Focussing of laser beams by means of a z-pinch formed plasma guiding system

It is shown that during a capillary discharge, plasma channels for guiding light from high-power lasers are temporarily formed. The experiments are performed with alumina capillaries of 2-mm radius and 14-cm length filled with about 100 Pa of helium, methane or hydrogen. The transmission of probe light is enhanced due to light guiding by a factor up to 20 during a time window of 5 ns. The results indicate the formation of a first channel, when the shock wave reaches the axis, and of a second channel after the reflection of the shock wave on the axis. Compared to simple focussing by a lens, in the guiding device the intensity–length product increases by almost two orders of magnitude. Guiding is achieved over 180 times the Rayleigh length. Received: 20 June 2000 / Revised version: 4 August 2000 / Published online: 6 September 2000     

Intensity enhancement of the C5+ Balmer radiation excited by capillary discharge pumping

The intensity of the 18.2 nm Balmer α-transition in C⁵⁺ excited in a capillary discharge using alumina and polyacetal tubes was studied. For discharge currents of up to 80 kA in tubes filled with C₂H₂, intense radiation from the excitation of C⁵⁺ ions and from the recombination of C⁶⁺ ions was observed. With increasing length of the discharge, the intensity in the falling edge of the recombination pulse rises faster than proportional. In contrast to previous investigations, gain by stimulated emission is excluded. The enhancement is ascribed to an optical guiding of the XUV radiation in the dense plasma created by ablation from the tube walls. Received: 1 April 1999 / Revised version: 22 July 1999 / Published online: 30 November 1999     

Theoretical determination of the X 2Σ+ and A 2Π potentials of CsO using relativistic effective core potentials

Theoretical potential energy curves are computed for the X ²Σ⁺ and A ²Π states of CsO using a relativistic effective core potential and a large valence Gaussian basis set. Seventeen electrons are correlated by a CI (SD ) calculation from each HF reference. We find the X ²Σ⁺ state lower by 497 and 726 cm^?1 at the HF and CI(SD) levels. Our calculated ω_e of 312 cm^?1 for the X ²Σ⁺ state agrees well with experimental values deduced from studies in matrices.     

Stieltjes imaging of atomic and molecular photoabsorption profiles

The effective transition frequencies and oscillator strenghts obtained from principal representations of spectral moments, or from variational procedures using appropriate square-integrable basis functions, are employed in the Stieltjes imaging of atomic and molecular photoabsorption profiles.     

DasK/β+-Verhältnis beim Zerfall J128→T128

In the decay of J¹²⁸ the intensity of a weak positron activity was measured to be (3.1±0.7)·10^?5 per disintegration. The remeasurement of the totalK capture was found to be 0.059±0.007 in agreement with the results of other authors. From this theK/β ⁺-ratio of the decay to the ground state of Te¹²⁸ is 1800±400. Theoretical calculations ofK/β ⁺ for allowed transitions show that the maximum energy of the positrons is 245±12 keV. In the decay of Rh¹⁰⁴ and Lu^176m no positrons more than 5·10^?6 could be detected.     

The slowing down of atoms with energies ofE<10 eV in crystals

The line shape of the 773 keV emission line emitted by¹⁸⁷Re depends on the slowing down of the recoil the atom receives in the precedingβ transition. Using the nuclear resonance fluorescence method the line profile was studied for sources where the radioactive atoms are embedded in single crystals of W and WSe₂ and in polycrystalline samples of Nb. The experimental results for W and Nb are in complete agreement with predictions by a theory in which the slowing down is calculated starting from a Born-v. Karman lattice model. The temperature dependence of the line shape yields the phonon lifetimes in W and Nb (W) to beτ(300 K)=(2.4±0.3)ps andτ(300 K)=(1.7±0.2) ps, respectively. Measurements for the slowing down of the recoiling atoms perpendicularly to and along the hexagonal axis of WSe₂ yielded an anisotropy in qualitative agreement with expected values.     

Die Linearpolarisation der Vernichtungsstrahlung von Positronen

A new measurement of the relative linear polarisation of the annihilation radiation of positrons is described. The angular resolution of the double-polarimeter used has been improved in comparison with earlier experiments by the construction of a fast fourfold-coincidence apparatus. Within an experimental error of 1,5% the results are in full agreement with the complete polarisation predicted by pair theory.     

The dissociation energy of ionic molecules; Selected oxides and fluorides: LiO,LiF, BeO,BeF, MgF,CaF and SrF

The dissociation energies, D₀, of several oxides and fluorides have been computed using a technique that has been shown to yield very accurate D₀ for ionic molecules. The theoretical values are LiO(3.84 eV), LiF(6.09). BeO(4.56). BeF(5.91), MgF(4.67). CaF(5.46), and SrF(5.50). These are in excellent agreement with experiment, except for LiO where our value is about 0.35 eV larger.