Isolation of the CH3˙ rotor in a thermally stable inert matrix: first characterization of the gradual transition from classical to quantum behaviour at low temperatures.

Matrix isolation is a method which plays a key role in isolating and characterizing highly reactive molecular radicals. However, the isolation matrices, usually composed of noble gases or small diamagnetic molecules, are stable only at very low temperatures, as they begin to desegregate even above a few tens of Kelvin. Here we report on the successful isolation of CH3˙ radicals in the cages of a nearly inert clathrate-SiO2 matrix. This host is found to exhibit a comparable inertness with respect to that of most conventional noble gas matrices but it is characterized by a peculiar thermal stability. The latter property is related to the covalent nature of the host material and gives the opportunity to study the confined radicals from a few degrees of Kelvin up to at least room temperature. Thanks to this advantage we were able to explore with continuity for the first time the CH3˙ rotor properties by electron paramagnetic resonance spectroscopy, starting from the quantum rotations which are observable only at the lowest temperatures (T ≈ 4 K), going through the gradual transition to the classical motion (4 K < T < 30 K), and ending with the properties of the fully classical rotor (T > 30 K). The method of isolation presented here is found to be very effective and promising, as it is expected to be applicable to a large variety of different molecular radicals.

Twofold co-ordinated Ge defects induced by gamma-ray irradiation in Ge-doped SiO2.

We report an experimental study by photoluminescence, optical absorption and Electron Paramagnetic Resonance measurements on the effects of exposure of Ge-doped amorphous SiO2 to gamma ray radiation at room temperature. We have evidenced that irradiation at doses of the order of 1 MGy is able to generate Ge-related defects, recognizable from their optical properties as twofold coordinated Ge centers. Until now, such centers, responsible for photosensitivity of Ge-doped SiO2, have been induced only in synthesis procedures of materials. The found result evidences a role played by gamma radiation in generating photosensitive defects and could furnish a novel basis for photosensitive pattern writing through ionizing radiation.

Annealing of radiation induced oxygen deficient point defects in amorphous silicon dioxide: evidence for a distribution of the reaction activation energies.

The selective annealing of point defects with different activation energies is studied, by performing sequences of thermal treatments on gamma irradiated silica samples in the temperature range 300-450 °C. Our experiments show that the dependence on time of the concentration of two irradiation induced point defects in silica, named ODC(II) (standing for oxygen deficient centre II) and the E(γ)(') centre, at a given temperature depends on the thermal history of the sample for both of the centres studied; moreover in the long time limit this concentration reaches an asymptotic value that depends on the treatment temperature alone. These results suggest the existence of a distribution of the activation energies of the reaction process responsible for the annealing of the defects investigated, intimately related to the intrinsic disorder of the amorphous lattice. Furthermore, our data show that the thermal treatment can modify this distribution of activation energies and as a consequence the thermal properties of the centre itself.

Generation of defects in amorphous SiO(2) assisted by two-step absorption on impurity sites.

Generation of the Si dangling bond defect in amorphous SiO(2) (E' centre) induced by tunable pulsed UV laser radiation was investigated by in situ optical absorption measurements. The defect generation efficiency peaks when the photon energy equals ∼5.1 eV, it depends quadratically on laser intensity and is correlated with the native linear absorption due to Ge impurities. We propose a model in which the generation of E' is assisted by a two-step absorption process occurring on Ge impurity sites.

Photoluminescence band at 4.4 eV in oxygen-deficient silica: temperature effects.

We report experimental results on the spectral properties and time behaviour of the 4.4 eV photoluminescence (PL) band in oxygen-deficient silica [Formula: see text]. Our measurements, performed both at T = 300 K and T = 10 K, show that at room temperature the PL features are independent of the particular excitation energy (5.0 eV, 6.8 eV and 7.6 eV) whereas at low temperature, upon excitation at 7.6 eV, the decay of the PL emission is faster than for lower excitation energies. This shortening of the PL lifetime is consistent with previously reported data, which were explained by hypothesizing an interconversion mechanism between two structural configurations of the same oxygen defect. Nevertheless, our results do not support the proposed mechanism and we tentatively suggest a different interpretation of the experimental data.

Transputer-based parallel system for acquisition and on-line analysis of single-fiber electromyographic signals.

We describe a transputer-based system suitable for accurate measurements of single-fiber electromyographic jitter. It consists of a conventional electromyograph, a home-made interface and a commercially available transputer-based board installed within a PC/AT compatible. Taking advantage of the concurrent operation of two transputer modules, the system features simultaneous data acquisition and statistical signal processing: while data are acquired and analyzed, a real-time visualization of the signal latency and its variability is provided. In the present configuration, the system can acquire and analyze up to 40,000 consecutive action potentials, which can be grouped into up to eight sets at different stimulation rates programmable up to 16 Hz. Since the determination of the electromyographic signal latency relies on least-squares smoothing and interpolation of the acquired data rather than on amplitude-threshold triggering, a low value (0.7 microsecond) of so called technical jitter is achieved. Computing power and memory can be easily extended by addition of transputer-based modules. Typical results of data acquisition and on-line analysis are reported.

The role of impurities in the irradiation induced densification of amorphous SiO(2).

In a recent work (Buscarino et al 2009 Phys. Rev. B 80 094202), by studying the properties of the (29)Si hyperfine structure of the E'(γ) point defect, we have proposed a model able to describe quantitatively the densification process taking place upon electron irradiation in amorphous SiO(2) (a-SiO(2)). In particular, we have shown that it proceeds heterogeneously, through the nucleation of confined densified regions statistically dispersed into the whole volume of the material. In the present experimental investigation, by using a similar approach on a wider set of materials, we explore how this process is influenced by impurities, such as OH and Cl, typically involved in relevant concentrations in commercial materials. Our results indicate that the degree of local densification within the structurally modified regions is influenced by the OH content of the material: the higher the OH content, the lower the local degree of densification of the irradiated materials. In contrast, no relevant contribution to the densification process induced by electron irradiation in a-SiO(2) can be ascribed to Cl impurities up to [Formula: see text] ppm by weight.

Influence of the drawing process on the defect generation in multistep-index germanium-doped optical fibers.

Variation of germanium lone pair center (GLPC) concentration in germanosilicate multistep-index optical fibers and preforms was studied using confocal microscopy luminescence technique. The experimental results provide evidence that in the central core region ([Ge] approximately 11 wt.%) of our specific canonical samples the ratio [GLPC]/[Ge] is five times larger in fiber than in preforms. The relative influence of the glass composition and of the drawing process on the generation efficiency of the GLPC defects that drive the glass photosensitivity is discussed. The radial distribution of these defects suggests a possible enhancement of the defect creation related to the internal stress of the fiber core.