Positron bound systems in benzene and in naphthalene

Summary Positron decay in liquid naphthalene and benzene was investigated through measurements of lifetimes and magnetic quenching. The obtained results when compared with those already known in solid phase suggest that positrons become bound in two different systems. The first has a lifetime τ₃≈1 ns which is unaffected by the melting, while the second (τ₄≈3 ns) arises above melting and behaves as a relaxed positronium. This work was supported by CISM (Centro Interuniversitario di Struttura della Materia) del Ministero della Pubblica Istruzione, and by GNSM (Gruppo Nazionale di Struttura della Materia) del CNR.     

Positron spin relaxation in solids

Summary The positron spin relaxation in a longitudinal magnetic field was investigated in some solids where Ps formation occurs. The experimental results indicate that this relaxation manifests itself in those organic compounds where the positron can excite low-lying triplet states. The values of positron slowing-down time, obtained by adopting a schematic model, range from 13 to 28 ps. This work was supported by CISM (Centro Interuniversitario di Struttura della Materia) of Ministero della Pubblica Istruzione, and by GNSM (Gruppo Nazionale di Struttura della Materia) of CNR.     

Positron spin relaxation in naphthalene

Summary The remaining spin polarizationP of positrons was measured in naphthalene against longitudinal magnetic-field strength. The measuring method was based on the relative yield of Ps in triplet and singlet state. In a zero fieldP is reduced to 50% of the initial polarization and approaches 100% upon increasing the applied field to larger values. This effect, which in teflon is quite absent, appears to be determined by the specific behaviour of positrons in naphthalene. This work was supported by CISM (Centro Interuniversitario di Struttura della Materia) of Ministero Pubblica Istruzione and by GNSM (Gruppo Nazionale di Struttura della Materia) of CNR.     

Positroniumlike system in quartz

Summary The ortho-state of the Ps-like system, which in crystalline quartz lives in Bloch states, has been detected in the time annihilation spectrum. Its lifetime and abundance are τ₃=0.49ns,I ₃=0.13. The magnetic quenching of this system has been measured and compared with that of Ps-like system living in other molecular media. This work was supported by CISM (Centro Interuniversitario di Struttura della Materia) of Ministero della Pubblica Istruzione and by GNSM (Gruppo Nazionale di Struttura della Materia) of CNR.     

Contact density and hyperfine splitting of positronium in naphthalene

The contact density |Ψ(0)|² and the hyperfine splitting of positronium in naphthalene have been determined by means of a magnetic quenching experiment. Results show that the perturbing effect of the medium on the hyperfine splitting is not interpretable as a change of the contact density alone, since on passing from vacuum to the medium, the contact density and the splitting do not scale together. The first is reduced by a factor of 0.25, the second by a factor of 0.067. A simple phenomenological model is presented. This work was entirely supported by CISM (Centro Interuniversitario di Struttura della Materia) of the Ministero della Pubblica Istruzione and by GNSM (Gruppo Nazionale di Struttura della Materia) of the Consiglio Nazionale delle Ricerche.     

On some measurements of the propagation properties of thermal waves in the Nb-H and Ta-H systems

Summary By means of an experimental technique based on a nonsteady-state method,i.e. on the propagation of thermal waves, we have measured at room temperature for oscillation frequencies between 6 and 30 mHz the thermal diffusivity, the lateral thermal-loss coefficient and the ?fractional heat loss? in Nb and Ta wires as a function of hydrogen doping. The appearance of hydride formation and precipitation notably changes the behaviour of these quantities. From these measurements it was also possible to calculate the propagation velocity and attenuation coefficient of thermal waves. It was found that the increase of the hydrogen concentration in the solid solution produces a gradual decrease in the velocity and increase in the attenuation coefficient until the solubility limit is reached. For larger hydrogen concentrations, the velocity showed a tendency to increase towards the value of the pure metal, while the attenuation coefficient decreased below the value of the pure metal. These results as a whole appear quite promising for studying the properties of hydrogenated systems. This work was supported by the Gruppo Nazionale di Struttura della Materia of the Consiglio Nazionale delle Ricerche and by the Centro Interuniversitario di Struttura della Materia of the M.P.I., Roma (Italia).     

