Optical polarization of neutron-rich sodium isotopes and β-NMR measurements of quadrupole moments

The nuclear quadrupole moments of neutron-rich sodium isotopes are being investigated with the help of in-beam polarization by optical pumping in combination with-NMR techniques. First measurements have yielded the quadrupole splittings of NMR signals in the lattice of LiNbO₃ for the isotopes²⁶Na,²⁷Na and²⁸Na. Interaction constants and ratios of the electric quadrupole moments are derived. In view of future experiments,-decay asymmetries for the sequence of isotopes up to theN=20 neutron shell closure,^26–31Na, have been measured.     

Implanted light dopants in ZnSe

Spin polarized radioactive ¹²B, ¹²N, and ⁸Li probe nuclei are implanted in nominally undoped ZnSe at stationary concentrations ≤ 10¹⁰ cm^-3. These light dopants, one donor and two acceptors, are characterized by β-radiation detected nuclear magnetic resonance (β-NMR) measurements. The donor ¹²B and the acceptor ¹²N show quite similar and rather simple implantation behaviour, the situation is markedly different for the second acceptor ⁸Li, however. This contrast indicates that in semi-insulating ZnSe the structure of dilute impurities is controlled by individual, element-specific properties. This revised version was published online in August 2006 with corrections to the Cover Date.     

Measurement of the electric quadrupole moments of 26–29Na

The nuclear electric quadrupole moments of the isotopes ²⁶Na, ²⁷Na, ²⁸Na and ²⁹Na were measured by -NMR spectroscopy in single crystals of LiNbO₃ and NaNO₃. High degrees of nuclear polarization were produced by optical pumping of the sodium atoms in a fast beam with a collinear laser beam. The polarized nuclei were implanted into the crystals and NMR signals were observed in the -decay asymmetries. Preparatory measurements also yielded improved values for the magnetic moments of ^27-31Na and confirmed the spin I=3/2 for ³¹Na. The results are discussed in comparison with large-basis shell model calculations. Received: 1 February 2000 / Accepted: 3 April 2000     

Harmonic lattice behavior of two-dimensional colloidal crystals

Using positional data from videomicroscopy and applying the equipartition theorem for harmonic Hamiltonians, we determine the wave-vector-dependent normal mode spring constants of a two-dimensional colloidal model crystal and compare the measured band structure to predictions of the harmonic lattice theory. We find good agreement for both the transversal and the longitudinal modes. For q-->0, the measured spring constants are consistent with the elastic moduli of the crystal.     

Interaction of atomic oxygen (N2O) with a clean Si(001) surface: O adsorption geometries as derived from the O KVV Auger lineshape

The room temperature adsorption of N₂O on the clean Si(001)2 × 1 surface was used as a model system in an Auger electron spectroscopy (AES) study presented in this paper. Earlier experimental and recent theoretical work have provided evidence that this reaction evolves in discernible stages each exhibiting different adsorption geometries for the oxygen atom. In this AES study the intensity ratio of the KL₁L₁ and KL_2,3L_2,3 O Auger transitions, , was measured as a function of the fractional oxygen coverage, θ, and compared with our calculated intensity ratios and binding energy measurements of the O 1s photoelectron from literature. As a result we have found, for the first time, that (θ) can be related to a specific adsorption geometry in the submonolayer range. Moreover, we have found experimental evidence for an intermediate stable O adsorption state on the dimer at low coverage (θ 0.2 monolayer), as proposed earlier from theoretical studies.     

Quantitative Auger depth profiling of LPCVD and PECVD silicon nitride films

Summary Thin silicon nitride films (100–210 nm) with refractive indices varying from 1.90 to 2.10 were deposited on silicon substrates by low pressure chemical vapour deposition (LPCVD) and plasma enhanced chemical vapour deposition (PECVD). Rutherford backscattering spectrometry (RBS), ellipsometry, surface profiling measurements and Auger electron spectroscopy (AES) in combination with Ar⁺ sputtering were used to characterize these films. We have found that the use of (p-p)heights of the Si LVV and N KLL Auger transitions in the first derivative of the energy distribution (dN(E)/dE) leads to an accurate determination of the silicon nitride composition in Auger depth profiles over a wide range of atomic Si/N ratios. Moreover, we have shown that the Si KLL Auger transition, generally considered to be a better probe than the low energy Si LVV Auger transition in determining the chemical composition of silicon nitride layers, leads to deviating results.

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Quantitative Auger-Tiefenprofilanalyse von LPCVD- und PECVD-Siliciumnitridfilmen

     

Correlation between dynamical heterogeneities, structure and potential-energy distribution in a 2D amorphous solid

We investigate the collective properties of particles in a 2D experimental system which consists of a bi-disperse mixture of colloidal particles confined at an air/water interface. We find a direct correlation between structure and dynamical heterogeneities in this system: particles belonging to locally ordered structures have lower potential energy and are slower than other particles. In a more general way we show that particles with high potential energy are dominating the dynamics especially in the α-relaxation regime.     

Carbon-carbon linkage of ethene by nickel-chelate complexes

Results will be described oligomerizing/polymerizing ethene. Using nickel catalysts in combination with P-O-bidentate (chelate) ligands ethene can be converted to high molecular weight polyethylene. The use of chelate ligands is essential, and emphasis is given to the importance of bidentate chelates.