Monochromatizationof synchrotron radiation down to about 10 eV at an energy of 14.4 keV has been achieved by double nuclear Bragg diffraction from "Fe-yttrium iron garnet single-crystal films set for the electronically forbidden (200) reflection. The monochromatized y quanta have been analyzed with respect to time delay and capability of resonance absorption. By setting of appropriate energy and time windows a pure beam of resonant y quanta at a rate of about 1 Hz is available.PACS numbers: 76.80.+y, 07.85.+ n, 61.10.Fr y quanta with an energy width of 10 6 to 10 s eV, which is typical for low-lying nuclear states, have a coherence length of 0.2 to 20 m. Thus a highly brilliant beam of such quanta would open new perspectives to experimental y optics. Unfortunately, the usual radioactive sources are monochromatic but not brilliant whereas synchrotron radiation is very brilliant but white.Resonant nuclear diffraction of synchrotron radiation as has been first pointed out by Ruby' is a suitable method to achieve the desired beam, which in addition would have a well-defined time structure and polarization pattern. A first attempt to observe resonant nuclear excitation in Fe has been published by Cohen, Miller, and West.In order to maintain the outstanding brilliance of the synchrotron radiation only coherent reflections come into consideration, i.e. , either grazing-incidence reflec-tions3 or nuclear Bragg and Laue diffraction. 4 5 Obviously the 14.4-keV resonance of Fe is a good choice because the excitation energy is easily available from any synchrotron radiation source which covers the xray range and because the mean life v = 140 ns of the excited state allows the application of delayedcoincidence techniques. Furthermore, the chemistry and crystallography of iron compounds is well known, so that the needed large-size single crystals can be made.While present experience with grazing-incidence antireflection films indicates that four sequential reflections are required in order to obtain the desired suppression of the nonresonant prompt radiation, only two nuclear Bragg reflections will be sufficient if pure nuclear reflections are used. The greatest progress with this technique has been published by Chechin et al. , who reported an enhancement of delayed counts behind an o. -Fe203 single crystal positioned for the I777) pure nuclear reflection.

Epitaxial (001) diamond film were grown on mirror-polished single-crystalline (001) silicon substrates by microwave plasma chemical vapor deposition from a methane/hydrogen gas mixture. The films were characterized by means of scanning electron microscopy, Raman spectroscopy, and x-ray analysis. The results show that the diamond crystallites are oriented to the silicon substrate with both the (001) planes and the [110] directions parallel to the silicon substrate.

No abstract

The nucleation of diamond on silicon (100) in a methane-in-hydrogen microwave plasma has been investigated by atomic force microscopy (AFM), scanning electron microscopy (SEM), and reflection high energy electron diffraction (RHEED). The nucleation of diamond was performed by application of an electrical substrate potential. It was found that three-dimensional non-facetted islands are initially formed whose sizes increase with the deposition time. In spite of their very small sizes of a few nanometers, RHEED reveals that the islands are of crystalline diamond structure. An induction time of about 6.5 min was necessary for the diamond nucleation, which is partly caused by the formation of a silicon carbide surface layer due to the carbon diffusion into the substrates. The time dependence of nucleation density was investigated and fitted with a model kinetics which considers the formation, destruction, and capture of active sites, germs, and nuclei. Analyses of the first nearest neighbor distance distributions reveal the formation of a depletion zone of nuclei around the existing islands. These results confirm the role of the adatom diffusion improved by the bias-enhanced ion bombardment. Analyses of the island size and the island height distributions show constant growth rate at beginning of the diamond deposition. From the nearly constant ratios of the island height to island size, independent of the nucleation time, one can deduce that the thermodynamics accounts not only for the growth mode, but also for the island shape during deposition. The bias-enhanced ion bombardment during deposition may increase also the diffusion of the surface atoms of the islands, allowing the system to approach equilibrium

Atmospheric‐pressure dielectric barrier discharges (DBDs) in argon with admixtures of small amounts of hexamethyldisiloxane (HMDSO) have been analysed by means of numerical modelling. A time‐dependent, spatially one‐dimensional fluid‐Poisson model has been used, which takes into account the spatial variation of the discharge plasma between the plane‐parallel dielectrics covering the electrodes. Main features of the model, including the reaction kinetics for HMDSO, are given. Good agreement with related experimental studies of the ignition voltage for HMDSO amounts of up to 200 ppm and the temporal course of the discharge current for conditions typical of deposition experiments is obtained by the model calculations when assuming that 30% of the reactions of HMDSO with excited argon atoms, with a rate coefficient of 5.0 × 10−10 cm3/s, lead to the production of electrons due to Penning ionization. The modelling results for constant frequency f = 86.2 kHz and applied voltage Ua = 4 kV show that the electrical energy dissipated in the DBD decreases with an increasing amount of HMDSO and enable the determination of the energy absorbed per HMDSO molecule on the basis of their energy balance. The analysis of the plasma–chemical processes also makes clear that collision processes of HMDSO with excited argon atoms and molecules leading to neutral reaction products are essential for the formation of thin polymer films.