Phase separation and partial devitrification in soda-lime silicate glasses

2 O-rich droplets dispersed throughout the SiO₂-rich matrix and nanoparticles of metallic silver. In exchanged specimens the joint effect of colloidal silver and electric field results in partial transformation of the amorphous droplets into crystalline Na₂O particles. Received: 11 February 1998 / Accepted: 20 November 1998 / Published online: 24 February 1999     

Theoretical and computer simulation study of density fluctuations in liquid binary alloys

The dynamical properties of liquid alloys are investigated by means of memory function equations and molecular-dynamics simulation. A simple model for the second-order memory function in a binary liquid, based on Mori's memory function formalism, is proposed and applied in numerical calculations of the time correlation functions and dynamic structure factor of liquid K_0.7Cs_0.3 and K_0.3Cs_0.7 alloys. Obtained results are discussed in comparison with the results of computer simulations. Received: 27 February 1998 / Revised: 24 July 1998 / Accepted: 27 July 1998     

Pulsed X-ray diode with a long-lifetime plasma cathode

6 shots, the X-ray dose, uniformity, and ionisation rate values guarantee an effective preionisation of excimer laser discharges. Owing to both the long lifetime and the substantial absence of maintenance, this X-ray diode seems suitable to preionise commercial gas lasers, such as excimer and TEA CO₂ lasers. Received: 28 August 1998 / Revised version: 16 November 1998 / Published online: 24 February 1999     

Martensitic transformations: first-principles calculations combined with molecular-dynamics simulations

Results are presented of first-principles total-energy calculations and molecular-dynamics simulations of structural transformations in magnetic transition metal alloys like Fe_1-xNi_x. While first-principles calculations allow to identify those structures having the lower total energy, molecular-dynamics simulations can be used to trace out the dependence of the transformation on temperature, composition, concentration of defects etc. We have used the method of the semi-empiric embedded-atom potential in the molecular-dynamics simulations which yields remarkable good results for the structural changes. Received: 3 February 1998 / Revised: 4 May 1998 / Accepted: 24 June 1998     

Excitations in the dilute lattice model: , and mass spectra

On the basis of features observed in the exact perturbation approach solution for the eigenspectrum of the dilute A₃ model, we propose expressions for excitations in the dilute A₄ and A₆ models. Principally, we require that these expressions satisfy the appropriate inversion relations. We demonstrate that they give the expected E₇ and E₆ mass spectra, and universal amplitudes, and agree with numerical expressions for the eigenvalues. Received: 17 February 1998 / Accepted: 30 April 1998     

Photoionization thresholds and structures of third group metals clustered with ammonia

A photoionization study of the Me(NH₃) clusters formed in the reaction of photoablated third group metal vapor with gaseous ammonia is reported. The photoionization spectra exhibit some features due to vibrational excitation of ionic clusters and to transitions to neutral Rydberg states leading to autoionization. DFT quantum chemical calculations are performed on the Me(NH₃). The cluster geometries are fully optimized imposing the C_3v symmetry. The calculated values of the IPs are in agreement with those experimentally determined. Received: 16 February 1998 / Revised and Accepted: 7 May 1998     

Calculation of X-ray natural circular dichroism

A method is presented to calculate the natural circular dichroism recently discovered in the X-ray range (XNCD). The basic formula represents XNCD as an odd second-rank tensor and leads to a sum rule that relates XNCD to the mixing of odd and even orbitals in the ground state. A multiple-scattering theory of XNCD is presented, and calculated spectra for the L-edges of iodine in LiIO₃ compare favorably with the experiments. Received: 20 October 1997 / Revised: 18 February 1998 / Accepted: 31 March 1998     

In situ optical monitoring of metalorganic vapor phase epitaxy growth of C-doped GaAs

20 cm^-3 and above) induce oscillations in the reflected intensity. The evolution of the layers morphology is shown to depend on two effects: (i)the locally high surface concentration of carbon which blocks locally the growth and hence induces holes at the surface, (ii)the occurrence of dislocations at thicknesses larger than the critical thickness which are revealed chlorides produced by the decomposition of CCl₄ (the carbon precursor) and form deep etch pits. Received: 13 February 1998 / Accepted: 26 October 1998     

