Strahlungsverhalten von Festkörperlasern hoher Pumpleistung

Ohne Zusammenfassung     

Determination of the deuterium abundance in water using a cw chemical DF laser

This paper describes a new method for the determination of the deuterium content in water samples. The absorption of the radiation of a cw deuterium fluoride laser by HDO in water vapor is measured by means of a nonresonant spectrophone (optoacoustic detector). This method enables one to evaluate the isotope ratio of hydrogen in a rather simple and straightforward way. Advantage is taken of the fact that HDO absorbs some of the DF laser lines whereas H₂O shows no absorption. Due to the small concentration in natural water deuterium is practically entirely present as HDO. Therefore a selective determination of the deuterium content is possible. In the course of the investigation the absorption coefficients of HDO for the different laser lines were determined. It was thereby established that there exists a strong coincidence of an HDO absorption line with the 2P2 DF laser line. The detection sensitivity of the method was then examined for some selected laser lines. The concentration of HDO in natural water samples could be determined with an accuracy of better than 10%. The experiments, however, show that with appropriate improvements it should be possible to achieve an accuracy of better than 1%. This makes the method very interesting as an analytical tool in hydrology.     

Search for ΔS = 2 decays of neutral Ξ-hyperons

A search for the decay Ξ⁰→pπ^? has been performed in a neutral hyperon beam derived from the 24 GeV/c external proton beam at CERN. The experiment gives as a 90% confidence upper limit for the branching ratio $\text{Γ(Ξ}{}^0\text{→pπ}{}^{\mathrm{?}}\text{)}\text{Γ(Ξ}{}^0\text{→Λπ}{}^0\text{)<3.6×10}{}^{\mathrm{?5}}$.     

Optimal control of coupled spin dynamics: design of NMR pulse sequences by gradient ascent algorithms

In this paper, we introduce optimal control algorithm for the design of pulse sequences in NMR spectroscopy. This methodology is used for designing pulse sequences that maximize the coherence transfer between coupled spins in a given specified time, minimize the relaxation effects in a given coherence transfer step or minimize the time required to produce a given unitary propagator, as desired. The application of these pulse engineering methods to design pulse sequences that are robust to experimentally important parameter variations, such as chemical shift dispersion or radiofrequency (rf) variations due to imperfections such as rf inhomogeneity is also explained.     

Construction of universal rotations from point-to-point transformations

For a desired range of offsets, universal rotations of arbitrary flip angle can be constructed based on point-to-point rotations of I(y) with half the flip angle. This approach allows, for example, creation of broadband or bandselective refocusing pulses from broadband or bandselective excitation pulses. Furthermore, universal rotations about any axis can be obtained from point-to-point transformations that can easily be optimized using optimal control algorithms. The construction procedure is demonstrated on the examples of a broadband refocusing pulse, a broadband 120(x) degrees rotation and a z-rotation with offset pattern.     

Optimized hole injection with strong electron acceptors at organic-metal interfaces

The energy-level alignment at interfaces between three electroactive conjugated organic materials and Au was systematically varied by adjusting the precoverage of the metal substrate with the electron acceptor tetrafluoro-tetracyanoquinodimethane (F4-TCNQ). Photoelectron spectroscopy revealed that electron transfer from Au to adsorbed F4-TCNQ was responsible for lowering the hole-injection barrier by as much as 1.2 eV. This novel interface modification scheme is independent of the charge transfer complex formation ability of the organic materials with the electron acceptor.     

Variation of the conformal radius

We study the change of the conformal radiusr(U) of a simply connected planar domainU versus the subdomainU _ε consisting of the points of distance at least ε to ∂U. We show that the smallest exponent λ such thatr(U)-r(U _t)=0(e ^λ) satisfies 0.59<λ<0.91. We also show that a well-known conjecture implies . Partially supported by NSF Grant DMS-9970398.     

Test by NMR of the phase coherence of electromagnetically induced transparency

Electromagnetically induced transparency is an effect observed in atomic systems, originating from quantum interference, in which electromagnetic transitions to and from a certain quantum state become suppressed. This dark state is also characterized by a quantum phase, relative to other states, which theoretically should stop evolving, but remain phase coherent, during transparency. We test this theoretical prediction using techniques developed for liquid-state nuclear magnetic resonance quantum computation, applied to a spin-7/2 nuclear spin system. A sequence of quantum operations is applied to create the dark state, and during transparency its phase evolution is measured relative to a reference state using Ramsey interferometry. Experimental measurements of the fringe visibility are in excellent agreement with theoretical expectations, taking into account measured decoherence rates.     

Atomic structure of a (2 x 1) reconstructed NiSi2/Si(001) interface

Nickel disilicide/silicon (001) interfaces were investigated by aberration corrected scanning transmission electron microscopy (STEM). The atomic structure was derived directly from the high spatial resolution high angle annular dark field STEM images without recourse to image simulation. It comprises fivefold coordinated silicon and sevenfold coordinated nickel sites at the interface and shows a 2 x 1 reconstruction. The proposed structure has not been experimentally observed before but has been recently predicted theoretically by others to be energetically favored.