Charge state and diffusion of hydrogen in the TiZrNi icosahedral alloy

The electronic structure of a hydrogen atom in a 1/1 approximant of the Ti-Zr-Ni icosahedral quasicrystal is investigated using ab initio methods based on the density functional theory. The charge state of the hydrogen atom in Ti₃₆Zr₃₂Ni₁₃ with different types of tetrahedral pores, as well as the charge state of hydrogen at a ratio H/M ≈ 1.7, is determined. The hydrogen atom is found to be in a nearly neutral state. The coefficient of hydrogen diffusion in Ti₃₆Zr₃₂Ni₁₃ is calculated.     

Measurement of the Barkas effect in hydrogen

The stopping power of gaseous hydrogen for positive and negative muons at energies ranging from 3 to 100 keV has been measured by time-of-flight. A pronounced Barkas effect was observed: the energy loss in hydrogen for negative muons was found to be significantly smaller than that for positive muons. Received: 23 April 1998 / Accepted: 7 May 1998     

Dynamics of muonic atom cascade in hydrogen-helium mixtures

The deexcitation of excited muonic protium and deuterium in the mixture of hydrogen and helium isotopes is considered. Methods of experimental determination of the probability of direct atomic muon capture by hydrogen and muon transfer rates from excited muonic hydrogen to helium are proposed. Theoretical results for the population of the muonic atoms in the ground state, , are compared with the existing experimental data. Results obtained for mixtures are of interest for investigation of nuclear fusion in muonic molecules. Received: 6 August 1998 / Revised: 1st October 1998 / Accepted: 2 October 1998     

Orientational photorefractive effects observed in poly(vinyl alcohol)/liquid crystal composites

60 ) as a photoconductive sensitizer under an applied dc field. Orientational photorefractivity was demonstrated by observing Raman–Nath diffraction beams with an external dc field. The photorefractive gratings were partially memorized even in the absence of the applied dc field. Received: 12 January 1998/Revised version: 2 February 1998     

Performance improvement of a holographic double-exposure interferometer by heating of the photorefractive recording crystal

A holographic double-exposure interferometer for quasi real-time analysis is realized, utilizing a photorefractive BTO:Cd crystal for hologram recording. Heating of the crystal reduces the time required to reach the steady-state working situation. Light-induced absorption losses are reduced by heating as well. Additionally, the diffraction efficiency is increased by this method. A sixfold increase in the light intensity of the reconstructed interferogram is achieved. Received: 2 December 1998 / Revised version: 28 January 1999 / Published online: 7 April 1999     

Elastic scattering of electrons by metastable hydrogen atoms in the presence of a resonant laser field

Within the framework of the rotating wave approximation the elastic scattering of electrons by metastable 2s state of hydrogen atoms is studied in the presence of a resonant laser field. The frequency of the circularly polarized laser field is chosen to match the 2s-3p transition frequency in the hydrogen atom. Variation of the cross section with laser intensity and with incident electron energy (50-150 eV) is investigated. Received: 18 July 1997 / Received in final form: 5 December 1997 / Accepted: 19 January 1998     

The effect of hydrogen on the magnetic properties of quenched Ti-Zr-Ni rods

Ti-Zr-Ni quenched rods of 3, 2 and 1.5 mm diameter, prepared by vacuum-casting, were analyzed by X-ray diffraction (XRD) and vibrating sample magnetometry (VSM) before and after hydrogenation. Samples with two different compositions were prepared, i.e., Ti₄₀Zr₄₀Ni₂₀ and Ti₅₃Zr₂₇Ni₂₀. The as-cast rods were pulverized and hydrogenated from the gas-phase at 45 bar and 300 °C for 1000 min. The mass spectra of the desorbed hydrogen revealed the distribution of the hydrogen desorption temperatures from these alloys, whereas using thermogravimetry we obtained the mass% of desorbed H. We found that the ratio between the i-phase and the C14 Laves phase depends on the master alloy composition and the cooling rate, i.e., the rod diameter. VSM measurements revealed an about 30% decrease in paramagnetic susceptibility upon hydrogenation for all six investigated samples.     

