Investigations on the dynamics of ferro- and antiferroelectric betaine compounds

Measurements of the dielectric permittivity around the ferroelectric transition of betaine arsenate and around the antiferroelectric one of deuterated betaine arsenate reveal relaxation-type dispersions in the upper MHz-region. The data analysis points to two mechanisms which contribute to the low temperature phase transitions of these crystals. In case of betaine phosphate Raman-spectra demonstrate that besides the antiferroelectric transition at 86 K a second structural transition exists at 81 K. Both transitions correlate with dielectric anomalies and the unit cell doubles at 81 K.Dedicated to Prof. Dr. H.E. Müser on the occasion of his 60th birthday     

A variable metric algorithm for unconstrained minimization without evaluation of derivatives

Summary This paper presents a modification of the BFGS-method for unconstrained minimization that avoids computation of derivatives. The gradients are approximated by the aid of differences of function values. These approximations are calculated in such a way that a complete convergence proof can be given. The presented algorithm is implementable, no exact line search is required. It is shown that, if the objective function is convex and some usually required conditions hold, the algorithm converges to a solution. If the Hessian matrix of the objective function is positive definite and satisfies a Lipschitz-condition in a neighbourhood of the solution, then the rate of convergence is superlinear.     

Test of an optical parametric oscillator (OPO) as a compact and fast tunable stokes source in coherent anti-stokes raman spectroscopy (CARS)

An optical parametric oscillator (OPO), as a novel kind of broadband Stokes source, is employed for coherent anti-Stokes Raman spectroscopy (CARS). Compared to the conventional dye laser configuration OPO-CARS offers practical advantages. The tunable OPO allows a fast and comfortable frequency tuning. The excitation bandwidth of about 35 cm^–1 (FWHM) limits the spectral range of effective and stable single pulse CARS generation but can be used to enhance selected spectral structures.     

Collision-induced dissociation of hydrated adenosine monophosphate nucleotide ions: protection of the ion in water nanoclusters

Fragmentation of singly charged anions of adenosine 5'-monophosphate (AMP-) induced by collisions with neutral atoms (Ne, Na) has been studied at a collision energy of 50 keV. The experiments were performed with isolated AMP- as well as with AMP- anions nanosolvated in a cluster with a given number m of water molecules. In the first case, the dominant fragmentation channels concern the loss of adenine, PO3- and H2PO4-. In the latter, loss of water molecules becomes the dominating process, and the AMP- ion is fully protected when m is larger than approximately 13. The observed fragment distributions are well described with the model of an evaporative ensemble.     

Isomorphous substitution in bimetallic oxide clusters

The geometric and electronic structure of bimetallic oxide clusters is studied as a function of their composition with gas phase vibrational spectroscopy. Infrared multiple photon dissociation spectra of titanium-vanadium oxide cluster anions are measured in the 500 to 1200 wave number range and assigned on the basis of harmonic frequencies calculated using density functional theory. Singly substituted (V(2)O(5))(n-1)(VTiO(5))(-) (n=2-4) cluster anions are shown to form polyhedral caged structures similar to those predicted for their isoelectronic counterparts, the neutral (V(2)O(5))(n) clusters. Upon systematic exchange of V by Ti atoms in V(4-n)Ti(n)O(-)(10) (n=1-4), the structure does not change. The stress induced by the isomorphous substitution results in an increased number of unpaired electrons (n-1) for the Ti-rich systems, leading to a quartet ground state for Ti(4)O(-)(10).     

Saturation of the all-optical Kerr effect in solids

We discuss the influence of the higher-order Kerr effect (HOKE) in wide bandgap solids at extreme intensities below the onset of optically induced damage. Using different theoretical models, we employ multiphoton absorption rates to compute the nonlinear refractive index by a Kramers-Kronig transform. Within this theoretical framework we provide an estimate for the appearance of significant deviations from the standard optical Kerr effect predicting a linear index change with intensity. We discuss the role of the observed saturation behavior in practically relevant situations, including Kerr lens mode-locking and supercontinuum generation in photonic crystal fibers. Furthermore, we present experimental data from a multiwave mixing experiment in BaF2, which can be explained by the appearance of the HOKE.     

Fatigue performance of glass/polyester laminates and the monitoring of material degradation

Experiments are performed on glass/polyester composites under fatigue loading. The stiffness degradation is recorded as an indirect measure of material damage. The stiffness decrease rate is described by a power dependence on stress; this relationship allows the fatigue law to be derived. Stiffness-controlled fatigue curves are generated and presented in an S-N-diagram based on normalized stress (equivalent to strain). Such stiffness controlled fatigue curves can be used as a basis for design criteria for components.Presented at the 9th International Conference on Mechanics of Composite Materials, 17–20 October 1995, Riga, Latvia.Materials Department, Risø National Laboratory, Roskilde, Denmark. Published in Mekhanika Kompozitnykh Materialov, Vol. 32, No. 1, pp. 32–41, January–February, 1996.