ALBERT

Notes: Far infrared laser magnetic resonance (FIR-LMR) spectra of the CHD radical in its X˜3A‘ electronic ground state were observed and assigned. The radicals were generated in the reaction of Na atoms with CHDBr2. LMR spectra were observed using seven laser lines at wavelengths around 100 to 200 μm. The spectra were assigned to six different rotational transitions and the molecular parameters of CHD were determined by a least squares fit. A number of additional transitions, observed using a laser line at λ=184.3 μm but not yet assigned in detail, were attributed to a coupling between the accidentally almost degenerate NKaKc=505 and 413 rotational levels induced by the εab term in the spin–rotation Hamiltonian and by the off-diagonal components of the hyperfine coupling tensor.

Notes: Single cyrstalline ZnTe layers have been successfully grown on (001) GaAs substrates by hot wall epitaxy at substrate temperatures between 280–370 °C. The vapor phase near the substrate surface was investigated by a quadrupole mass spectrometer. Transmission electron microscopy, reflectivity, and photoluminescence were used for growth optimization, impurity identification, and strain determination. The biaxial inplane strain is about ε(parallel)=−0.5% at the interface and ε(parallel) (320 °C)=−0.06% at the surface for thicknesses of 1–5 μm. Two luminescence peaks can be assigned to As and N.

Notes: Wide gap II-VI semiconductors have strong second order susceptibilities χ(2) and are therefore promising materials for efficient second harmonic generation. We have grown high quality single crystalline ZnSe/ZnTe/ZnSe/GaAs (001) waveguides by metalorganic-vapor-phase-epitaxy. Using end-fire coupling we observe a phase matched signal of the 1170.5 nm fundamental wave. The fundamental beam is generated by a tuneable KTP optical-parametric-oscillator pumped by a ps-Ti:sapphire laser system. Phase matching is achieved by coupling the TE0 fundamental mode with the TM2 second harmonic mode within the symmetric ZnTe waveguide. © 1995 American Institute of Physics.

Notes: The effect of particulate volume fraction vp and diameter dp on the composite Young's modulus Ec is studied both experimentally, using a silica bead/epoxy system, as well as with the help of computer simulations. The experimental and simulation results show that for a given particulate size, the overall Ec vs vp curve displays a concave upward shape and not a linear shape. This superlinear trend of the data implies that the average strain normalized to the applied strain λ=ε¯p/εc transferred to the particulates increases with volume fraction. The above finding is explained in terms of a mean-field picture, where a single particle interacts with an effective medium consisting of the remaining particles embedded in the matrix. As the modulus of the effective medium surrounding a reference particle increases with vp, the modulus mismatch between the reference particulate and the medium is consequently reduced. This leads to an overall increase in the normalized average strain λ transferred to each particulate as vp is increased. The experimental results using silica particulates with various sizes dp, as well as the simulation results, show that smaller particulates provide an increased composite modulus as compared to larger particulates, at constant vp. General equations are developed, which relate the composite modulus to the average particle stress or strain, given only information about the volume fraction and the Young's modulus of each of the phases present.Through the application of these relations, it is found that smaller particulates display a greater amount of normalized average strain λ transferred than larger particulates. The effect of particulate Young's modulus Ep in combination with particulate size on the resulting Ec is also studied using simulations only. It is found that for a low particulate to matrix modulus ratio Ep/Em, the particulate size has very little influence on Ec. Moreover, the shape of the Ec vs vp curve can be well approximated by a straight line up to large values of vp. On the other hand, as the ratio Ep/Em is increased, the superlinear trend of the composite modulus Ec vs vp data is more apparent. This results in a smaller range of the Ec vs vp curve, which can be approximated by a linear function. It is also found that the extent of this linear region also decreases with particle size.

Notes: The long-term compositional stability of hydrogenated and deuterated amorphous germanium was studied by infrared absorption measurements. The results reveal for substrate temperatures below about 200 °C the growth of a void-rich material which is oxidized rapidly by ambient oxygen and water.

Notes: A brief reply to the Comment is provided. Both the procedure I suggested and the tables presented by Carroll [Textile Res. J. 47, 56 (1977)] allow easy and fast estimation of contact angles.

