Quantitative analysis of solid mixtures by diffuse reflectance measurements

The factors affecting the reflectance of an absorbing substance mixed with material not absorbing at the wavelength of measurement were investigated and it was found that the effect of particle size could be eliminated by prolonged grinding. It is shown that the percentage by weight of the absorbing substance is directly proportional to the quantity (A/R)^1.383 (R = percentage reflectance ; A = 100 — R) and that this quantity is additive for 2 absorbing substances. A method is developed for the accurate determination of low concentrations of one and two absorbing substances mixed with non-absorbing material. The method is applicable to poorly crystallized substances which give no characteristic X-ray powder diffraction patterns as well as to crystalline solids. The reflectance of some substances is shown to be dependent on the previous history of the particular sample. The method is not applicable if there is physical interaction between the components of the mixture.     

Intra-cavity generation of superpositions of Laguerre–Gaussian beams

In this paper we demonstrate experimentally the intra-cavity generation of a coherent superposition of Laguerre–Gaussian modes of zero radial order but opposite azimuthal order. The superposition is created with a simple intra-cavity stop that creates equal losses for the two azimuthal modes, and we show that by adjustment of the stop we can produce modes up to azimuthal order 8. The fact that we have a coherent superposition rather than an incoherent superposition is verified by intensity measurements, propagation measurements and a decomposition of the field by an inner product executed on a phase-only spatial light modulator. Such fields have relevance in quantum information and optical trapping.     

An electron spin resonance study of dimeric,trimeric, and tetrameric paramagnetic silver atom clusters

The low temperature γ-irradiation of frozen benzene solutions containing AgClO₄ leads directly to the formation of the paramagnetic species Ag₂ ⁺, no Ag⁰ centre being detected. When benzene or toluene solutions containing AgBF₄ were similarly treated and the matrix carefully annealed, well-resolved E.S.R. spectra attributed to paramagnetic clusters containing three and four equivalent silver atoms were detected. The lack of resolved anisotropy in the spectrum of the trimer and tetramer are thought to argue in favour of a triangular and tetrahedral geometry respectively rather than linear chains.     

An array model for the field adsorption of helium on tungsten (111)

A theory of field adsorption has been developed, using a charged-surface model comprising an infinite regular array of superimposed monopoles and dipoles, together with a distant array of monopoles of the opposite polarity. Previous theoretical treatments are shown to be oversimplified, and the effects of various corrections to the present treatment are explored. For helium on tungsten (111) in an external field of 56 V/nm, the conventionally-defined field-induced contribution to the short-range binding energy is estimated to probably lie between 40 and 75 meV, and the total differential short-range binding energy is estimated to probably lie between 30 and 50 meV. As far as is known, this is the first detailed theoretical discussion of field adsorption on a specific crystal face, and a paradigm situation has been chosen for investigation.     

A fresh look at the electric-field dependence of surface-atom binding energy

This paper aims to clarify the theoretical concepts associated with the field dependence of surface-atom binding energy. The various coefficients involved are defined, and orbital-polarization and charge-transfer effects are distinguished. It is demonstrated that the latter, as well as the former, may have an F² form. A new empirical method of estimating the polarization-energy coefficient for a field-evaporating surface atom is described: this is based on the joint measurements of activation energy and onset appearance energy made by Ernst. His data for rhodium are reanalysed to give the value 1.05 ± 0.3 meV V^?2 nm². This value is then used to show that polarization-type effects will influence the interpretation of field-sensitivity data. Further experiments are called for.     

Acoustical Klein-Gordon equation: a time-independent perturbation analysis

The perturbation analysis of an ideal acoustical duct was first made by Rayleigh in 1878 and the result has since stood in the literature. However, the analysis is based on the assumption of potential and kinetic energy densities that remain constant as a change in cross section occurs, whereas, in fact, they may fluctuate significantly in comparison to the slowly varying "wave function," Psi(x,t), of the acoustical Klein-Gordon equation. The square of the time-independent eigenfunction, psi(2)(x), is directly proportional to the potential energy per unit length of fluid, and it is shown that it is precisely the perturbation in potential energy that defines correctly the eigenvalue shifts.     

Primordial galactic magnetic fields from domain walls at the QCD phase transition

We propose a mechanism to generate large-scale magnetic fields with correlation lengths of 100 kpc. Domain walls with QCD-scale internal structure form and coalesce, obtaining Hubble-scale correlations while aligning nucleon spins. Because of strong CP violation, the walls are ferromagnetic, which induces electromagnetic fields with Hubble-size correlations. The CP violation also induces a maximal helicity (Chern-Simons) which supports an "inverse cascade," allowing the initial correlations to grow to 100 kpc today. We estimate the generated electromagnetic fields in terms of the QCD parameters and discuss the effects of the resulting fields.     

A model for the channel noise of MESFETs including hot electron effects

Noise is an important consideration in the reliability of microelectronic circuits determining the sensitivity of the circuits and placing a lower limit on the regions of operation. Proper modeling of noise in integrated circuits is essential for reliable operation. A derivation is given for the channel noise coefficient of FET’s operating in the saturation region. Some simple approximations are made for hot electron effects which can be incorporated into the derivation and accounted for by a numerical integration technique. Experimental results of measured and calculated noise coefficients are compared for depletion mode MESFETs of different gate lengths. This model gives a much more realistic representation of the channel noise coefficients for short gate length devices rather than the simple 2/3 value currently used in circuit simulations.