Highly anisotropic elasticity of a dendrimeric liquid crystal

Magnetic and electrical Freedericksz measurements were performed on the second generation monodendritic liquid crystal G₂(OH). The deduced elastic constants were found to be exceptionally anisotropic: the splay elastic constant K₁₁ is more than an order of magnitude larger than the bend elastic constant K₃₃, and the twist constant K₂₂ is approximately twice K₃₃. The results are discussed in terms of molecular conformations. Received: 23 February 1998 / Revised and Accepted: 26 May 1998     

Correlation functions in the Calogero-Sutherland model with open boundaries

Calogero-Sutherland models of type BC_N are known to be relevant to the physics of one-dimensional quantum impurity effects. Here we represent certain correlation functions of these models in terms of generalized hypergeometric functions. Their asymptotic behaviour supports the predictions of (boundary) conformal field theory for the orthogonality catastrophy and Friedel oscillations. Received: 12 February 1998 / Accepted: 17 March 1998     

Optical characterization of thermally evaporated thin films of As40S40Se20 chalcogenide glass by reflectance measurements

40 S₄₀Se₂₀, deposited by thermal evaporation, were obtained in the 400 nm to 2200 nm spectral region. The optical constants of this amorphous material were computed using an optical characterization method based mainly on the ideas of Minkov and Swanepoel of utilising the upper and lower envelopes of the spectrum, which allows us to obtain both the real and imaginary parts of the complex refractive index, and the film thickness. Thickness measurements made by a surface-profiling stylus have been carried out to cross-check the results obtained by the optical method. The dispersion of the refractive index is discussed in terms of the single-oscillator Wemple–DiDomenico model. The optical band gap has been determined from absorption coefficient data by Tauc’s procedure. Finally, the photo-induced and thermally induced changes in the optical properties of a-As₄₀S₄₀Se₂₀ thin films were also studied, using both transmission and reflection spectra. Received: 11 February 1998/Accepted: 16 February 1998     

Monolayers of diblock copolymer at the air-water interface: the attractive monomer-surface case

We have studied both experimentally and theoretically the surface pressure isotherms of copolymers of polystyrene-polyethyleneoxide (PS-PEO) at the air-water interface. The SCMF (single chain mean-field) theory provides a very good agreement with the experiments for the entire range of surface densities and is consistent with the experiments if an adsorption energy per PEO monomer at the air-water interface of about one k_{B T} is taken. In addition, the chain density profile has been calculated for a variety of surface densities, from the dilute to the very dense ones. The SCMF approach has been complemented by a mean-field approach in the low density regime, where the PEO chains act as a two-dimensional layer. Both theoretical calculations agree with the experiments in this region. Received: 19 June 1997 / Revised: 2 February 1998 / Accepted: 11 February 1998     

Performance of a diode-pumped 5 W Nd3+:GdVO4 microchip laser at 1.06 μm

4 as a host for neodymium has several advantages for diode pumping in comparison with other crystals. The absorption cross section of neodymium in GdVO₄ is considerably stronger and broader than in YAG. This allows for the construction of very compact monolithical microchip lasers. In our paper, we report for the first time on a diode-pumped monolithical Nd³⁺([%at.]1.3):GdVO₄ microchip laser at 1.06 μm. A maximum output power of 5 W is achieved. The temporal and the spectral emission properties are described. The beam propagation properties are studied in detail. Received: 23 July 1998 / Revised version: 9 November 1998 / Published online: 24 February 1999     

Dissociative excitation of the fluoromethanes by electron impact

Emission spectra following electron impact on molecules of the homologous series of fluoromethanes CH_xF_4-x with x=0-4 have been investigated from the near infrared at 700 nm to the ultraviolet VUV-spectral region at 100 nm. Earlier experimental data for the visible and ultraviolet spectral region were revised and evaluated again on the basis of reliable new data for the dynamic viscosity of the molecules. The measurement of absolute and relative cross-sections were systematically extended into the VUV region from 100 nm to 200 nm. The examination of atomic lines as well as molecular band systems in the VUV gives further insight into the dissociation mechanism and shows that many excited levels even of atomic and molecular species cannot be populated directly, but only by transitions from higher-lying energy levels. Simple steric effects can be distinguished from more complex transition phenomena. Received: 25 February 1998 / Revised: 29 May 1998 and 18 June 1998 / Accepted: 23 June 1998     

