An inverse eigenvalue method for frequency isolation in spring–mass systems

The action of external vibrating forces on mechanical structures can cause severe damages when resonance occurs. The removal of natural frequencies of the structure from resonance bands is therefore of great importance. This problem is called frequency isolation problem and is the subject of this paper. A new inverse eigenvalue method is proposed and applied to spring–mass systems, which have generated much interest in the literature as prototypes of vibrating structures. The novelty of the method lies in using the zeros of the frequency response function at the last mass as control variables in an optimization problem to minimize the impact of redesign. Numerically accurate algorithms for computing the sensitivity with respect to the control variables are presented, which form the basis of an efficient multidimensional search strategy to solve the frequency isolation problem. Copyright © 2001 by John Wiley & Sons, Ltd.     

Tunable coherent light sources

The physical principles of tunable sources of coherent light are briefly described. Characteristic data and the present stage of development are reviewed.     

Zur Konformation von 3-Chlormethylsubstituierten 2,3-Dihydro-1,4-benzoxathiinen

The preferential conformation of some 3-substituted 1,4-benzoxathianes has been determined by means of ¹H n.m.r. spectroscopy.     

Paramagnetische Resonanz des dreiwertigen Gadoliniums in YPO4, YVO4 und YAsO4

The EPR spectra of Gd³⁺-doped isostructural single crystals of YPO₄, YVO₄, and YAsO₄ have been measured in X-band at room temperature and at nitrogen and helium temperatures. The zero-field splitting can be described by the five crystal-field parametersb ₂ ⁰ ,b ₄ ⁰ ,b _6/0,b ₄ ⁴ , and b ₆ ⁴ fitted to a tetragonal spin-Hamiltonian corresponding to the point symmetry D_2d of the Gd³⁺ site. The values ofg and b ₂ ⁰ vary significantly with host lattice and with temperature.     

Subpicosecond carrier trapping in high-defect-density amorphous Si and GaAs

The relaxation of photoexcited carriers in a-Si and a-GaAs has been studied with subpicosecond time resolution by excite-and-probe experiments. The measured time dependence of the induced reflectivity and absorption reveals that the initial trapping of mobile carriers in high-defect-density materials occurs on a subpicosecond time scale.     

Wave functions,nodal domains,flow, and vortices in open microwave systems

The signatures of wave functions in open cylindrical microwave billiards are investigated. The wave functions are obtained by means of a transmission measurement from an attached lead to a probe antenna inside the billiard. One can deduce complex wave functions and current densities of the system from the measurement. We investigate distributions and correlations of wave functions, currents and other quantities and compare this results with predictions from the random plane wave approach. Vortices and saddles are created and annihilated as a function of frequency and show a rich dynamic. Additionally we investigate nodal domains of the real and imaginary part of the wave function, their relation to the phase rigidity, and its parametric dependance on global phase shifts.     

Toward generation of μJ range sub-ps THz pulses by optical rectification

Recently the possibility of scaling up the energy of sub-ps THz pulses generated in lithium-niobate by tilted pulse front excitation was demonstrated. Using 500 μJ energy pump pulses at 780 nm center wavelength, we achieved THz pulses with energy up to 240 nJ. In this article, results of calculations using a simple model predict the possibility of increasing the THz pulse energy above 1 μJ and the quantum efficiency up to 50% by decreasing the temperature. The dependence of the THz pulse energy and the maximum achievable electric field on the crystal length and the pump pulse duration is also presented. According to the calculations, generation of the maximum THz energy needs a specific pump pulse duration, because of increasing dispersion and absorption with increasing frequency. Not only longer, but also shorter pulses lead to a degradation of the THz energy. Results of calculations for GaSe, GaP and ZnTe are also presented. PACS 07.57.Hm; 42.65.Ky; 42.79.Nv     

Double-beam fourier spectroscopy with interferometric background compensation

A double-beam multiplex spectrometer is described which is specially suited for measuring small variations in absorptivity or reflectivity, or narrow spectral lines, against a strong and varying background. By a modification of the conventional Fourier spectrometer, the interferogram beam and the radiation normally going back to the source — the anti-interferogram beam — are combined in the sensitive area of a single detector. If a sample and a reference standard are placed in the interferogram and the anti-interferogram paths respectively, the Fourier transform of the interferogram of the combined intensity directly yields the difference between the spectra of the sample and the reference standard with the optical cancellation of the common features. The advantages and possible applications of this technique are discussed.     

Search for E2 strength in electrofission of 238U and 232Th

We have measured the cross-section ratio σ^?/σ⁺ for the electron and positron induced fission of uranium and thorium in the energy range 10–40 MeV with detectors at 90° and in a 2π geometry. The observed periodic structure is discussed with respect to E2 admixtures and an anisotropy of the angular distribution, after the onset of each higher chance fission threshold, as well as with respect to the shape of the photofission cross section.     

Study of the shock-induced acceleration of hexane droplets

An experimental study of the interaction of a shock wave with a hexane droplet is presented. The main goal of the experiments was to record images of the process and measure basic parameters describing movement, dispersion and evaporation of the droplets engulfed by a shock wave propagating in air. A shock tube with a visualization section was used for this research. Photography of the process allowed one to measure the positions, velocities and sizes of mist clouds created by the interaction processes. Analysis of the pictures shows that there is no qualitative difference between cases for different size droplets, but shock Mach number had a significant effect on the process. Quantitative analysis shows that under certain conditions, a catastrophic breakup mechanism of dispersion occurred. The droplets are shattered into a mist cloud before they achieve mechanical equilibrium with the surrounding gas. The approximate time for the complete dispersion and acceleration of the fuel droplet varies from 300 to 500 μs, and depends both on the droplet diameter and shock velocity. The dispersion time is controlled principally by the droplet diameter, and to a lesser extent, the shock Mach number. This paper is based on work that was presented at the 20th International Colloquium on the Dynamics of Explosions and Reactive Systems, Montreal, Canada, July 31–August 5, 2005.