A Fourier optics model of two-beam scanning laser interferometers

The article illustrates the use of Fourier optics to describe the operation of two-beam scanning laser interferometers. It deals with the effect of diffraction on the spatial periodicity of a monochromatic and coherent beam. Particular attention is given to the analysis of systematic errors in high-accuracy laser metrology. The article reviews the special case of plane wave and Gaussian illuminations, examines how beam truncation affects the period of traveling fringes and presents a general relation between the relative wavelength deviation and the impulse standard deviation of the photons. Received: 18 February 1998 / Revised: 21 October 1998 / Accepted: 16 November 1998     

Etalon effect in wave-mixing experiments with photorefractive crystals

We present investigations of the etalon effect in sillenite photorefractive crystals with a pulsed frequency-doubled Nd:YAG laser and show its influence on two- and four-wave mixing. In four-wave mixing the etalon effect allows changes of the diffraction efficiency by more than 80%. Results are supported by theoretical considerations. Received: 15 April 1998/Revised version: 19 June 1998     

Adiabatic following in standing-wave diffraction of atoms

We report experiments on the diffraction of atoms from a standing light wave in the channeling regime, characterized by long interaction time and large potential height. The observed far-field diffraction patterns depend specifically on the way in which the potential is switched on and off. For fast switching, the evolution is non-adiabatic and many diffraction orders are populated. For slow switching, however, the evolution is adiabatic and the number of populated diffraction orders decreases dramatically. The experiments are performed in two different setups employing rubidium and argon atoms, respectively. In one of the setups, we study the dependence of the diffraction pattern on the interaction time, in the other setup that on the incidence angle. Received: 30 November 1998 / Revised version: 5 July 1999 / Published online: 8 September 1999     

On the implicit integral character of Roothaan's expansion

The molecular generator coordinate Hartree-Fock method is reviewed. The connection between a quadrature solution of the generator coordinate Hartree-Fock equations and Roothaan's equations is stressed. The relation between linear expansion coefficients and generator coordinate weight functions is discussed and a numerical and analytical example is provided for the 1s orbital of the hydrogen atom represented as the integral transform of a Gaussian function. For the same example, the Gauss-Labatto quadrature is employed to emphasize the implicit integral character of Roothaan's equations. As a major conclusion, the interpretation that every LCAO calculation is actually performing integrations of the Griffin-Wheeler equations is advanced. Basis sets are therefore abscissas of the implicit quadrature used in the integration, whereas the linear coefficients automatically incorporate the corresponding weights. Subsequently, it is shown how to extract the generator coordinate weight function from the LCAO coefficients which has the advantage of being a characteristic of the physical system under study and not of the particular calculation being carried out. As such, basis set design becomes how to efficiently sample the weight function. Received: 13 June 1998 / Received in final form: 12 August 1998 / Accepted: 14 September 1998     

Translational order in liquid-expanded lipid monolayers functionalized with nucleosides

The monolayer behavior of a lipid containing two unsaturated alkyl chains and a nucleoside derivative as polar headgroup has been investigated by the Langmuir technique. From the surface pressure vs. molecular area isotherm, the monolayer appears as a pure liquid-expanded phase and should be then considered as a two-dimensional liquid. However, grazing incidence X-ray diffraction experiments evidence a translational order that does not exist when the lipid headgroup is a choline moiety. Since unsaturated chains are expected to induce a fluid state of the monolayer at the temperature considered, this order is likely to originate from the natural tendency nucleosides have to establish among themselves -stacking interactions between the bases. The collected X-ray data are consistent with the geometrical requirements for bases stacking. Received: 3 August 1998 / Revised: 16 September 1998 / Accepted: 21 September 1998     

Photoionization of the hydrogen atom in strong magnetic fields of White Dwarfs

The photoionization cross-sections of the hydrogen atom in strong magnetic fields of magnetic White Dwarf stars were calculated with a direct numerical integration method using the Landau basis as a high field approach. The validity regime of these solutions overlap with those of the spherical symmetry and complement other recently developed methods. An important result is the relation between the density of states and the normalization for the more than one open channel regime leading to additional coupling terms not taken into account by multi channel quantum defect methods. Received: 16 March 1998 / Revised and Accepted: 14 July 1998     

A new stable organic metal: -(BETS)C(CN). The first -type radical cation salt with a planar-triangular discrete organic anion

The -(BETS)₂C(CN)₃ radical cation salt was prepared by electrocrystallization, and its crystal structure was determined by single crystal X-ray diffraction. The electronic structure of -(BETS)₂C(CN)₃ was studied by means of the extended Hückel tight binding method. The electrical conductivity of this salt as a function of temperature shows a metallic behaviour down to 1.3 K. Shubnikov-de Haas oscillations reveal both the classical and magnetic breakdown orbits on the Fermi surface typical of the -type organic conductors. In addition, a low frequency oscillation (250 T) which is not predicted by the band structure calculations has been found in the oscillation spectrum. Received: 11 March 1998 / Revised: 9 June 1998 / Accepted: 11 June 1998     

