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.     

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.     

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     

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.     

Exchange collisions between positronium and Cu2+ ions

Summary The fate and properties of the ortho-Ps in aqueous solutions of CuSO₄ and CuCl₂ have been investigated through lifetime measurements and through a magnetic-quenching experiment. The obtained results are interpreted in terms of exchange collisions between Ps and unpaired electron. An upper limit is given for the chemical-quenching rate. Our measurements have shown that the presence of paramagnetic centres determines a strong shift of the levels of the Ps hyperfine structure. This work was entirely supported by CISM (Centro Interuniversitario di Struttura della Materia) of the Ministero della Publica 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.     

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 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.     

Biosystem response to soft-X-rays irradiation: non-monotonic effects in the relevant biological parameters of yeast cells

Summary Soft-X-rays irradiation of yeast cells allows selective interference with different cellular structures. The monitoring of different physical parameters leads to substantial variations in the response to X-rays showing that monotonicity should not be taken for granted. Also at INFM (Istituto Nazionale di Fisica della Materia).     

Frequency measurements of far infrared laser lines by means of MIM point contact diodes

We present frequency measurements of 85 Far Infrared (FIR) laser lines ranging from 2 748 GHz to 421 GHz. We use the heterodyne technique of mixing FIR laser transitions and microwave radiation on MIM point contact diodes to determine the FIR frequencies.Work supported by Consiglio Nazionale delle Ricerche -Italia and INFM (Consorzio Interuniversitario di Fisica della Materia)     

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).     

Positron polarization in chiral media

Summary The positron spin depolarization, relative to the direction of a static magnetic field, was investigated in some chiral media where Ps formation occurs. The results indicate that helical positrons are able to distinguish one enantiomer from the other: the remaining polarizations—just before Ps is formed—differ by about 20% on passing from one enantiomer to the other of the same chiral species. Moreover the existence of a process—strictly connected with chirality and able to hinder any depolarizing process more efficiently than the field itself—has been made evident at low fields. This work was supported by INFM (Consorzio Nazionale di Fisica della Materia).     

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.     

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.     

Search for spin effect in positron interaction with optically active camphor

Summary The depolarization, relative to the direction of a magnetic field, which accompanies the slowing-down of positrons in the two enantiomers of camphor, down to the positronium formation energy, has been found to be greater (27%) in L-camphor. The measuring method was based on the Ps relative yield, in the magnetic field, in triplet and singlet state. All other annihilation features, as well as the intrinsic properties of Ps once formed, do not display significant differences between the two enantiomers. This work was supported by INFM (Consorzio Nazionale di Fisica della Materia).     

The influence of thes-f interaction on the conduction band behaviour in ferromagnetic semiconductors. A theory of the red shift effect

The mechanism of the thermomechanical instability of an elastically slowly strained solid is explored, by a normal mode analysis of temperature and strain fluctuations. The strain dependence of the Grüneisen parameter is found to play a major role, since it controls the rates of damping of the lowest wavevector acoustic excitations pumped by the imposed strain. For -titanium the instability is found to occur at a critical strain comparable with that of the elastic limit. The instability can be envisaged in terms of a fast growing deformation, presumably associated with an acoustic emission.Gruppo Nazionale di Struttura della Materia del Consiglio Nazionale delle Ricerche, UR7     

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.     

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).     

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     

“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.