Lattice dynamics of the si(111) surface: folding effects in the 2×1 structure

We report a bond-charge-model calculation for Si(111):H(1×1). This configuration may be assumed to represent the “ideal” (1×1) surface, as long as the saturation of the dangling bonds by H atoms produces a slight perturbation of the dynamical matrix. We display also the geometric folding of these dispersion curves into the (2×1) configuration and compare our results to recent He scattering data and “ab initio” calculations. Thus we are in the position to discuss which dynamical features of the 2×1 pattern arise from the mere folding of the (1×1) pattern and which are due to the physical changes produced by the specific reconstruction.     

Photosensitized oxidation of 3,6-dimethyloctane

In this paper an interesting mechanism involving a Barton reaction which leads to a propagation of photooxidation processes between γ positioned tertiary carbons is described.     

Carbon dioxide fixation by electrochemical reduction in water to oxalate and glyoxylate

Carbon dioxide, in aqueous solution containing tetramethylammonium ions at pH 9 was reduced in two steps to yield oxalic acid at −0.9 V and glyoxylic acid at −1.8 V on carbon and on mercury. Products were analysed by HPLC and confirmed by specific colorimetric measurements.     

Two signal processing techniques for the enhancement of the flaw-to-grain echo ratio

Two signal processing techniques are developed to improve the signal-to-noise ratio in the testing of coarse-grained materials. They are: the space minimization—frequency minimization (SMFM) algorithm and the space minimization—frequency threshold (SMFT) algorithm. Experimental data for austenitic stainless steel samples are presented which show encouraging flaw detection capabilities of the two algorithms.     

Similarity analysis and non-linear wave propagation

We characterize the most general 2 × 2 first order quasilinear hyperbolic systems in conservative form which are invariant with respect to the stretching group of transformation. The invariant solutions satisfy, under suitable conditions, an autonomous first order system of ordinary differential equations. A procedure is given to characterize the profile of the velocities of the strong and weak discontinuities.     

Parametric resonance in magnetic fluids

A new resonance effect in the nonlinear behaviour of magnetically anisotropic objects in an alternating external magnetic field is proposed. Ferromagnetic particles with a “frozen” magnetic moment (due to a strong magnetic anisotropy), when located in an external alternating magnetic field, are able to rotate (or vibrate) and to transfer energy from the external field to the medium. The numerical solution of the appropriate parametrically driven nonlinear equation shows all types of nonlinear dynamic behaviour, including transition to chaos. The sensitivity of the proposed phenemenon could be used for an experimental analysis of the size distribution of the ferromagnetic particles in a ferrofluid or of the size of “magnetic holes”.