An analysis of the three-valence model of photorefraction

In this paper we present an analysis of the three-valence model of photorefraction. The basic equations of the model are studied and solved to find zero- and first-order solutions in the steady state, including expressions for the space-charge field and the effective trap density. The intensity dependence of the solutions for the population densities and space-charge field are analysed and discussed for varying proportions of the different valence states in the material, and the influence of some important material parameters considered. The possible applications of these results to the experimental characterisation of three-valence-state materials are discussed and some comparisons are made between the three-valence and the two-independent-centre models of photorefraction. Received: 17 November 1998 / Revised version: 8 January 1999 / Published online: 31 March 1999     

Characteristics of Decisions in Decision Analysis Practice

This paper presents summary statistics regarding the characteristics of decisions identified in a recent review of 86 applications of decision analysis. The goals of this summary are (1) to identify values for important parameters which characterize the structure of decisions which have been analysed using decision analysis; (2) to see what proportion of studies use the various available analysis tools; and (3) to draw implications from these results, particularly in terms of simplifying applied decision analysis. The results show that decisions tend to be multiattributed, involve a modest number of alternatives, and uncertainties appear to be the major source of influence on the chosen alternative. Furthermore, applications generally appear to take advantage of tools dealing with problem structuring, assessing uncertainty and performing sensitivity analysis.     

Anomalous viscosity behavior of some poly(methyl methacrylate)s prepared by anionic polymerization

A series of poly(methyl methacrylate)s with molecular weights in the range 3.0 × 10⁴–1.03 × 10⁶ have been prepared by anionic polymerization and their limiting viscosity numbers determined in a variety of solvents. It was found that whereas the higher-molecular-weight polymers behaved normally, the lower-molecular-weight polymers showed anomalous behavior. First, the limiting viscosity numbers of the lower-molecular-weight polymers were much higher than expected, and, second, their number-average molecular weights as measured by gel permeation chromatography were considerably smaller than those determined by membrane osmometry.     

Properties of ferromagnetic micropowders

Very small particles of iron, cobalt, and iron-cobalt alloys have been prepared by an evaporation-condensation technique. The crystal structure and shape of these particles have been investigated by X-ray diffraction and electron microscopy.The iron and alloy particles were all found to be bcc and the cobalt particles fcc. No hcp cobalt was observed. The shape of all particles was predominantly spherical. The electron microscope showed that the particles possessed a distribution of sizes and were linked together to form chains.Magnetic measurements have been carried out at 290 K on assemblies of these particles. Coercivity, saturation and remanence curves obtained using a Faraday balance magnetometer and pulsed field magnetometer are presented. These show the particles to have fairly high coercivities (typically 1 kOe) but fairly low remanence to saturation ratios (typically 0.25).     

Production Bonus Schemes: A Cautionary Tale

A case study is presented showing how a simple logical analysis of the causes of a problem, rather than its symptoms, efficiently solved some production and accounting difficulties.Discrepancies between accounting records and physical stocks, and substandard production were found to be caused by a badly designed factory bonus scheme. Modification of the scheme solved both problems.     

Preparation,fractionation, and dilute-solution behavior of ABA poly(methyl methacrylate-b-α-methylstyrene) block copolymers

The preparation by anionic polymerization of six ABA poly(methyl methacrylate-b-α-methylstyrene) block copolymers and of sixteen poly(α-methylstyrene)s is described. The block copolymers, of similar molecular weight but with different chemical compositions, were fractionated by preparative gel permeation chromatography and their behavior in dilute solution was investigated using viscometry. The results obtained indicate that the intramolecular phase separation does not occur under the conditions utilized, the block copolymers assuming randomcoil configurations in all of the copolymer/solvent systems studied. Consequently the block copolymer molecules are more expanded than homopolymers of the same molecular weight. The series of poly(α-methylstyrene)s covered the molecular weight range 2.7 × 10³–1.3 × 10⁶ and enabled the determination of Mark–Houwink–Sakurada constants for poly(α-methylstyrene) in the solvents chosen for the block copolymer studies.     

Self-referencing, spectrally, or spatially encoded spectral interferometry for the complete characterization of attosecond electromagnetic pulses

We propose a method for the complete characterization of attosecond duration electromagnetic pulses produced by high harmonic generation in an atomic gas. Our method is based on self-referencing spectral interferometry of two spectrally sheared extreme ultraviolet pulses, which is achieved by pumping the harmonic source with two sheared optical driving pulses. The resulting interferogram contains sufficient information to completely reconstruct the temporal behavior of the electric field. We demonstrate that such a method is feasible, and outline two possible experimental configurations.     

Sum frequency generation at 309 nm using a violet and a near-IR DFB diode laser for detection of OH

We have used a violet diode laser at 404 nm and a distributed feedback diode laser at 1320 nm to produce 0.8 nW of radiation at 309 nm by sum frequency generation in beta-barium borate. The UV radiation was tuned mode-hop-free over 30 GHz and used to detect OH radicals produced in a microwave discharge. By chopping the UV light at 500 Hz, we observed a concentration of 2×10¹² cm^-3 with a signal to noise ratio of 30:1. Received: 16 November 2001 / Revised version: 23 January 2002 / Published online: 14 March 2002