Lattice reordering in Pb implanted Ge crystals

Channeling effect and sheet resistivity techniques have been used to investigate the damage induced by 40 keV Pb implantation in Ge crystals. At 450° the reordering of the Ge lattice occurs simultaneously to the out-diffusion of the implanted Pb atoms while the recovery of the sheet resistivity occurs at higher temperatures. 90% of the implanted atoms still retained in the crystal are located in substitutional sites. Work supported in part by Centro Siciliano di Fisica Nucleare e di Structura della Materia and by Gruppo Nazionale di Struttura della Materia del Consiglio Nazionale delle Ricerche.     

A simple criterion for forecasting off-centre configuration of monovalent impurity ions in alkali halide crystals

Summary With the aid of the Sanderson model for nonmolecular structures a simple criterion has been derived for forecasting off-centre configuration of monovalent impurity ions in alkali halide crystals based on the effect that the Sanderson partial charges on atoms have on their radii. Work jointly supported by Ministero della Pubblica Istruzione and by Consiglio Nazionale delle Ricerche, Gruppo Nazionale di Struttura della Materia.     

“Laser snow” effect in CS2 vapour induced by krypton laser

We report on the laser snow effect observed in CS₂ vapour illuminated by uv lines of a krypton laser. We discuss the effect of diffusion and convection processes on the threshold value of the CS₂ vapour pressure. We discuss also the kinetics of the formation process related to the proposed model of chemical reactions induced by laser light.Work partially supported by the Gruppo Nazionale di Struttura della Materia del CNR and by the INFN.     

On the possibility of a forecast for off-centre configuration of Ag+ and Cu+ in alkali halide crystals

Summary The simple cut criterion based on the accurate determination of the radii of the ions in alkali halides and previously introduced by the authors for forecasting off-centre configuration of Li⁺ and F⁻ has been extended to heavy ions (Ag⁺ and Cu⁺). It has been found that this criterion is valid for the Ag⁺ ion, whereas for Cu⁺ gives a less precise forecast because of the lack in knowledge of the effective partial charge on Cu⁺ ion. It has been evidenced that the critical value of the ratior ₊ ^* /r ₊ between impurity and host ion radius which allows off-centre configuration is dependent on the impurity ion mass. Work jointly supported by the Ministero della Pubblica Istruzione and by Consiglio Nazionale delle Ricerche, Gruppo Nazionale di Struttura della Materia.     

Analysis of a qualitative criterion for forecasting off-centre displacement of impurity ions in ionic crystals

Summary With the aid of the Sanderson model for nonmolecular structure a qualitative criterion is derived for forecasting off-centre configurations of monovalent impurity ions (Li⁺, F⁻, Cl⁻, Na⁺, Ag⁺, Cu⁺) in alkali halide crystals. The same criterion is checked for Mn²⁺ impurity ions in some oxides. We also present a critical review of the criteria introduced in the past, as well as of the experimental results up to now known in the literature. Due to the relevance of its scientific content, this paper has been given priority by the Journal Direction. Work jointly supported by the Ministero della Pubblica Istruzione and by Consiglio Nazionale delle Ricerche, Gruppo Nazionale di Struttura della Materia.     

A GMSA calculation of the radial distribution functions of molten RbCl

Summary The generalized mean spherical approximation (GMSA) for a fluid of charged hard spheres of equal diameters is used for calculating the radial distribution functions of molten RbCl. The results are compared to neutron diffraction data and a good overall agreement is found between the experimental and the theoreticalg _ij (r). Further application of this same approach to the case of ionic melts with ionic species of different diameters is discussed. Work supported in part by Comitato Regionale Ricerche Nucleari and Struttura della Materia (CRRNSM), Sicily, Italy.     

Optical visualization of deformations with nematic liquid crystals

Nematic liquid crystals are shown to be able to visualize elastic deformations even if no dynamic scattering is present. At low values of the elastic energy density, by observing the excited birefringence it is possible to map the normal modes of the elastic disturbance. At higher energy density a cellular motion (rolls) is produced which, in the homeotropic geometry, gives rise to a domain pattern which allows to visualize the velocity field. Work supported by Gruppo Nazionale di Struttura della Materia (GNSM) of CNR.     

Solution of the Hartree-Fock equations

We suggest to use the Newton iteration method for constructing a (locally unique) solution of the atomic and nuclear Hartree-Fock equations for an arbitrary number of particles. Our proposal is based on a theorem by Kantorovi and rests on the following points: 1) the two-body potential must satisfy a boundedness condition; 2) the zero-order approximation, used to start the iteration sequence, must satisfy certain conditions, to be proved numerically. Condition 1) holds, for instance, for all local potentials, defined by a bounded function and for a class of nonlocal potentials; it does not hold for local potentials, behaving as 1/r near the origin.This work has been supported in part by Istituto Nazionale di Fisica Nucleare (Sezione di Catania) and by Centro Siciliano di Fisica Nucleare e di Struttura della Materia (Catania).     

