UV cathodoluminescence of crystalline α-quartz at low temperatures

Two luminescence bands in the UV range were detected in crystalline α-quartz under electron beam excitation (6 kV, 3-5 μA). One band is situated at 5 eV and could be observed in pure samples. Its intensity increases with cooling below 100 K and undergoes saturation below 40 K alongside a slow growth with the time of irradiation at 9 K. The decay curve of the band at 5 eV contains two components, a fast (<10 ns) and a slow one in the range of 200 μs. The photoluminescence band at 5 eV with a similar temperature dependence was found in previously neutron-irradiated crystalline α-quartz. Therefore, the band at 5 eV was attributed to host material defects in both irradiation cases. The creation mechanism of such defects by electrons, the energy of which is lower than the threshold for a knock-out mechanism of defect creation, is discussed. Another band at 6 eV, containing subbands in different samples, appears in the samples containing aluminum, lithium and sodium ions. This luminescence is ascribed to a tunnel radiative transition in an association of (alkali atom)⁰-[AlO₄]⁺ that is formed after the trapping of an electron and a hole by Li⁺ (or Na⁺) and AlO₄.     

The CO fundamental-band laser as secondary frequency standard at 5 µm

We present a very high-resolution heterodyne spectrometer based on a CO laser which operates down to fundamental-band transitions of the molecule. This allows us to detect saturated absorption signals on these transitions at very low pressure (0.4 Pa) and laser intensity (< 1 mW/cm²), yielding a linewidth of about 250 kHz. With the CO fundamental-band laser stabilized to these saturation signals we have measured the transition frequencies of the fundamental bands of three isotopic species to an accuracy of typically 20 kHz (v/v 3 × 10^–10), referenced to the CO₂ frequency standard. Together with additional frequency measurements of the first hot bands, these provide the first heterodyne frequency data of sub-Doppler accuracy for transitions in low lying bands of CO. They now represent the most accurate secondary frequency standard in the spectral region around 5 µm (60 THz).     

Accurate calculation of the elastic scattering properties of ^7Li atoms at ultralow temperatures

The elastic scattering properties for collisions between two ^7Li atoms are investigated in the cold and ultracold regimes separately. Based on recent theoretical and experimental results, we present the improved hybrid potentials for the singlet X^1 ∑g^＋ and triplet a^3 ∑u^＋ ground states of the Li2, Our calculated values for the scattering lengths α and the effective ranges re are compared with previous ones, and found them to be in good agreement. The scattering lengths are 34.6α0 for the singlet state and -27.6α0 for the triplet state. Shape resonances occur in the collisions at low energies. We also calculate the total cross sections and the energy positions of shape resonances for both X^1 ∑g^＋ and a^3 ∑u^＋ states.     

Prediction of thermodynamic and surface properties of Pb−Hg liquid alloys at different temperatures

The thermodynamic properties, such as free energy of mixing, heat of mixing, activity and structural properties, such as concentration fluctuation in long wavelength limit, short-range order parameter of Pb–Hg liquid alloy at 600 K have been calculated using theoretical modelling. It has then been correlated with modified Butler model to compute the surface tension of the alloys at different temperatures. The Pb–Hg system at 600 K is found to be ordering at higher concentration of Pb.     

Intensities and H2-broadened half-widths of Germane lines around 4.7 μm at temperatures relevant to Jupiter's atmosphere

Absolute intensities and H₂-broadened half-widths of R(0) and R(1) of ⁷²GeH₄ have been measured at 119.5, 150.3, 199.0, 250.0, and 296.0 K using a tunable diode laser and the sweep integration technique. The combined intensity of the v₃ bands of all of the five isotopic species of GeH₄ as well as that of the Q-branches alone have been measured independently at 294 K using the Wilson-Wells-Penner-Weber technique.     

Calculations of O2 absorption and fluorescence at elevated temperatures for a broadband argon—fluoride laser source at 193 nm

Calculations have been made of O₂ absorption and fluorescence in the Schumann-Runge B³Σ^-_u←X³Σ^-_g band system for excitation by a broadband argon fluoride excimer laser at 193 nm. Results are presented for line strengths, spectral absorption coefficients, relative fluorescence spectra, total fluorescence and integrated absorption coefficients. The calculations have been performed for 300, 500, 1000, 1500 and 2000 K, a range of temperatures typically found in combustion flows. The absorption coefficients and fluorescence yields found are large enough to encourage use of argon fluoride lasers for O₂ measurements in a variety of flows.     

The Intensities of Methane in the 3-5 µm Region Revisited

The analysis of the linestrengths of the infrared spectrum of methane (12 and 13) in the 3-5 μm region has been revisited on the basis of new measurements from Fourier transform spectra recorded at Kitt Peak under various optical densities. A simultaneous fit of these new data with previously reported tunable difference-frequency laser data has been done. An effective transition moment model in tensorial form up to the third order of approximation within the Pentad scheme has been used. The standard deviations achieved are very close to the experimental precision: 3 and 1.5%, respectively, for the two sets of data for the (12)CH(4) molecule, representing a substantial improvement with respect to earlier studies. The integrated bandstrengths obtained in the present work differ from previously reported values by factors ranging from -5 to +6%. The correction for the nu(3) band, the strongest band of the Pentad system, is +2% with respect to the study of Hilico et al. [J. C. Hilico, J. P. Champion, S. Toumi, V. G. Tyuterev, and S. A. Tashkun, J. Mol. Spectrosc. 168, 455-476 (1994)]. Copyright 2000 Academic Press.     

