The stability of metallic glasses under ion irradiation

Abstract

Metallic glasses of Fe₈₀B₂₀ and Fe₈₀P₁₃C₇ prepared by rapid quenching have been implanted with 40 keV Ar⁺, P⁺ and B⁺ ions at doses up to 2.10¹⁶ ions cm^?2 at 273 K. The structure of the glasses was examined by X-ray diffraction. Crystallites of α-Fe and Fe₃B, present in F₈₀B₂₀ samples, either become highly defective or are amorphised. Implantation also produces structural changes in the amorphous Fe₈₀B₂₀ (a-Fe₈₀B₂₀) phase as revealed by changes in the width β of the principal diffraction halo. Both disordering and relaxation effects are observed, dependent on the initial degree of order within the a-Fe₈₀B₂₀ phase before irradiation. Implantation of F₈₀p₁₃C₇ results in an increase in the content of γ-Fe and Fe₃P crystallites, already present before irradiation, and in the nucleation of α-Fe.     

(2 + 1) Resonance enhanced multiphoton ionization of hydrogen chloride in a pulsed supersonic jet: Vacuum wavenumbers of rotational lines with detailed band analysis for excited electronic states of HCl

A comprehensive study of the excited electronic states of HCl is reported. Using resonance enhanced multiphoton ionization ((2 + 1) REMPI) and time-of-flight mass spectrometry (TOFMS) over 120 band systems are analyzed. Supersonic jet techniques are used to prepare rotationally cold species for laser spectroscopy in the 77 000 to 96 000 cm⁻¹ region. At least 50 new electronic states are characterized as well as several features only tentatively assigned previously. A long vibrational progression consisting of 29 vibrational levels of the deeply bound V¹Σ⁺(0⁺) valence/ion-pair state is studied. We also extend the identification and analysis to high vibrational levels of low-lying Rydberg states. The assignments of [²Π_i] Rydberg state complexes are evaluated in terms of spin-orbit coupling and united-atom calculations. In several band systems, the spectra exhibit anomalous rotational line intensities and broadened linewidths which are consistent with predissociation. Multiphoton ionization with mass spectrometry permits the investigation of isotope effects as well as the appearance of fragment species associated with repulsive potential curves.     

Time-division multiplexing of pump wavelengths to achieve ultrabroadband, flat, backward-pumped Raman gain

We describe a simple scheme, which we call the Raman smart pump, to allow for the achievement of flat gain over broad bands with backward-pumped Raman amplification. The proposed method, based on time-division multiplexing, also removes all interaction among the various pump wavelengths and allows for dynamic gain control through simple electronic means.     

Wafer and epilayer improvement for integrated optics devices on InP: the ESPRIT project 2518

Ga _x In_1–x As _y P_1–y alloys lattice matched to InP substrates are currently used to fabricate optoelectronic and integrated optics devices. To achieve devices with high performances and high fabrication yield, the uniformity and reproducibility of the Ga _x In_1–x As _y P_1–y epitaxial layers (composition, thickness, doping, etc.) have become key parameters. These problems have been addressed in the frame of ESPRIT project 2518 and are presented in this paper. Several aspects have been considered starting from the optimization of InP substrates, the MOVPE growth of uniform GalnAsP layers, the material characterization to the validation of material uniformity on passive optical waveguides. Both scanning photoluminescence analysis and waveguide losses measurements performed on 2 inch wafers with a high lateral resolution have shown that high quality uniform GalnAsP layers can be obtained reproducibly on 2 InP substrates using a commercially available LP-MOCVD growth process. In particular, more than 60% of 36 mm long, 3m wide and 100m spaced rib waveguides exhibit losses below 0.8dBcm^–1.     

Predicting future duration from present age: A critical assessment

Using a temporal version of the Copernican principle, Gott has proposed a statistical predictor of future longevity based on present age (Gott III, J. R., 1993, Nature, 363, 315) and applied the predictor to a variety of examples, including the longevity of the human species. Although Gott's proposal contains a grain of truth, it does not have the universal predictive power that he attributes to it.     

