Multielement atomic absorption analysis using Hadamard transform spectroscopy with a new computation and superposition procedure

The application of Hadamard transform spectroscopy (HTS) to analytical atomic spectroscopy has been investigated. A theoretical examination of the signal to noise ratio indicates that HTS offers a slight multiplex advantage over single slit scanning of the spectrum for the measurement of intense lines, as in atomic absorption, but is disadvantageous for measuring small signals in atomic emission and fluorescence. Using a simple HT spectrometer Mg and Pb were determined simultaneously by flame atomic absorption spectrometry. Sensitivities similar to those of conventional systems were obtained but, owing to instrumental imperfections and a short data collection time, detection limits were worse. Optimum system performance was obtained when the mask interval width was equal to the width of the image of the spectrometer entrance slit. Greater spectral detail was revealed by superposition of a set of computed spectra in which the starting point of each spectrum was displaced from the others by a distance less than the interval width of the Hadamard mask. This approach gave improved spectra without increased instrumental complexity.     

Balanced sets in an independence structure induced by a submodular function

A submodular (and non-decreasing) function on a set induces an independence structure; the notion of a “balanced” set in this situation helps us determine whether a given independence structure is induced by any submodular function other than its own rank function, answering a question of U. S. R. Murty and I. Simon. The notion “balanced” also has a natural meaning when one independence structure is induced from another across a bipartite graph.     

The mass spectra of alkylbenzenes—II. The formation of [M ? methyl]+ from tri- and tetra-methylbenzenes

A study of the [M ? methyl]⁺ ion in the mass spectra of ring-deuterated tri- and tetramethylbenzenes has confirmed the partial randomisation of hydrogen and deuterium atoms observed by other workers. The results can be explained by postulating randomisation of the peripheral entities, followed by several cycles of ring expansion and contraction, prior to fragmentation.     

Surface complexation of DNA with insoluble monolayers. Influence of divalent counterions

DNA interacts with insoluble monolayers made of cationic amphiphiles as well as with monolayers of zwitterionic lipids in the presence of divalent ions. Binding to dioctadecyldimethylammonium bromide (DODAB) or distearoyl-sn-glycero-3-phosphocholine (DSPC) monolayers in the presence of calcium is accompanied by monolayer expansion. For the positively charged DODAB monolayer, this causes a decrease of surface potential, while an increase is observed for the DSPC monolayers. Binding to dipalmitoyl-sn-glycero-3-phosphocholine preserves most of the liquid expanded-liquid condensed coexistence region. The liquid condensed domains adopt an elongated morphology in the presence of DNA, especially in the presence of calcium. The interaction of DNA with phospholipid monolayers is ion specific: the presence of calcium leads to a stronger interaction than magnesium and barium. These results were confirmed by bulk complexation studies.     

Spatially homogeneous random evolutions

Spatially homogeneous random evolutions arise in the study of the growth of a population in a spatially homogeneous random environment. The random evolution is obtained as the solution of a bilinear stochastic evolution equation. The main results are concerned with the asymptotic behavior of the solution for large times. In particular, conditions for the existence of a stationary random field are established. Furthermore space-time renormalization limit theorems are obtained which lead to either Gaussian or non-Gaussian generalized processes depending on the case under consideration.     

Silicon doping of MBE-grown GaAs films

Two concentration ranges of silicon doping in MBE-grown GaAs films have been investigated in some detail. In lightly doped films, with a free-electron concentration of ≈10¹⁶cm^?3, low-temperature photoluminescence spectra have been analysed to develop a model to account for spectral features previously attributed to Ge and Si acceptor levels. In heavily doped films, a maximum free-electron concentration of ≈7×10¹⁸ cm^?3 has been obtained, which is only rather weakly dependent on growth conditions and the nature of the arsenic species (As₂ or As₄). Transmission electron microscopy has shown that no significant precipitation effects occur when higher Si fluxes are used but there is evidence for autocompensation. The maximum PL intensity (300 K) is found at a lower free electron concentration then with Sn-doped films, and is more sharply peaked, but there is no evidence for an anomalous Moss-Burstein shift.     

Rear leit observation

We have observed light emission from the rear of CaF₂-roughened Al-I-Au tunnel sandwiches. Like the forward emitted light, its spectral intensity shows a sharp drop at energies > 2.5 eV. We interpret the results in terms of both emissions being mediated by a common surface or interface plasmon; the plasmon is damped above 2.5 eV by excitation of an interband transition in Au.     

Density functional theories which are upper bounds to the Hartree-fock limit in one dimension

We begin by constructing the Hartree-Fock density matrix for two model systems, polynomial wavefunctions and the equal-density orbitals. The former yields the generalization of a well-known results for the harmonic oscillator whilst the latter permits the derivation of a density functional theory.