Bibliothek

Notizen: Vacuum scanning tunneling microscopy has been used to investigate the hydrogen-terminated Si(111) surfaces obtained upon dissolution of the native oxide in HF and NH4F solutions. Whereas etching in aqueous HF acid produces an atomically rough surface, comparable treatment in NH4F results in atomically flat surfaces. These atomically flat surfaces are extremely well ordered and exhibit terraces which extend thousands of angstroms.

Notizen: NI in argon and in argon/10% N2 matrices has been excited into b 1Σ+, v'=0–4 with a pulsed tunable dye laser. The regular line structure in the fluorescence excitation and emission spectra in pure argon matrices is attributed to a local phonon progression of 15 cm−1, both in the X 3Σ− and b 1Σ+ states. "Hot'' local phonon lines, which originate in the thermally populated levels n'=1 and 2 of the local phonon mode at elevated temperatures, have been identified. The matrix lifetime of the vibrationally relaxed b state, which does not relax via the a state in pure argon, extrapolates to a radiative lifetime of 15 μs in vacuo, in satisfactory agreement with a recent gas phase determination. The a state of NI, which emits at 1185 and 1173 nm, could be populated via the b state only in mixed matrices of argon and N2. The lifetime in these matrices is 20.5±2 ms, which extrapolates to a gas phase radiative lifetime of approximately 38 ms. The b state of NI is completely quenched in pure nitrogen matrices, but emission from the a state is still detected with a lifetime of 4.6±0.5 ms. On the basis of these results recently reported a-state lifetimes of NCl and NBr in the ms to sub-ms range appear unreasonably low.

Notizen: NBr in solid argon has been excited to the b 1Σ+ state with a pulsed tunable dye laser. In addition to the well-known fluorescence spectrum of the b state, infrared emission from the vibrationally relaxed a 1Δ state was detected at 1086 nm. Three major trapping sites were resolved in absorption and fluorescence excitation spectra of the b–X system. Additional sites could be identified in the excitation spectrum of the a state, which is populated via the b state by internal conversion. There is a weak site effect on the lifetime of the vibrationally relaxed b state, which varies between 40 and 30 μs. Vibrational relaxation rates in the b state show a stronger site dependence. Internal conversion from b 1Σ+ to a 1Δ accounts for 〈0.1% to at least 17% of the decay rate of NBr b 1Σ+, v=0. Orbitally forbidden transitions to the X10+ component of the ground state were identified 23.0 to 25.5 cm−1 on the high-energy side of the more intense a 1Δ→X21± transition of several sites. The lifetime of the a state varies between 152±12 ms in the least perturbed site and 121±15 ms in the site with the strongest a 1Δ→X10+ forbidden component. NBr trapped in double vacancies of pure fcc argon, of single stacking faults, and of multiple stacking faults (hcp pockets) in argon, can account for the sites.

Notizen: The photodissociation of F2 in crystalline Kr is subject to only a minor cage effect—quantum yield of 0.5 at 308 nm, at T=12 K. Two-photon-induced harpooning in the same system leads to dissociation with near unit quantum efficiency; it is shown that this absorption is coherent with a giant cross section of 3(±2)×10−45 cm4 s at 275 nm. Excitation and emission spectra of charge-transfer transitions in solid Kr doped with F atoms, and solid Ar multiply doped with F/Kr are reported. The vertical excitation in F/Kr leads to a state delocalized over immediate neighbors—assigned to Kr+6F−. This state localizes on a subpicosecond time scale to the triatomic Kr+2F− configuration. The latter relaxes radiatively (τr=140 ns), to the repulsive wall of the ground state (∼1 eV above ground). The excitation in F:Kr/Ar solids shows the diatomic KrF(B←X) resonances. From the analysis of the spectrum, a harmonic frequency ω=75(±3) cm−1 is extracted for F atoms in the ground state, consistent with molecular-dynamics simulations of an octahedrally trapped F atom [ω=70(±1) cm−1]. The diatomic KrF(B) state relaxes on a time scale of 20 fs〈t〈500 fs to the mixed triatomic configuration, (ArKr)+F−, which then relaxes radiatively (τr=60 ns). The kinetic energy released upon the radiative dissociation of triatomic exciplexes generates acoustic waves, and defects, and leads to mass transport. The modes of propagation of the acoustic waves, their subsequent thermalization, and self-annealing of the solid, are directly monitored by spatially and temporally resolved transient scattering experiments. A transient loss (σ(approximately-equal-to)10−17 cm2), the origin of which is not fully understood, is observed for Kr2F isolated in solid Kr. The mass transport, and long-range migration of F atoms (∼10 lattice sites), is directly monitored in multiply doped Ar solids in which the shuttle of F atoms between Xe and Kr centers can be demonstrated.

