Bibliothek

Notizen: Guided ion-beam techniques are used to measure the cross sections for reaction of CF4 with Ar+, Ne+, and He+ from thermal to 50 eV. Dissociative charge transfer followed by successive loss of F atoms are the major processes observed. Only CF+x (x=1–3) products are observed in the reactions of Ar+ and Ne+. With He+, in addition to the CF+x products, both C+ and F+ are seen at high kinetic energies. Reaction rates for these reactions are also given and compared with previous measurements. It is found that the energy dependence of the cross sections can be understood by considering the energies needed to access specific electronic states of the CF+4 ion.

Notizen: Absolute cross sections are measured for the reactions of O2+(X 2Πg) with H2 , D2 , and HD from thermal energies to over 4 eV. The OH++OH, HO2++H, O++H2O, and H2O++O product channels (and the corresponding isotopic analogs) are observed, although H2++O2 is not. While the first three products appear at their thermodynamic thresholds, formation of H2O++O, the least endothermic channel, exhibits a barrier to reaction. In the HD system, the DO2+ product ion is strongly favored over the HO2+ product. Results for internally excited O2+ reactants, probably the a 4Πu state, are also presented. Analysis of the excitation functions, molecular orbital arguments, and statistical kinetic theories are used to understand the mechanisms and dynamics of this reaction. It is shown that the inefficiency of the O+ product channel is due to spin and symmetry constraints. The other three product channels proceed through a long-lived intermediate, but formation of this intermediate from reactants requires surmounting a barrier measured to be 1.1±0.1 eV. The intramolecular isotope effects are shown to be due to statistical and dynamic effects.

Notizen: We present beam measurements which have been carried out using a CHORDIS ion source supplied with a modified extraction system. This system is designed to handle extraction voltages up to 60 kV. For beam diagnostics, a Faraday cup and a sector magnet have been used. Presented are measurements on beam current, mass, and energy spectra in dependence on various source parameters as gas pressure, arc voltage, and extraction voltage.

Notizen: Thin films of the noble metals Ag and Au are prospective candidates for application as electrical contacts in high temperature superconductor (HTSC) thin film devices. We present first results on intrinsic stress in Ag and Au films measured during and after their UHV deposition onto YBa2Cu3O7−x thin film substrates. At 110 K film growth proceeds by columnar grain growth with tensile stresses as huge as the tensile strength of the bulk phase. At room temperature and up to 600 K the films grow via island growth developing much smaller film stress. At temperatures above 550 K a compressive stress contribution due to interdiffusion of oxygen from the HTSC substrates into the contact films is observed.

Notizen: Traveltime and amplitude residuals of P waves from teleseismic events show a positive correlation at the stations of the German seismic broad-band array GRF. Fast arrivals consistently have reduced amplitudes in the four frequency ranges considered (short-period bandpass, WWSSN-SP filter, broad-band velocity and WWSSN-LP filter).With the exception of the WWSSN-LP filter, 40–60 per cent of the average of the relative traveltime and almost 100 per cent of the average of the amplitude residuals at individual stations, with respect to the reference station A1, can be explained by near-surface sediment layers. These shallow structures are known from local and regional geology and are documented in borehole, local refraction and polarization studies. This aximuthal- and distance-independent term (station average) reaches values of up to -0.55 s and -0.15 units in log amplitude (magnitude).Even after the removal of these station averages, which also contain a trend of increasing crustal and uppermost mantle velocity from north to south, large azimuthal- and distance-dependent relative traveltime and magnitude residuals can be observed across the array. These residuals vary between -0.8 and +1.1 s and -0.54 and +0.72 magnitude units. Negative (fast) traveltime residuals are again related to negative magnitude residuals (small amplitudes). The azimuthal- and distance-dependent patterns of these residuals are identical within three distinct clusters of stations. The main features of the residual patterns correlate well with the main fault systems and inhomogeneities near the array, some of which have been postulated previously. The velocity anomalies causing the azimuthal- and distance-dependent residuals are located in crust and uppermost mantle under the array and its vicinity.The fact that local effects like sedimentary covers are responsible for 40 per cent or more of the station average residuals demonstrates again that site effects should be removed before tomographic methods are employed. The simple stations corrections usually applied in global tomography might severely underestimate the influence of local and regional velocity anomalies, since even after the removal of the station average residual large azimuthal- and distance-dependent traveltime and amplitude anomalies remain between stations less than 100 km apart. These anomalies due to shallow inhomogeneities might then erroneously be mapped into features at greater depth.