Flame propagation of nano/micron-sized aluminum particles and ice (ALICE) mixtures

Flame propagation of aluminum–ice (ALICE) mixtures is studied theoretically and experimentally. Both a mono distribution of nano aluminum particles and a bimodal distribution of nano- and micron-sized aluminum particles are considered over a pressure range of 1–10 MPa. A multi-zone theoretical framework is established to predict the burning rate and temperature distribution by solving the energy equation in each zone and matching the temperature and heat flux at the interfacial boundaries. The burning rates are measured experimentally by burning aluminum–ice strands in a constant-volume vessel. For stoichiometric ALICE mixtures with 80 nm particles, the burning rate shows a pressure dependence of r_b = aPⁿ, with an exponent of 0.33. If a portion of 80 nm particles is replaced with 5 and 20 μm particles, the burning rate is not significantly affected for a loading density up to 15–25% and decreases significantly beyond this value. The flame thickness of a bimodal-particle mixture is greater than its counterpart of a mono-dispersed particle mixture. The theoretical and experimental results support the hypothesis that the combustion of aluminum–ice mixtures is controlled by diffusion processes across the oxide layers of particles.     

The contribution of fundamental frequency, amplitude envelope, and voicing duration cues to speechreading in normal-hearing subjects

The ability to combine speechreading (i.e., lipreading) with prosodic information extracted from the low-frequency regions of speech was evaluated with three normally hearing subjects. The subjects were tested in a connected discourse tracking procedure which measures the rate at which spoken text can be repeated back without any errors. Receptive conditions included speechreading alone (SA), speechreading plus amplitude envelope cues (AM), speechreading plus fundamental frequency cues (FM), and speechreading plus intensity-modulated fundamental frequency cues (AM + FM). In a second experiment, one subject was further tested in a speechreading plus voicing duration cue condition (DUR). Speechreading performance was best in the AM + FM condition (83.6 words per minute,) and worst in the SA condition (41.1 words per minute). Tracking levels in the AM, FM, and DUR conditions were 73.7, 73.6, and 65.4 words per minute, respectively. The average tracking rate obtained when subjects were allowed to listen to the talker's normal (unfiltered) speech (NS condition) was 108.3 words per minute. These results demonstrate that speechreaders can use information related to the rhythm, stress, and intonation patterns of speech to improve their speechreading performance.     

Gamow-Teller beta decay of the very neutron-deficientN=50 nuclide98Cd

For the first time, detailed decay-spectroscopic investigations were performed for the very neutron-deficientN=50 nuclide⁹⁸Cd. The⁹⁸Cd activity was produced in spallation reactions between 600 MeV protons and a natural tin target, yielding a⁹⁸Cd beam intensity of 10 to 60 atoms/s at the collector of the ISOLDE massseparator. By means of-ray and conversion-electron spectroscopy, 19 transitions were found to follow the ⁺/EC decay⁹⁸Cd⁹⁸Ag. The transitions at 61 and 107 keV were shown to beM1(+E2) andE2, respectively, and the⁹⁸Cd half-life was measured as 9.2±0.3 s. TheQ _EC value of⁹⁸Cd is determined semiempirically and is compared to model predictions together with the measuredQ _EC values of the neighbouring cadmium isotopes^100,102Cd and theN=50 isotones⁹²Mo,⁹⁴Ru, and⁹⁶Pd, taken from the literature. The newly established decay scheme of⁹⁸Cd includes 9 excited states of⁹⁸Ag. Four states at 1691, 1861, 2164, and 2544 keV are directly fed by 0⁺ 1⁺ Gamow-Teller beta transitions with a summed strength of 3.5 _–0.7 ^+0.8 . This value corresponds to 25±5% of the strength predicted for the GT transformation of a g_9/2 proton (in⁹⁸Cd) into a g_7/2 neutron (in⁹⁸Ag) by the extreme single-particle shell model. The GT-strength splitting and quenching, observed for⁹⁸Cd, are compared with the corresponding data for lighter even-even N=50 isotones, and are discussed with reference to the predictions of more sophisticated nuclear models. We find that only in some cases it is possible to explain qualitatively the observed GT strength distribution and its total magnitude without renormalizing the free-neutron value of the axial-vector coupling constant.Dedicated to Prof. P. Armbruster on the occasion of his 60th birthday