A SAXS study of the nanostructural characteristics of TEOS-derived sonogels upon heat treatment up to 1100 °C

Xerogels obtained from the acid-catalyzed and ultrasound stimulated hydrolysis of TEOS were submitted to heat treatment at temperatures ranging from 60 to 1100 °C and studied by small-angle X-ray scattering (SAXS). The SAXS intensity as a function of the modulus of the scattering vector q was obtained in the range from q₀=0.19 to q_m=4.4 nm⁻¹. At 60 °C the xerogels exhibit an apparent surface fractal structure with a fractal dimension D_S∼2.5 in a length scale ranging from 1/q₁∼1 to 1/q_m∼0.22 nm. This structure becomes extremely rough at 120 °C (D_S∼3) and at 150 °C, it apparently converts to a mass fractal with a fractal dimension D∼2.4. This may mean an emptying of the pores with preservation of a share of the original mass fractal structure of the wet aged gel, for it had presented a mass fractal dimension D∼2.2. A well characterized porous structure formed by 2.0 nm mean size pores with smooth surface of about 380 m²/g is formed at 300 °C and remains stable until approximately 800 °C. At 900 °C the SAXS intensity vanishes indicating the disappearance of the pores in the probed length scale. The elimination of the nanopores occurs by a mechanism in which the number of pores diminishes keeping constant their mean size. The xerogels exhibit a foaming phenomenon above 900 °C and scatter following Porod's law as does a surface formed by a coarse structure.     

Effect of the Arsenic Concentration on the Characteristics of the Growth Step Echelon in GaAs Epitaxy

The effect of the arsenic concentration in the vapor phase on the growth step distribution over the surface of GaAs epitaxial layers grown in a chlorine-hydride vapor-transport system on substrates with 4° (111)A and (113)A orientations is studied. It is demonstrated that the average distance between steps in the echelon depends on the arsenic concentration and increases with it up to a certain constant value. It is assumed that this is connected with the change in the kink density at the steps.     

Entangled state generation and modulation of vacuum squeezing by classical optical fields in birefringent fiber

Evolution of the vacuum fluctuations in the two-wave mixing of the optical fields propagating in a birefringent fiber is studied. The two-wave mixing in the birefringent fiber was suggested as a possible scheme for the entangled-state generation. Our treatment in studying the entangled-state generation uses depleted pump approximation and enables one to trace the influence of the input conditions of classical optical fields on the evolution of vacuum squeezing. We report the periodical modulation of the vacuum squeezing when the input relative phase of coherent waves varies. The measure of nonclassical correlations imposed on the generated light is calculated.     

Foliations by complex curves and the geometry of real surfaces of finite type

We show that if the Levi form of a smooth CR manifold is de-generate in every conormal direction, then on a dense open set, the manifold is foliated by complex curves. As a consequence we show that every real analytic manifold of finite D'Angelo type can be stratified so that each stratum locally is contained in a Levi nondegenerate hypersurface. Received in final form: 11 June 2001 / Published online: 28 February 2002     

Simulation of time-resolved vibronic spectra and the possibility of analyzing molecules with similar spectral properties

The possibility of using time-resolved vibronic spectroscopy for spectral analysis of mixtures of chemical compounds with similar optical properties, when traditional methods (based on stedy-state spectra) are inefficient, is demonstrated by using the method of computer simulation. The analysis is carried out by the example of molecules of a series of polyenes (butadiene, hexadiene, octatetraene, decapentaene, and decatetraene), their various cis-and trans-rotational isomers, and phenyl-substituted polyenes. Ranges of relative concentrations of molecules similar in their spectral properties, where reliable interpretation of time-resolved spectra of mixtures and both qualitative and quantitative analyses are possible, are determined. The use of computer simulation methods for optimizing full-scale experiments in femtosecond spectroscopy is shown to hold much promise.     

Quark masses from regularized integrals of vector current spectral functions

We give estimates of quark masses from a comparison of two methods of regularizing the coefficient of the Schwinger term. The masses of the first radial excited states of ?, ω, and ? would have to be slightly higher than indicated by the new Orsay data for this method to yield real solutions for the masses of theu, d, ands quarks.