Notizen: The GRF array is situated on Jurassic limestone of the Franconian Alb in SE Germany. The mislocation vectors show symmetry axes in their slowness and azimuth components. For the slowness the line of separation is at about 95d̀ against north. The azimuth pattern shows a symmetry axis nearly perpendicular to the axis in the slowness pattern. Waves arriving from NE have a reduced slowness, whereas waves from SW have a larger slowness. The largest azimuth anomalies are found in the directions where the slowness components change direction. These effects can to a large extent be modelled by a low-velocity sedimentary layer dipping to NNE with about 0.8d̀ dip. Such a sedimentary wedge correlates well with the geological data, is able to reproduce the observed mislocation vector pattern and explains a major part of the observed traveltime residuals. It furthermore demonstrates that local effects, like the influence of sedimentary covers, should be removed before inversion procedures and tomographic methods are applied.

Notizen: The aim of this study is to demonstrate what effects subduction zones, i.e. laterally inhomogeneous, high-velocity anomalies in the Earth's mantle, can have on the traveltimes and the amplitudes of teleseismic recordings, and to show that the amplitude anomalies are large enough that they can be used in the determination of the structure of subducting slabs.The method used for the computation of the seismograms in these laterally inhomogeneous media is the Gaussian beam method which, in contrast to the ray method usually used, gives correct amplitudes at caustics and in critical regions. The restriction to 2-D models in the downdip direction of the slab, i.e. the assumption that there is no change along strike of the subduction zone, reduces the number of free parameters. For the short-period waves considered here it is sufficient that the section of the slab be uniform along strike for a few hundreds of kilometres. This geometry is satisfied by many subduction zones. Furthermore we expect pronounced effects of the subduction zones to be in the downdip direction because the wave will then sample the anomaly to a large extent. The influence of the following parameters of the slab on teleseismically recorded traveltimes and amplitudes is studied in detail: maximum velocity anomaly, depth of penetration, thickness of the slab and dip change. The influence of the source location, i.e. depth and epicentre, is also considered.We compare seismograms from this modelling to those computed from a radially symmetric reference earth and observe the following effects: negative traveltime residuals, occurrence of caustics and focusing h2reasing the magnitude by up to +0.7 magnitude units, defocusing and the creation of shadow zones. Receiver regions with large amplitude anomalies are not necessarily regions where large traveltime anomalies occur. Regions with little change in their traveltime anomalies can show dramatic changes in amplitude (larger than Δmb=± 1). Small structures of subduction zones are better resolved by amplitude information than by traveltime information. A combined interpretation of traveltimes and amplitudes yields much better resolution of the structure of subduction zones because traveltimes and amplitudes contain independent information about the structure studied. This helps to eliminate ambiguities which may not be resolved when using only one type of information (traveltimes or amplitudes) especially in aseismic parts of slabs where no additional information from seismicity is available.Observations of amplitudes for deep Kurile events recorded at European stations show anomalies that can be explained by a slab with a penetration depth of 670 km, thus indicating the usefulness of this approach in the determination of subduction zone structure.