Perturbative renormalization of multi-channel Kondo-type models

The poor man's scaling is extended to higher order by the use of the open-shell Rayleigh-Schr?dinger perturbation theory. A generalized Kondo-type model with the SU(n)SU(m) symmetry is proposed and renormalized to the third order. It is shown that the model has both local Fermi-liquid and non-Fermi-liquid fixed points, and that the latter becomes unstable in the special case of n=m=2. Possible relevance of the model to the newly found phase IV in Ce_xLa_1-xB₆ is discussed. Received: 24 February 1998 / Accepted: 17 April 1998     

Femtosecond nonresonant degenerate four-wave mixing at atmospheric pressure and in a free jet

2 , N₂, and CO₂. For CO₂ additional experiments have been performed at reduced pressure and in a molecular beam. By delaying the probe pulse a periodic recovery of the DFWM signal is observed. The period of these transients can be assigned unambiguously to rotational Raman transitions of the ground state within the laser bandwidth. The decay of the transients yields the collisional dephasing of the Raman-induced polarization. At zero delay also optical-field-induced birefringence of electronic nature contributes to the signal. The different time scales of the Raman and electronic effects allow us to estimate their relative strength. Received: 3 August 1998 / Revised version: 21 October 1998 / Published online: 24 February 1999     

Photorefractive effects in long, narrow BSO crystals with applied electric field

In a photorefractive Bi₁₂SiO₂₀ crystal with high applied electric ac field of square-wave shape a fast two-wave coupling response (less than 1 s) and a slow hologram readout decay (minutes) was found for a wavelength of 633 nm. This can be explained by electron–hole transport with two trap levels. An intensity dependence of the slower complementary grating was found. Illuminating with the readout wave without applied electric field leads to a very slow grating decay (many hours). Received: 18 November 1998 / Revised version: 3 February 1999 / Published online: 7 April 1999     

Near-infrared trace-gas sensors based on room-temperature diode lasers

2 monitor designed for field applications using room-temperature diode lasers are presented. Near-infrared DFB lasers operating at 1.57 μm and around 2.0 μm have been used for CO₂ measurements. At ambient concentration levels a resolution of more than two orders of magnitude has been demonstrated at 1.57 μm, at 2 μm the precision is in the order of 0.1 ppm CO₂, and for trace analysis a detection limit of 10 ppb has been obtained. The measurements demonstrate the capability of near-infrared DFB diode lasers for the precise determination of CO₂ concentrations as required for climatological, medical, or industrial applications. Received: 24 February 1998/Revised version: 27 April 1998     

Photodissociation dynamics of Cl in a xenon cluster

The photodissociation of a chlorine molecule in the environment of a xenon cluster has been studied experimentally using the real time pump and probe technique through the formation of an XeCl reaction product. The photodissociating system is probed in such a way that the movement of a single chlorine atom in the xenon environment is detected. Various Xe_nCl₂ cluster sizes have been investigated leading to the distinction between uncapped, half-capped and doubly capped structures for these clusters. These structures have a profound influence on the photodissociation dynamics. Retrapping of one chlorine atomic fragment and stabilization of the XeCl reaction product is only observed for the half and doubly capped clusters. The experimental work is complemented by classical molecular dynamics calculations to get a full picture of the photodissociation. Received: 17 February 1998 / Received in final form and Accepted: 28 July 1998     

Flow birefringence and stress optical law of viscoelastic solutions of cationic surfactants and sodium salicylate

The optical and rheological properties of different viscoelastic solutions of surfactant are studied in order to gather experimental data used to calculate the value of the stress optical coefficient C. Three surfactants of the same family (CTAB) have been chosen; they differ by the length of the hydrocarbon chain; it concerns the dodecyltrimethylammonium bromide (C₁₅H₃₄BrN or DoTAB), the myristyltrimethylammonium bromide (C₁₇H₃₈BrN or MyTAB), and the hexadecyltrimethylammonium bromide (C₁₉H₄₂BrN or CTAB). Different parameters like the temperature of the solution and the salinity of the solvent have been made to vary. Flow birefringence experiments and rheological measurements are performed on these solutions in order to study the dependence of the extinction angle , of the birefringence intensity and of the shear stress with the shear rate . These data are used to check the stress optical law which turns out to be valid in a wide range of shear rates. The stress optical coefficient C is then computed: it is found to vary with the salinity of the solvent and the temperature of the solution for a given surfactant. Then, for all solutions of this work the variations of C are related to the variations of the polarizability anisotropy and the persistence length. Received: 18 February 1998 / Revised: 23 June 1998 / Accepted: 22 July 1998