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Notes: The band structure of hexagonal boron nitride (BN) has been calculated using an ab initio linear combination of pseudoatomic-orbitals method. The calculated band structure confirms a previous finding that this material is an indirect band-gap insulator and has two empty interlayer bands. Projected densities of states are compared with the experimental x-ray emission spectra of B and N K edges and the agreement is good. This good agreement between the present ground-state calculation and the experimental x-ray emission spectra supports our previous finding that there should be very little valence electron relaxation effect on x-ray emission spectra. A real-space Green's function technique and the Z+1 approximation have been used to calculate the exciton spectra of B and N K edges. The first peak at 192 eV in B K edge is found to be a bound exciton with a binding energy of 1.7±0.4 eV. Only resonance is found for the N K edge. The calculated exciton spectra agree very well with the experimental quantum-yield and electron-energy-loss spectra.

Notes: Various approximations in the calculations of electron energy-loss cross section and x-ray absorption coefficient have been evaluated using the Si L2,3 edge as an example. It was found that the random phase approximation, for which the cross terms are neglected in the expressions of electron energy-loss cross section and x-ray absorption coefficient, is a very good approximation. The dipole approximation in the calculation of the electron energy-loss cross section was found to begin to break down even at small acceptance half angles, such as 12.5 mrad. For energy losses larger than that of the Si L2,3 edge the dipole approximation will break down faster. This approximation also fails for a large scattering angle even at a small energy loss. The electron energy-loss cross section and x-ray absorption coefficient of the Si L2,3 edge has been calculated and compared. The large difference in relative intensity reported experimentally between the two spectra is not confirmed in this study. More experimental comparisons between electron energy loss and x-ray absorption spectra are recommended for Si and other compounds.

Notes: The properties of double layers produced in current filaments have been studied. The experiments were performed in a magnetized triple plasma device in which the diameter of the central plasma column could be varied. The scaling of the double layer potential drop Vdl with the parameter jd2, where j is the current density in the double layer and d is the double layer thickness, was determined for several values of the current filament radius R0. For relatively large values of R0, the one-dimensional (Langmuir) scaling was obeyed, Vdl∼(jd2)2/3. When R0 was decreased, a departure from the strictly one-dimensional scaling was observed, with Vdl becoming less and less dependent on the parameter jd2.

Notes: SUMMARY.— The records of 554 consecutive patients attending hospitals in the Cambridge region with urticaria have been analysed with the aid of a computer. These patients represent only a small minority of all patients with urticaria in the region. In 79%, the aetiology was unknown although in many cases aggravating factors, for example, psychological stress, aspirin, or infection, were detected.A past or family history of atopic disorders was not found more frequently in patients with urticaria than in controls and the course of the disease in patients with a background of atopy did not differ from the remainder, suggesting that an undetected allergy is not responsible for many of these undiagnosed cases. Allergy is relatively more important in acute urticaria and in patients who do not attend hospital.The frequency of attacks and total duration of the disease are different in patients with urticaria alone, angio-oedema alone and both together but there was little difference in the aetiological factors, supporting the concept that urticaria and angio-oedema are fundamentally similar.The prognosis is expressed in the form of life tables.

Notes: Valence shell electronic excitations in silicon, and its carbide, nitride, and oxide are studied using electron energy loss microspectroscopy in a transmission electron microscope, at 1-eV resolution with a 100-keV electron beam. This so-called "low-loss'' region of the spectrum is normalized to scattering cross section per valence electron for each compound, and a large, sequential, variation in the cross section across the series of compounds is reported. The low-loss cross section is calculated using the Born approximation by including single-electron transitions in the target. Both band-to-band and band-to-continuum transitions are included, by using extended Hückel band calculations to obtain the valence shell crystal states, using the electron gas model for the ionization states, and using the random phase approximation to obtain differential cross sections. The calculations yield semiquantitative results, and allow for a chemical interpretation of the experimental results. The relation of the present model to that for plasmon excitation is also briefly discussed.

