Ge Reflection Research Articles

High-resolution study of (222, 113) three-beam diffraction in Ge

The results of high-resolution analysis of the (222, >113) three-beam diffraction in Ge are presented. For monochromatization and angular collimation of the incident synchrotron beam a multi-crystal arrangement in a dispersive setup in both vertical and horizontal planes was used in an attempt to experimentally approach plane-wave incident conditions. Using this setup, for various azimuthal angles the polar angular curves which are very close to theoretical computer simulations for the plane monochromatic wave were measured. The effect of the strong two-beam 222 diffraction was observed for the first time with the maximum reflectivity close to 60% even though the total reflection of the incident beam into a forbidden reflection was not achieved owing to absorption. The structure factor of the 222 reflection in Ge was experimentally determined.

Phonon effects in resonant “forbidden” reflections

The influence of atomic vibrations on the ”forbidden” Bragg reflections, excited near absorption edges owing to anisotropic resonant X-ray scattering, is discussed. It is shown that in spite of the difference of X-rays and phonons energies the “forbidden” reflections can be extremely sensitive to the atomic vibrations and their correlations. The numerical calculations are performed taking into account the correlations of atomic thermal displacements, which give an adequate description of the thermal growth of the 600 forbidden reflection in Ge and describe drastic changes of the “forbidden” reflections spectra in ZnO. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Electrochemically- and Photo-Induced IR Absorption of Low Band-Gap Polydithienothiophenes: A Comparative Study

ABSTRACTPoly(dithieno[3,4-b:3',4'-d]thiophene) (pDTT1), poly(dithieno[3,4-b:3',2'-d]thiophene (pDTT2) and poly(dithieno[3,4-b:2',3'-d]thiophene) (pDTT3) are low band-gap polymers that undergo both p- (oxidation) and n- (reduction) type electrochemical reversible doping in organic electrolytes. In this work we report on the in-situ IR spectroscopic characterization of both doping types using attenuated total reflection (ATR) FTIR spectroelectrochemistry. Thin polymer films were prepared on Ge reflection elements by electrosynthesis. During electrochemical potential cycling experiments in a spectroelectrochemical cell, the evolution of infrared active vibrational (IRAV) modes, correlated with the generation of charge within the polymer chain, was studied. In addition to electrochemical doping, the polymers exhibit also charged excitations by photodoping. FTIR spectra recorded by illumination/dark cycles show photoinduced IRAV bands which are similar to those found for the electrochemically p-doped materials. The infrared spectroscopic results are compared with Raman spectra of the neutral polymers. The relationship between vibrational properties and structure of the polymers is discussed in terms of different contribution of the φ-electrons within the repeating units to the electronic structure of the conjugated backbone.

Determination of phase using multiple-beam effects

It is shown that the absolute phase of a weak reflection can be obtained from an experimental integrated intensity measurement by taking into account multiple-beam effects. The absolute structure factor of the basis-forbidden 622 reflection in Ge has been measured. The contribution from weakly excited reflections whose phases are known acts as a reference wave for the reflection in question. It is further shown that, even in the limit of weak excitation of multiple beams, the intensity distribution in reciprocal space is determined by dynamical, not kinematical, theory. The remarkable result is demonstrated that several hundred beams are required to fully calculate X-ray multiple-beam effects, and an efficient approximation is presented for multiple-beam dynamical calculations with weak reflections. The intensity of weak multiple-beam effects, at least in the diamond structure, is roughly proportional to Z4, where Z is the atomic number.

Diffraction studies of the (222) reflection in Ge and Si: Anharmonicity and the bonding electron

We present the results of a combined x-ray and neutron diffraction study of the temperature dependence of the forbidden (222) reflection in germanium. Integrated intensities were measured from room temperature to 850\ifmmode^\circ\else\textdegree\fi{}C with x rays and to 904\ifmmode^\circ\else\textdegree\fi{}C with neutrons. In addition, an earlier x-ray study of the silicon (222) reflection has been improved and extended to 800\ifmmode^\circ\else\textdegree\fi{}C and a correction has been made in the corresponding neutron results. The germanium and silicon data are interpreted in terms of anharmonic atomic vibrations and anticentrosymmetric valence-charge distributions. Contrary to our previous reports, the temperature dependence of the scattering from the bonding electrons is found to be similar to that from the core electrons.