[en] A molecular cluster containing a coplanar ring of iron(III) ions with spin 5/2 was investigated by Moessbauer spectroscopy. The iron spins are antiferromagnetically coupled so that the ground state has total spin S=0. Spectra in the absence of an applied magnetic field consisted of a quadrupolar doublet, the linewidth of which monotonically increased with the temperature. A quadrupolar splitting of about 0.35 mm/s was found. Calculations of the iron-orbital electronic populations were carried out and the quadrupolar splitting was estimated. Its value was in agreement with the experimental one. In addition, the trend of the linewidth was explained in terms of isotropic spin fluctuations. Spectra in the presence of a 4.5 T longitudinal magnetic field were also collected. The hyperfine field was obtained from their fitting. Differences with respect to the hyperfine field obtained from susceptibility data were also interpreted in terms of spin fluctuations

[en] The angular averaged mean-square displacement of ¹⁵¹Eu in EuBa₂Cu₃O_7-δ was measured as a function of temperature by Moessbauer spectroscopy using the absorption area method. Large low-temperature anharmonicity was found; i.e. the adiabatic potential experienced by Eu³⁺ ions presents a 'wine-bottle bottom' shape with a flat region about 0.1 AA wide. Comparisons with other experimental results are made. (author)

[en] A general method to calculate electronic Green's functions in the tight-binding scheme is presented. The integrations are systematically reduced to the irreducible volume of the first Brillouin zone, so that a very short time is necessary for the computer's work. The importance of symmetry planes, lines and points is stressed

[en] A dc motor electronic controller, driving a rotating chopper, is presented. The controller uses an integral feedback technique, consisting of charging a storage capacitor by means of width modulated pulses. These pulses are obtained by comparing the revolution time of the chopper with a fixed time. It is shown that the steady-state angular speed is independent both of the friction coefficient and the feedback loop gain. The stability condition of the feedback loop gain is also derived

No abstract available

[en] A method for designing a 10-500 eV alkali ion source is described. By observing computer-calculated ion paths, the source geometry and electrode potentials are optimized. The source program set can be used without modification to calculate ion trajectories in any four-electrode configuration with axial symmetry. (orig.)

[en] In the last years an appreciable experimental effort has been devoted to the ion-atom low energy collisional range investigation. Charge transfer reactions have been studied also with particular interest and applications to astrophysics, ionospheric physics and plasma physics. In spite of the experimental and theoretical efforts, cross section parameters for many charge transfer reactions are still affected by large uncertainty. In the energy range 10-10⁴ eV the experimental technique, which seems to permit the highest accuracy for cross section measurements, is the crossed beam technique, which was here used in the attempt to develope and improve new systems for the investigation of reaction mechanisms occurring in the interstellar medium and in planetary atmospheres. Therefore a novel device was designed for charge transfer reactions in the energy range 20-200 eV and the system was adapted to the molecular beam apparatus of universal type. (Auth.)

[en] The utilization of cryogenerators for Moessbauer measurements at low temperature may give trouble owing to the velocity noise caused by spurious vibrations. A theoretical analysis of the velocity noise effect and a way of handling it has already been carried out by the authors. The present work concerns the applications of the theoretical method and illustrates the results obtained applying it to the spectra collected with a new Moessbauer cryostat cooled by a Gifford-McMahon closed cycle refrigerator. (orig.)

[en] Moessbauer parameters of ¹⁵¹Eu in Eu_1+xBa_2-xCu₃O_7+δ have been studied as a function of temperature, for x = 0.15 and x = 0.25. The areas of the absorption spectra for the two samples are almost equal. This could mean that the f-Moessbauer factor for ¹⁵¹Eu at the barium sites is much smaller than that for ¹⁵¹Eu at the rare earth sites, i.e. we only observe the spectra of the latter sites. According to this, the fits of the spectra with a single quadrupolar pattern and with two quadrupolar patterns have given equivalent χ² values. (orig.)

[en] This paper represents the conclusive part of a study on the behaviour of a prototype of a cryostat for Moessbauer spectroscopy cooled by a Gifford-McMahon closed-cycle refrigerator. It reports the experimental results which pointed out the main source of velocity noise for the Moessbauer measurements and allowed us to improve the performance of the device to a significant degree by means of appropriate changes. (orig.)