A transition from a doubly periodic flow to chaos via successive frequency lockings

In a four dimensional dynamical system depending on a parameter a transition from doubly periodic to aperiodic behaviour occurs through a sequence of frequency lockings with rotation numbers _n =n/2n+1. The aperiodic motion sets in when the unstable hyperbolic orbits associated with the final locked regime (rotation number =1/2) collide with the strange attractor present at higher values of the parameter.Partially supported from CRRNSM (Comitato Regionale per le Ricerche Nucleari e di Struttura della Materia)     

Speculations on the absorption and emission properties of the F-centre in alkali halides

Summary A review of acquisitions and of unresolved problems concerning F-centres in alkali halides is presented. The main conclusions area) the temperature dependence of the bands is mainly due to the thermal expansion for the absorption process and to electron-phonon interaction for the emission process: for the latter half-width and Huang-Rhys factor values are related to the transverse optical frequency ones;b) the lifetime τ_R of the relaxed excited state is not intrinsic at low temperature because of the interaction between F-and α-centres always present in real samples;c) even though the absolute quantum yield decreases when F-centre concentration increased, the relative quantum yield η_r is unaffected by change on F-centre concentration, strongly evidencing a nonhomogenous distribution of F-centres in real samples;d) a temperature-dependent, fine structure in τ_R (an η_r) values is observed but only when temperature control of the samples is better than a few hundredth of a degree: this effect has been interpreted in terms of interactions which involve F-, α-and F′-centres. Work jointly supported by the Ministero della Pubblica Istruzione and by Consiglio Nazionale delle Ricerche, Gruppo Nazionale di Struttura della Materia.     

Temperature gradients effects in low-power laser annealing of ion implantedα-Ge

The influence of temperature gradients in laser annealing of-Ge, obtained below the melting threshold, was tested by irradiating the samples with a linear fringe pattern from a pulsed ruby laser. The peak temperature reached on the specimen surface was calculated to be well below the melting threshold of the material. Temperature gradients are observed to enhance the crystallization process in the material. Spontaneous periodic structures, or ripples, having a period equal to the laser wavelength, are frequently observed in areas corresponding to minima of the fringe pattern.Gruppo Nazionale di Struttura della Materia     

Surface polarization effect on positron backdiffusion from dielectrics

The reduction of the thermal positron backdiffusion probability from a dielectric, due to the electrostatic polarization of the surface, has been calculated in the framework of an isothermal diffusion model. Our results show that this reduction can pass from levels of only a few percent (e.g., Si and Ge at room temperature) to almost complete suppression for substances with short positron diffusion length and at low temperatures. It is also shown that the surface polarization effect can be ignored in measurements of the positron diffusion constant with beam techniques if the low-energy part of the backdiffusion probability vs. beam energy curve is not included in the analysis.Work jointly supported by the Ministero della Pubblica Istruzione and by the Consiglio Nazionale delle Ricerche, Gruppo Nazionale di Struttura della Materia     

Two-photon transitions between discretenS states in atomic He

Summary The two-photon transition probabilities from the ground state 1¹ S and from the metastable states 2¹ S and 2³ S to the lowest excited states are computed in He. A configuration interaction approach is used, and a detailed comparison between the ?length? and the ?velocity? gauges leads to an estimate of 1% accuracy. Correlation effects are found to account for about 20% of the transition probabilities. The life-time of the 2¹ S state is ≅19.6 ms. The two-photon transition probabilities display a dramatic dependence on frequency, characterized by resonances and transparencies. Based on work supported in part by the Gruppo Nazionale di Struttura della Materia of the C.N.R. and by the Frascati Synchrotron Radiation Contract (P.U.L.S.).     

A thermoelastic method to determine the thermal diffusivity

We present a method to measure the thermal diffusivity of metals. A homogeneous small heat source is fed into a cyclindrical sample of titanium by thermoelastic compression. This heating process is followed by a thermal relaxation controlled by the thermal diffusivity. The resulting temperature variation is detected by a miniature temperature sensor mounted on the lateral surface of the sample. The value so obtained for the thermal diffusivity of titanium is (6.6±0.2)·10⁻⁶ m²/s. Gruppo Nazionale Struttura della Materia del Consiglio Nazionale delle Ricerche