Gas chromatographic determination of aromatic amines in water samples after solid-phase extraction and derivatization with iodine: I. Derivatization

A new method for the selective determination of aromatic amines is presented, which is based on the solid-phase extraction at pH 9 and subsequent derivatization of the analytes to the corresponding iodobenzenes. These can selectively and sensitively be determined with gas chromatography and electron-capture detection. Separation of at least 30 compounds in a single chromatographic run in 30 min is possible. With this method, 56 aromatic amines were investigated, and only in six cases no derivatives were obtained. Limits of quantitation were between 0.5 and 8 μg l⁻¹, but may still be lowered with higher sample volumes or different injection techniques. The application to water samples revealed the suitability for the investigation of ground, leachate and wastewater.     

Band gap uniformity and layer stability of HgTe-CdTe superlattices grown by photon-assisted molecular beam epitaxy

Two material properties important to the application of HgTe/CdTe superlattices for device fabrication are band gap uniformity and thermal stability. In this paper, we present the results of an infrared photoluminescence study of (211)B HgTe/CdTe superlattices grown by photon-assisted molecular beam epitaxy which show that cut-off wavelength uniformity can be controlled to a level commensurate with the demands of advanced infrared detector fabrication. Infrared photoluminescence and transmission electron microscopy were also employed to demonstrate that (211)B HgTe/CdTe superlattices are less prone to interdiffusion than previously believed.     

Structure and methyl groups internal rotation of difluorodimethylsilane

The rotational spectra of the normal and ¹³C isotopomers of difluorodimethylsilane have been measured by molecular beam Fourier-transform microwave (FTMW) and free-jet absorption millimeter-wave (FJ-AMMW) spectroscopy. Each rotational transition is split into several component lines due to the two methyl groups internal rotations. The corresponding V₃ barrier has been determined to be 5.256(3) kJ mol⁻¹. A r₀ structure of the heavy atoms skeleton has been obtained. The silicon atom lies—accidentally—on the center of mass of the molecule.     

A model for GRIN-SCH-SQW diode lasers

A comprehensive model for graded-index, separate-confinement-heterostructure, single-quantum-well (GRIN-SCH-SQW) Al_xGa_1-xAs diode lasers is presented, and compared with experimental data. The model combines many individual features not heretofore included together, and gives good agreement with gain-vs.-current density data for different structure variations. In addition, the threshold temperature dependence agrees well with data for typical laser conditions, and the high-gain kink in T₀ versus temperature is qualitatively explained     

Convergence conditions for restarted conjugate gradient methods with inaccurate line searches

Convergence properties of restarted conjugate gradient methods are investigated for the case where the usual requirement that an exact line search be performed at each iteration is relaxed.The objective function is assumed to have continuous second derivatives and the eigenvalues of the Hessian are assumed to be bounded above and below by positive constants. It is further assumed that a Lipschitz condition on the second derivatives is satisfied at the location of the minimum.A class of descent methods is described which exhibitn-step quadratic convergence when restarted even though errors are permitted in the line search. It is then shown that two conjugate gradient methods belong to this class.Sponsored by the United States Army under Contract No. DA-31-124-ARO-D-462.     

Linear and widely linear filtering applied to iterative detection of generalized MIMO signals

To suppress the co-antenna interference in multiple input multiple output (MIMO) systems, an iterative receiver with a linear detector for complex symbols is investigated. We show that the considered generalized MIMO system, i.e., a system that transmits complex conjugate repetitions in addition to the pure data, requires the application of a widely linear (wl) detector. Awl detector consists of four real filters which are represented by two complex filters for the received signal and its complex conjugate, respectively. Furthermore, we present approximations of the detector that significantly reduce computational complexity with only little loss in frame-error rate performance. Simulation results show that the proposed MIMO system achieves large gains over standard solutions.     

Investigation of monolayer roughness in HgTe-CdTe superlattices

Infrared photoluminescence (PL) measurements were performed on (2ID-oriented superlattices with energy gaps in the range 110–495 meV. Most of the samples with thinner HgTe quantum wells displayed two PL peaks separated by Δhω ≈ 30–65 meV (which generally increased with decreasing well thickness). Both peak energies (E_p) sometimes varied gradually with location on the surface, and in one case three peaks of approximately equal spacing were observed in some locations. The data are consistent with a model which assumes the presence of randomly distributed islands having well thicknesses varying by approximately one monolayer. We find that Ahco and the variations of the spectra with temperature agree well with calculations based on this simple model.