Notizen: In the Basle region of Switzerland we demonstrate that rockfall blocks can be dated accurately using the radiocarbon method. Soil beneath rockfall blocks containing organic macro-remains was sampled at four cliff sites in the Tabular Jura south-east of Basle. A lightweight drilling rig drills a 101-mm-diameter hole through rockfall blocks up to 6 m in height. A downhole sampler is used to recover soil samples from below the blocks. Eighteen radiocarbon dates on charcoal, wood, needles of conifers and organic macro-remains from 11 blocks gave dates between 970 and 440 bp. Calibrated ages range between ad 1210 and ad 1450. These results, along with field observations and historical research, suggest that a large number of blocks toppled as rockfalls within a short time interval in the epicentral area of the ad 1356 Basle earthquake.

Notizen: In the past, the doorstopper method has been used successfully for determining in situ tectonic stresses. Here an example is given about the application of the borehole slotter in the domain of tectonic stress determinations, which is an innovative strain relief technique without overcoring. Data are presented that test the reliability of the strain measurements by the borehole slotter under well-known stress conditions, that indicate the reproducibility of borehole slotter strain data compared with those from doorstopper measurements, and that demonstrate the utility of the borehole slotter even for the determination of weak tectonic strains. In northern Switzerland the borehole slotter was used in a horizontal borehole under lithostatic loading conditions only, in a rock slope area with very heterogeneous stress conditions, and in a vertical borehole where tectonic stress should be dominant. The results show that the borehole slotter is a reliable technique, which allows precise determination of in situ strains. In addition, borehole slotter measurements are less time-consuming and therefore less costly than most other stress-measuring techniques—an important aspect in a future of reduced research budgets.〈section xml:id="abs1-2"〉〈title type="main"〉SummaryThe borehole slotter is a strain relief technique, using a recoverable strain sensor that need not be glued to the rock surface and need not be overcored. It is quicker to use than most other stress or strain measuring techniques, especially in comparison to standard overcoring techniques (Fig. 14).The borehole slotter supplied convincing results with respect to principal stress orientations, in good agreement with:(i) principal stress orientations related to lithostatic stress conditions (Dittingen 2 site),(ii) palaeostress orientations during the main stage of Jura folding only 13 to 7 Ma (Laufen 1 site),(iii) the results of neighboring door-stopper measurements (Dittingen 1 site).The borehole slotter tests do not supply stress magnitudes directly. Thus, the elastic properties of the rock mass have to be determined independently. Stress ratios from borehole dotter tests in Dittingen 2 are realistic.In conclusion, the borehole slotter is a fully developed probe to measure in situ strains in boreholes. It allows the reliable determination of a large number of data within a short time even in weakly stressed rocks. This makes the borehole slotter a powerful tool for the determination of contemporary regional tectonic stress fields.

Notizen: Abstract— This paper presents the results of an experimental testing programme to examine the uniaxial creep, low cycle fatigue and creep/fatigue interaction behaviour of a Ni-base superalloy at 700°C. The material is used in the manufacture of aeroengine turbine discs. A creep continuum damage mechanics model is shown to be capable of accurately predicting the creep and creep rupture behaviour of the material. A healing term has been incorporated into the damage mechanics model to allow the behaviour under creep/fatigue conditions to be described.