Notes: The characterization of the physicochemical nature of interfaces is a key problem in the field of advanced fibrous composites. The macroscopic regime contact angle, which reflects the energetics of wetting at the solid-liquid interface, is difficult to measure by usual methods in the case of very thin cylindrical fibers, but it may be calculated from the shape of a liquid droplet spread onto a cylindrical monofilament using a method developed by Yamaki and Katayama [J. Appl. Polym. Sci. 19, 2897 (1975)], and B. J. Carroll [J. Coll. Interf. Sci. 57, 488 (1976)]. Unfortunately, measurements of the contact angle based on this method are, so far, unable to provide an accuracy of better than about 5°. In the present article two simple extensions of the method of Yamaki and Katayama and Carroll, are presented, from which highly accurate values of the contact angle may be obtained. This is demonstrated experimentally from the spreading of glycerol droplets on carbon fibers and epoxy droplets on aramid fibers.

Notes: Investigations of the relationship between changes in climate and the variation of composition of the atmosphere can be performed most directly and with the highest resolution possible by analyzing the air sampled and preserved in the polar ice sheets for hundreds of thousands of years by nature itself. For the determination of CO2, CH4, N2O, and CO in air samples of 1–3 cm3 extracted from ice cores, a high-frequency modulated tunable diode laser absorption spectrometer has been developed. The instrument can measure CO2, CH4, N2O, and CO at ambient mixing ratios of 300 ppmv (1 ppmv=10−6 volume mixing ratio), 1700, 300, and 100 ppbv (1 ppbv=10−9 volume mixing ratio), respectively, with a precision of 1%–2%. The measurement of high CO2 mixing ratios is not constrained by the detection limit. For other gases mixing ratios of 20 ppbv of CH4, 0.9 ppbv of N2O, and 1.6 ppbv of CO are detectable with the instrument in 2 cm3 standard temperature and pressure. These detection limits and the measurement precision are sufficient for the determination of past changes in atmospheric composition. The technique is also suitable for other applications in which several infrared active trace gases have to be determined in the low ppbv and possibly even in the upper pptv range in air samples of a few cm3 or small samples of other matrices. The sensitivity or the reproducibility of the instrument can be improved by another order of magnitude by using more powerful lasers and by averaging over longer periods. Such improvement could result in a capability to detect sub-ppbv mixing ratios in samples of the present size or ppbv measurements in even smaller air samples. The higher reproducibility would also make the technique interesting for measurements of isotope ratios. © 1997 American Institute of Physics.

Notes: Color detectors based on multilayers of amorphous–silicon alloys facilitate the detection of the three fundamental components of visible light in one single pixel of a sensor array. In order to achieve sensitivity for the blue, green, and red components of light, three different bias voltages are applied to the device. By switching them sequentially the detector is read out. n-i-p-i-i-n structures with a controlled band gap and mobility–lifetime product exhibit excellent stationary properties, namely: good color separation and have dynamic behaviors above 95 dB. Besides the stationary behavior the transient response of a color detector is a further optimization criterion. The experimentally found transient photocurrent response after switching on monochromatic light at different applied bias voltages showed reasonable delay times in the range of tens of milliseconds before reaching steady state. Numerical simulations have been carried out which reproduce this characteristic behavior and facilitate a study of time dependent processes within the device, such as charge transport and storage in localized states. The delay times can be explained by the recharging of electrical defect states in the amorphous material. Consequently, the electrical potential within the device changes, which remarkably affects the carrier transport. Based on these results optimization criteria for the transient behavior of the color detectors are discussed. © 1999 American Institute of Physics.

Notes: The formation of microcrystalline silicon was investigated employing the layer-by-layer technique with very high frequency plasma excitation. Etching of the deposited layers was found to be a dominant effect during hydrogen plasma treatment. Microcrystalline growth occurred when the thickness of the deposited layer was reduced to a certain value in each cycle. The etching stopped when a noticeable crystalline volume fraction was produced; for long hydrogen treatment times no film growth was observed. © 1999 American Institute of Physics.