Notizen: Seismic P and S waves recorded at the GRF array in Germany are used to study the inhomogeneous structure of the boundary layer D″ at the base of the mantle. The use of the seismic array allows the detection of small-scale anomalies in the lowermost mantle. The lateral resolution attainable is about 100 to 400 km, i.e. more than 10 times better than with tomographic methods.The analysis of 13 years of GRF broad-band array data yields 255 events with high signal-to-noise ratio which are used to map the lowermost mantle. 74 of these events show anomalous P waves (PdP) which arrive 3-6 s after the direct P wave and have a slowness 0.7–0.8 s/° smaller than the slowness of direct P. Other events close to the events with PdP do not have such an anomalous phase.Using slowness, traveltime, amplitude and waveform information it is demonstrated that PdP is caused by an anomalous lower mantle velocity structure below the turning point of the P wave. If the pp phase (i.e. a P wave first reflected at the free surface near the source) is used together with the P phase, distinct and well-sepaarted P-velocity anomalies can be determined under the Nansen Basin, the Kara Sea and northern Siberia. The areas of the bounce points of PdP in the lower mantle have a lateral extension of about 100 by 200 km, but this is not the size of the anomaly, since the resolution of P waves at 1 Hz in this depth is 130 km by 260 km (Fresnel zone). The accuracy to which the depth of the reflector (2612 km under the Nansen Basin and 2605 km under northern Siberia respectively) can be determined for 1-D models is ±10 to 20 km. The velocity contrast at the lower mantle discontinuity is about 3 per cent ±1 to 1.5 per cent. Areas which have velocity fluctuations smaller than about 1 per cent can not be detected as anomalous areas. 2-D models of the anomalies reveal the range of adequate models and possible trade-offs. If the lateral extension of the anomaly is about 7° the reflector has to be 40 km deeper than in the 1-D model. If the dip of the reflector in the lowermost mantle is only about 1.5° it is difficult to resolve if the reflector is tilted towards the source or towards the receiver. For the anomaly under the Nansen Basin deviations from the great circle path are observed for PdP, indicating 3-D effects. Below the three anomalies the core-mantle boundary (CMB) can be located by PcP with a depth in agreement with standard earth models under northern Siberia has a region where P- and S-velocity anomalies coincide, but also a region where only an S-velocity anomaly but no P-velocity anomaly is observed. This results in changes of the Poisson ratio from +5.9 per cent to -4.8 per cent (±1.5 per cent) across the discontinuity in the lowermost mantle for the regions studied.The analysis of the S waves (S, SdS and ScS) reveals two S-velocity anomalies in the lowermost mantle under northern Siberia. The first anomaly coincides with the P velocity anomaly under northern Siberia and can be explained by a 1-D model with a reflector depth of 2610 km ± 15 km and a velocity contrast of 2.3 per cent. The second S-velocity anomaly is in an area where no P-velocity anomaly can be detected. The corresponding 1-D S-velocity model has a reflector depth of 2575 km ± 15 km and an S-velocity contrast of 2.6 per cent. The smallest structures that can be resolved with the S waves in this depth are about twice as large as for the P waves, i.e. 230 km by 460 km (Fresnel zone).The joint analysis of P and S waves therefore shows a region with a P-velocity anomaly together with weak indications for an S-velocity anomaly (Nansen Basin) and a second region with a P but no S anomaly (Kara Sea). The lowermost mantle

Notizen: Examples of Gräfenberg-array data showing anomalous P-waves which typically arrive 3–5 s after the direct P-wave and which have a slowness 0.7–0,8 s deg-1 smaller than direct P are presented. This additional phase is most frequently observed for events located in the NE portion of the southern Kurile Island subduction zone 73°-80° from Gräfenberg, but systematically disappears for events in the SW portion of this zone.Because of the magnitude of the slowness difference, these observations cannot be attributed to a complex source rupture process nor to multipathing through the descending slab. Likewise, they may not be accounted for by near-receiver structure because these phases are not seen for all Kurile events. If present they appear at all stations of the array but they follow direct P too closely to be a multiple from the Mono. Therefore, we conclude they are very likely caused by lower mantle velocity structure.The most likely explanation is the presence of a P velocity jump of about 3 per cent approximately 290km above the core-mantle boundary, since such a reflector in the lowermost mantle not only gives a good fit of traveltimes and slowness but is also able to model the waveform and the amplitudes of this additional P phase. the distribution of bounce points on this reflector for the Kurile events indicates a lateral extension of this velocity anomaly under northern Siberia of about 150 km by at least 200 km. the best fitting S-wave model has a reflector in the same depth, but the velocity contrast seems to be only about 2 per cent suggesting a different behaviour of the P and S velocity in D″.Few events from other regions in this distance range are suitable for a definitive analysis of this kind. From among this group some observations indicate a lower mantle anomaly under the Lomonosow Ridge and under northern Greenland; but since the lower mantle under western Siberia, northern Novaya Zemlya, the Azores Islands region and the USSR-Afghanistan border region does not produce an additional phase in the Gräfenberg recordings it is very unlikely that such a velocity anomaly in the lowermost mantle is a global feature.