Photochemical free radical formation from aliphatic dicarboxylic acids in solution as studied by electron spin resonance spectroscopy

Various free radicals formed during UV.-irradiation of aliphatic dicarboxylic acids in aqueous and methanolic solution are identified by ESR.-spectroscopy. Their structures point to α-cleavage and photoreduction as the dominant primary photochemical decay processes. The relative contributions of these reactions to the overall photodecomposition depend on solvent and degree of α-alkylation of the acid. Emission ESR.-spectra are found for radicals formed by C, CO-bond cleavage of α-dimethyl substituted acids. The polarization is referred to the triplet mechanism of CIDEP. and indicates this cleavage reaction occurs from a triplet molecular state.     

Investigation of persistent photoconductivity in Si-dopedn-Al x Ga1−x as grown by molecular beam epitaxy

The persistent photoconductivity effect in Si-dopedn-Al _x Ga_1?x As layers grown by molecular beam epitaxy on (100)GaAs substrates has been investigated by detailed Halleffect and capacitance measurements at 10–300 K. In the alloy composition range 0.25<x <0.40 the electrical properties ofn-Al _x Ga_1?x As are governed by a deep electron trap having an emission barrier of 0.34–0.40 eV (depending on the doping concentration), as determined by admittance measurements. The concentration of deep electron traps, deduced from low-temperature capacitance measurements, is found to coincide with the amount of persistent photoconductivity observed in the material. Consequently, the earlier proposed population of two-dimensional subbands at the Al _x Ga_1?x As/GaAs-substrate hetero-interface, i.e. charge separation bymacroscopic barriers, can not account for the measured high overall number of persistent photoexcited carriers. Instead, the vanishing small capture rates of photoexcited electrons result frommicroscopic capture barriers. The dominant deep electron trap, which we attribute to deep donor-type (DX) centers, is found to be homogeneously distributed throughout the Al _x Ga_1?x As layer depth. From our Hall effect measurements a trap depth of 0.05–0.12 eV (depending on the doping concentration) below the conduction band is derived. The capture barrier is thus in the order of 0.30 eV. This value is in excellent agreement with data obtained from liquid phase epitaxially grown Si-dopedn-Al _x Ga_1?x As.     

In situ characterization of MBE grown GaAs and Al x Ga1−x As films using RHEED,SIMS, and AES techniques

A combination of the surface diagnostic techniques Auger electron spectroscopy (AES), reflection high energy electron diffraction (RHEED), and secondary ion mass spectroscopy (SIMS) was used in order to get more detailed information on basic processes which lead to the formation of high quality monocrystalline GaAs and Al _x Ga_1−x As films by molecular beam epitaxy (MBE) under ultra-high vacuum conditions. The formation and changes of reconstructed surface structures on (100) GaAs as a function of growth parameters were observedduring growth by RHEED. AES was used to determine the relative ratio of Ga/As on the surface for different reconstructed structures, to investigate the impurity contamination on substrate surfaces and grown films, and to study the surface segregation of Sn in MBE GaAs during doping. Finally, intentional and unintentional impurities incorporated during the growth of GaAs and Al _x Ga_1−x As by MBE were detected by the SIMS technique immediately after growth within the reaction chamber.     

Stress-induced magnetic easy axis in amorphous Fe40Ni40P14B6

In a ribbon of amorphous Fe₄₀Ni₄₀P₁₄B₆ (Metglas 2826) the iron spins tend to be parallel to the plane of the ribbon, but the distribution of spin directions within the plane is nearly random. When a uniaxial tensile stress is applied to the ribbon the spins become almost completely aligned parallel to the applied stress. The technique of Mössbauer polarimetry was used to detect and measure this effect.     

Second harmonic generation with noncollinear focusing

The propagation of off-axis Gaussian beams through an optical system is described in the paraxial approximation. The formulae for the second harmonic power for stationary focused noncollinear SHG is given. Numerical results and analytical approximations for limiting cases are provided. The theory is applied to the problem of background of noncollinear SHG in pulse duration measurement techniques.     

Characterization of polymeric dielectric insulation by thermomechanical analysis

Use of a thermomechanical analyzer, TMA, affords a rapid, precise method for determining linear expansion coefficients and linear expansivities of cross-linked polymeric insulation material. Completed cable samples were dissassembled and insulation specimens were cut longitudinally and examined with repeated temperature cycling: ethylene—propylene copolymer rubbers and filled and unfilled crosslinked polyethylenes. The TMA results indicated that all specimes were oriented, likely incurred from extrusion processing. The facility of TMA for investigating potential anisotropic behavior from prior orientation is illustrated by examples of TMA measurements on radial specimens, similarly free of mechanical constraints.     

Discrete degenerate representations of non-compact unitary groups

Two degenerate principal series of irreducible unitary representations of an arbitrary non-compact unitary groupU(p,1) are derived. These series are determined by the eigenvalues of the first and second-order invariant operators, which are shown to possess a discrete spectrum. The explicit form of the corresponding harmonic functions is derived and the properties of the discrete representations are discussed in detail. Moreover, in the Appendix, we derive the properties of the corresponding degenerate representations of an arbitrary compactU(p) group.On leave of absence from Institute of Nuclear Research, Warsaw, Poland.On leave of absence from Institute of Physics of the Czechoslovak Academy of Sciences, Prague, Czechoslovakia.     

Microwave study of proximity effects in superconductivity

The microwave surface resistance of a thin layer of copper in close contact with the surface of bulk lead is investigated. It is found that the basic ideas about proximity effects in superconductivity developed in recent years are capable of explaining the observed behaviour very satisfactorily. Our data give evidence of a complex excitation spectrum in the copper, with low energy excitations above a true gap and a group of high energy excitations which is usually observed in tunneling experiments. The response to a magnetic field shows that scattering at the free copper surface is diffuse. This leads at low fields, where a Meissner shielding current flows at the copper surface, to a decrease of the depairing energy by a large amount of first order in the vector potentialA instead of only a small second order amount. This also explains an initial decrease of the absorption when a weak magnetic field is applied. For fields over a large range below the critical field of lead superconductivity is effectively suppressed in copper. The data also give information on the increase of the penetration depth in lead because of the coating with copper and on the effective electron-electron interaction in copper: $$\lambda \to 2\lambda _L and (NV)_{Cu} \cong 0.05.$$