Hydrogen storage in sonicated carbon materials

The hydrogen storage in purified single-wall carbon nanotubes (SWNTs), graphite and diamond powder was investigated at room temperature and ambient pressure. The samples were sonicated in 5 M HNO₃ for various periods of time using an ultrasonic probe of the alloy Ti-6Al-4V. The goal of this treatment was to open the carbon nanotubes. The maximum value of overall hydrogen storage was found to be 1.5 wt %, as determined by thermal desorption spectroscopy. The storage capacity increases with sonication time. The sonication treatment introduces particles of the Ti alloy into the samples, as shown by X-ray diffraction, transmission electron microscopy, and chemical analysis. All of the hydrogen uptake can be explained by the assumption that the hydrogen is only stored in the Ti-alloy particles. The presence of Ti-alloy particles does not allow the determination of whether a small amount of hydrogen possibly is stored in the SWNTs themselves, and the fraction of nanotubes opened by the sonication treatment is unknown. Received: 18 December 2000 / Accepted: 18 December 2000 / Published online: 9 February 2001     

Cubic superspace symmetry and inflation rules in metastable MgAl alloy

The diffraction properties of a quenched Al-Mg alloy which has been recently termed as a “cubic quasicrystal” are quantitatively reanalyzed. It is shown that the phase can be interpreted within the superspace formalism as an ordinary incommensurately modulated structure. The cubic six-dimensional superspace group that describes its symmetry properties has been determined. The additional inflation symmetry features exhibited by the diffraction diagram can be summed up by its invariance for the inflation factor , but this property has its origin in the specific value of the modulus of the modulation wave vectors, which is composition dependent. Other particular values of this modulus can give rise to similar scaling properties. Further experiments are required to ellucidate if the mentioned inflation symmetry is a fortuitous situation in a composition dependent wave vector, or has indeed the physical significance which would allow to describe the system as a “cubic quasicrystal”. Received: 25 June 1998 / Received in final form and Accepted: 23 September 1998     

Detection of hydrogen isotope atoms by mass spectrometry combined with resonant ionization

We examined the application of mass spectrometric methods using resonant ionization by a tunable laser and proposed its use for analyzing hydrogen isotopes. We conducted resonance ionization mass spectrometry (RIMS) to detect gas-phase hydrogen isotope atoms. The ionization efficiency was increased by more than 1000 times that obtained with conventional methods using nonresonant ionization. Resonant laser ablation mass spectrometry (RLAMS) was applied for deuterium detection in solid samples. A graphite substrate implanted with deuterium was used for ordinary laser ablation mass spectrometry (LAMS) and RLAMS. The deuterium signal was observed very clearly by RLAMS, in contrast to LAMS. Mass spectrometry combined with resonance ionization was very useful for hydrogen isotope detection, because components with equal mass numbers were resolved and the method demonstrated higher ionization efficiency. Received: 4 November 1998 / Revised version: 12 January 1999 / Published online: 7 April 1999     

On the continuity in energy of the free-bound one-photon cross-sections

For a charged particle in a central field of force we discuss the continuity in energy of the cross-sections for the related processes of ground state photoeffect and radiative capture. This continuity follows from general arguments, while for a Coulomb potential, where exact analytic expressions are available, the continuity can also be traced explicitly through the calculation. Our analysis allows us to identify an error in papers on radiative muon capture in the ground state which claim the existence of a discontinuity in the cross-section, occurring in hydrogen for muon energies in the neighbourhood of 2.8 keV. As a discontinuity in radiative capture would imply a discontinuity in atomic photoeffect, we note that in the latter case there are extensive experimental results in good qualitative agreement with the usual continuous result (Stobbe formula) for the cross-section. Received: 13 February 1998 / Accepted: 14 May 1998     

Fringe contrast in three grating Mach-Zehnder atomic interferometers

Several three-grating Mach-Zehnder atomic interferometers have been built and operated in recent years but no general theory of the contrast of the fringes produced by these apparatus is available. The purpose of this paper is to develop this theory, based on the Fresnel-Kirchoff approximate treatment of diffraction. Such a theory has been developed by Turchette et al. [JOSA B 9, 1601 (1992)] but because the necessary multiple integrals were evaluated in a purely numerical way, this treatment was not fully general. We show here how to reduce the computation by analytic means and we are thus able to calculate the contrast with a modest numerical effort. Moreover, we get a simple insight of the contrast reduction related to several defects of a real apparatus. We apply our calculations to existing interferometers as well as to an apparatus working with lithium which is under construction in our laboratory. Received: 24 April 1998 / Revised: 25 October 1998 / Accepted: 11 December 1998     

Polarisation effect of laser field in inelastic electron - hydrogen collisions

We study the influence of the laser polarization on the electron impact excitation of atomic hydrogen. Our method takes into account the “dressing” of the target states by including the laser-atom interaction to first order time-dependent perturbation theory, while the interaction of the laser field with the incident electron is treated to all orders by using the non relativist Volkov function. The interaction of the fast projectile with the target atom is treated in the first Born approximation. The calculations are performed via two distinct computations. The first one is based on a direct calculation, the second based on a Sturmian approach. Important differences appear between the angular distributions depending on the polarization chosen. Received : 17 february 1998 / Revised : 20 july 1998 / Accepted : 2 september 1998