at low temperature. The analysis of the observed spectral diffusion leads one to assume different energy transfer mechanisms depending on the CO concentration.