Notes: Core-shell electronic absorption edges from thin specimens of silicon, α-silicon carbide, β-silicon nitride, and amorphous silica are studied by using electron-energy-loss spectroscopy in a transmission electron microscope. The elemental and chemical effects in the near-edge regions of the Si L2,3 and C, N, and O K edges are calculated by using some semiempirical models. The chemical effects in the region of the edges near-edge onset are due to valence-shell excited states, which we have modeled as linear combinations of atomic orbitals using the extended Hückel method, with the effects of translational periodicity in crystals included by using Bloch wave functions. Population analyses of valence-shell electronic structure and cross sections for bound→bound atomic transitions are used to interpret and calculate theoretical near-edge fine structure for direct comparison with experiment. The near-edge ionization region is calculated by using a plane-wave excited state to account for elemental effects. Chemical effects in the ionization region are accounted for by including contributions from the elastic backscattering of outgoing waves by the atoms that neighbor the excited atom. The elemental and chemical effects in the edges are shown to be separable to a large extent by using these models, and calculated cross sections are in good semiquantitative agreement with experimental results.

Notes: A modified parametrization of the complete neglect of the differential overlap molecular orbital method is applied to energy-band calculations of solids. Results of the band calculation are used to calculate the near-edge fine-structure of core-shell electronic absorption edges in electron energy loss spectra. Preliminary application is made to silicon crystal.

Notes: Transmission electron energy loss spectra were obtained from small (approximately 100-A(ring) diam) regions of a series of single-phase silicon-containing specimens at 100-keV incident beam energy, using a field emission source transmission electron microscope fitted with a magnetic sector spectrometer. The specimen foils were diamond cubic silicon, α-silicon carbide, α-silicon nitride, and amorphous silica. The SiL near-edge structure depends markedly upon the chemical environment of the silicon. In this paper we show that the changes in near-L-edge structure, including threshold onset energy shift and edge profile, result from bond-induced changes in the valence shell electronic structure of the specimen materials. Extended Hückel molecular orbital theory was used to calculate the valence shell electronic structure of five-atom tetrahedral clusters, with silicon as the central atom and the other atoms noted above in corner positions. Inelastic electron scattering cross sections for silicon 2p and 2s core shell transitions to valence shell excited states were then calculated using the first Born approximation.

Notes: Objective To evaluate the predictive value of transvaginal Doppler ultrasound studies of the uterine and umbilical arteries in early pregnancy, in identifying pregnant women at risk of subsequently developing pre-eclampsia, or the delivery of a small for gestational age infant.Design A multivariate logistic regression of Z scores of Doppler indices obtained from the uterine and umbilical arteries of 652 women with singleton pregnancies at 12 to 16 weeks of gestation. Measurements included the presence or absence of a notch, bilateral (right and left waveform) notching, vessel diameter, the resistance index, the pulsatility index, time averaged mean velocity (cm/s), maximum systolic velocity (cm/s), and volume flow (mL/min). Stepwise logistic regression and multivariate analysis of all the parameters measured was used to construct several scoring systems.Main outcome measures Pre-eclampsia, birthweight, preterm delivery.Results In women that developed complications, there was a trend towards increased resistance and reduced velocity and volume flow. If bilateral notches were present there was an increased risk of pre-eclampsia (odds ratio [OR] 21.99, 95% CI 6.55–73.79), premature delivery (OR 2.38, 95% CI 1.19–4.75), and the delivery of a small for gestational age baby (OR 8.63, 95% CI 3.95–18.84). Using multivariate analysis, a seven parameter model was selected (after removal of vessel diameter, uterine and umbilical resistance index). This model produces a scoring system with a sensitivity of 92.9% and a specificity of 85.1% for the prediction of pre-eclampsia. A three parameter model (bilateral notches, uterine resistance index, umbilical pulsatility index) provides similar sensitivities, but lower specificities, when compared with the seven parameter model.Conclusion These data indicate that there are differences in uterine and umbilical artery Doppler blood flow indices at 12 to 16 weeks, in pregnancies with a normal or complicated outcome. Scoring systems derived from multivariate analysis of Doppler indices demonstrate the potential of being able to identify, in early pregnancy, a group of women at increased risk of the subsequent development of pre-eclampsia, premature delivery, or the birth of a small for gestational age baby.