Triply excited three-electron systems--semiclassical model

We calculate rovibronic intrashell spectra of the triply highly excited atomic hydrogen dianion, helium anion and lithium atom, within a simple semiclassical model. Zero-order electronic energy levels and half-lives are calculated for a number of principal quantum numbers and approximate thresholds for the appearance of vibronic modes are estimated. Since no quantum-mechanical and experimental data are available for the highly excited levels (N > 5), where the semiclassical models apply, no comparison with other results are possible at present. The problem of comparing semiclassical and quantum-mechanical calculations for moderately large quantum numbers, which seem attainable by the present day experimental technique, has been discussed. Received: 16 September 1998 / Received in final form: 16 March 1999     

Impedance spectroscopic study of Si/HF-electrolyte system during Si dissolution

Porous silicon (PS) has been synthesized by anodic etching in HF and ethanol electrolyte. The in situ current–voltage characteristic clearly identify the pore formation, transition and electropolishing regimes. The experimentally observed impedance and phase as a function of frequency (Bode plots) reasonably agree with theoretically simulated Bode plots drawn by considering the equivalent circuit for PS/electrolyte. Received: 10 December 1999 / Accepted: 12 February 2000 / Published online: 13 September 2000     

Grain boundaries and the law of corresponding cones in smectics

Focal Conic Domains (FCDs) in smectic phases often assemble according to a particular rule, experimentally discovered by G. Friedel, the law of corresponding cones (l.c.c.). This paper reports various results relating to this type of association. First we show that a l.c.c. contact between 2 focal conic domains has a vanishing energy, yielding metastable local equilibrium. Then we use some projective properties of conic sections to extend the celebrated Apollonian tiling, which describes a tilt grain boundary (TiGB) of vanishing disorientation made of toric focal conic domains, to any TiGB. Finally we present a realistic model of the energy of the TiGB, which we compare to the energy of a TiGB split into dislocations, and to the energy of a curvature wall. This model explains why FCD tilings show macroscopic zones not filled with FCDs. Received 21 June 1999 and Received in final form 10 September 1999     

Propagation characteristics of the rectangular flattened Gaussian beams through circular apertured and misaligned optical systems

Based on the fact that a hard aperture function can be expanded into a finite sum of complex Gaussian functions, the approximate analytical expression for the output field distribution of a rectangular flattened Gaussian beam passing through a circular apertured and misaligned paraxial ABCD system is derived. The result brings more convenient for studying its propagation than the usual way by using diffraction integral directly. Some numerical simulations are also given for illustrating the propagation properties of a rectangular flattened Gaussian beam through a circular apertured and misaligned optical system.     

The electric field modulation by lateral crack coated with HfO2/SiO2 high reflector

High-dielectric constant multilayer coatings are commonly used on optics to increase the laser damage resistance of optics. In this work, the three-dimensional finite-difference time-domain is improved to simulate the electric field intensity distribution in the silica–hafnia multilayer coatings on fused silica subsurface with lateral crack. Results reveal that as the substrate is defect-free, the largest intensification is at the surface layer, especially in the hafnia layer. As the lateral crack appears on the subsurface of fused silica, the intensified sites are mainly located at distorted coatings layers. Dependences of peak LIEF on crack's parameters are detailed investigated. Results show that laser intensification is more sensitive to its width, and the modulation of lateral defect with silica–hafnia multilayer is greater than nodular defect as its width larger than 5λ.     

Propagation characteristics of higher-order annular Gaussian beams in atmospheric turbulence

The propagation characteristics of higher-order annular Gaussian (HOAG) beams in turbulence are investigated. From a HOAG source plane excitation, the average intensity of the receiver plane is developed analytically. This formulation is verified against the previously derived HOAG beam solution in free space. The graphical outputs indicate that, upon traveling in turbulent atmosphere, the HOAG beam will undergo different stages of evolution. At intermediate propagation distances, it will attempt to concentrate the energy near the origin. In this process, the appearance of the single higher-order primary beam will be encountered. Eventually HOAG originated beam will become a pure Gaussian beam after propagating an excessive distance in the turbulent medium.     

Theoretical Investigation of Decay Process in a Doubly Excited 2s^2 ^1S0 State of He-Like Ions

In the framework of multi-configuration Dirac-Fock theory, a detailed calculation is performed for the decay rates and the energies of the doubly excited 2s^2 ^1So state of He-like ions, of which atomic number Z ranges from 6 to 92. The 2s^2 ^1So - ls ^2S1/2 Auger decay is predominant at low Z regime, whereas the 2s^2 ^1So - 1s2p ^1,3P1 two-electron one-photon transitions become quite important in moderate and high Z regimes. For heavy ions with Z ≥ 72, the contribution of 2s^2 ^1So - 1s2s ^3S1 M1 transition is significant. The Breit interaction considerably enhances the 2s^2 ^1So - 1s ^2S1/2 Auger rate at high Z regime.     

A tuneable, single-mode titanium-doped-sapphire laser source with variable pulse duration in the nanosecond regime

We built and characterized a high-energy, injection-seeded, single-longitudinal-mode pulsed titanium-doped-sapphire laser with tuneable wavelength in the near-infrared and variable pulse temporal duration in the nanosecond regime. We show experimentally how the pulse duration can be easily varied either by changing the cavity length or by changing the pump energy. We successfully interpreted these results on the basis of a theoretical model which treats the operation of this type of laser as a gain-switching technique. Also, as far as the stabilization of the laser cavity, compared to the traditional techniques we employed a novel simplified solution involving the use of an avalanche photodiode.     

Influence of negative pressure on thermoelectric properties of Sb2Te3

We investigated the influence of negative pressure on the electrical conductivity, the Seebeck coefficient, and the power factor of Sb₂Te₃. We performed first-principles calculations with the linearized-augmented plane-wave method considering negative hydrostatic pressure in the range from zero to −2 GPa and doping for electrons and holes up to 10²⁰ cm⁻³. Our results predict a significant increase of the Seebeck coefficient and the power factor under negative pressure for certain doping concentrations.     

Collisional excitation soft X-ray laser at 23.6 nm in a laser-produced cylindrical target

We have tested soft X-ray lasing in neon-like germanium with cylindrical targets where wave guiding and plasma confinement may affect lasing. An intense soft X-ray laser beam of 0.05 MW peak power and a narrow beam divergence (8 mrad) was produced at 23.6 nm with a 4 cm long straight cylindrical target of 0.72 mm inner diameter. Bending the cylindrical target to form a toroidal shape increased the lasing intensity by a factor of 3 accompanied with reduction of the beam divergence from 8 to 6 mrad.