Femtosecond Infrared Laser Spectroscopy of Characteristic Molecular Vibrations in Bacteria in the 6-µm Spectral Range

JETP Letters - The transmission spectra of femtosecond laser pulses in the mid-IR range (5–6.6 μm) were investigated of submonolayer of Staphylococcus aureus and Pseudomonas aeruginosa...     

Mössbauer spectroscopy of perovskite-related oxide fluorides of composition Ba0.5Sr0.5FeO2F at elevated temperatures

The discovery of superconductivity in perovskite-related oxide fluorides of composition Sr₂CuO₂F_2?+?x has stimulated interest in the synthesis and characterisation of other inorganic oxide fluorides with related structures. Recently a new low temperature preparation of inorganic oxide fluorides has been reported, which entails heating the precursor oxide with the polymer poly (vinylidene fluoride) to fluorinate oxygen deficient strontium ferrite, SrFeO_{3??? δ} . The oxygen-deficient compounds Sr_0.5Ba_0.5FeO_{3??? δ} has been fluorinated to give a compound of composition Sr_0.5Ba_0.5FeO₂F. The magnetic transition temperature for this compound is determined by Mössbauer spectroscopy to be 670(±10) K.     

Low frequency elastic properties of glasses at low temperatures—implications on the tunneling model

We have measured the low frequency elastic properties of dielectric, normal conducting and superconducting metallic glasses at audio-frequencies (f1 kHz) and temperatures down to 10 mK. Our results are discussed in the framework of the tunneling model of glasses. The major assumption of the tunneling model regarding the tunneling states with long relaxation time has been verified, but discrepancies to high frequency measurements have been found. In addition, our experiments on superconducting metallic glasses seem to indicate that the present treatment of the electron-tunneling state interaction is not sufficient.     

Operation of Tm3+ fiber laser at λ = 2.3 μm coupled to a silica fiber Bragg grating

A wavelength tunable all fibre laser which utilizes Tm³⁺-doped fluoride fibre as a gain mechanism emitting around a wavelength of = 2.3 m is reported. Tm³⁺-doped fluoride fibre was coupled to a fibre Bragg grating inscribed in silica. This laser was evaluated by employing it as a wavelength tunable source in a methane gas optical sensor. A minimum gas detectivity of 100 ppm m limited by the ability to prepare low concentrations of gases was achieved. Emission wavelength control was implemented by thermally tuning the grating, although this method may prove to be too slow and cumbersome for practical use. A model describing the coupling between the silica fibre Bragg grating and the Tm³⁺-doped fibre was developed and integrated with a fibre laser model. This theoretical framework was used to examine the optimum silica and fluoride fibre parameters to achieve the maximum coupling between the fibres, with the aim of further reducing the fibre laser threshold.     

Analysis of the Internal Optical Losses of the Vertical-Cavity Surface-Emitting Laser of the Spectral Range of 1.55 µm Formed by a Plate Sintering Technique

Optics and Spectroscopy - The results of a study of internal optical losses and current injection efficiency in vertical-emitting lasers of a spectral range of 1.55 µm obtained by sintering...     

Loss characteristics of an O-type polyethylene waveguide at 337 μm wavelength

The loss characteristics of O-type dielectric waveguides measured experimentally at 337 μm wavelength are discussed. Dielectric waveguides, including single mode guides, have been fabricated using polyethylene, and their attenuation constants measured about twice as much as the theoretical ones.     

Postprocessing treatments to improve the laser damage resistance of fused silica optical surfaces and SiO2 coatings

The combination of deep wet etching and a magneto-rheological finishing(MRF) process is investigated to simultaneously improve laser damage resistance of a fused-silica surface at 355 nm. The subsequently deposited SiO2 coatings are researched to clarify the impact of substrate finishing technology on the coatings. It is revealed that a deep removal proceeding from the single side or double side had a significant impact on the laser-induced damage threshold(LIDT) of the fused silica, especially for the rear surface. After the deep etching, the MRF process that followed does not actually increase the LIDT, but it does ameliorate the surface qualities without additional LIDT degradation. The combination guarantee both the integrity of the surface’s finish and the laser damage resistance of the fused silica and subsequent SiO2 coatings.     

Infrared multiphoton dissociation of unsubstituted metal carbonyls at 5 μm

A frequency-doubled carbon dioxide laser of modest output energy (1 mJ) has been used to study, for the first time, the infrared multiphoton absorption by, and dissociation of, the unsubstituted carbonyls of vanadium, chromium, iron, nickel, molybdenum, and tungsten. The multiphoton absorption cross-sections measured for Ni(CO)₄, Fe(CO)₅, Cr(CO)₆, Mo(CO)₆, and V(CO)₆ are high (2×10^–17) and ensure facile multiphoton dissociation. In focussed beams a pressure independent reaction yield proportional to the 1.5 power of the beam energy is observed for Fe(CO)₅, Cr(CO)₆, and Mo(CO)₆ implying threshold fluences of only 32, 25, and 26 mJ cm^–2, respectively. The stoichiometry of the reaction, observed by a pressure measurement technique, is consistent with production of metal atoms and carbon monoxide as final products for Ni(CO)₄, Fe(CO)₅, Cr(CO)₆, and Mo(CO)₆. This extensive decarbonylation along the ground state surface is consistent with recent studies of the photochemistry of these molecules from excited electronic states.NRC/Heriot-Watt Summer Student 1982On sabbatical leave from the Department of Chemistry, Heriot-Watt University, Edinburgh, United Kingdom     

The Effect of Active Region Heating on Dynamic and Power Characteristics of Quantum Cascade Lasers Emitting at a Wavelength of 4.8 µm at Room Temperature

Optics and Spectroscopy - Results of studying stripe quantum cascade lasers emitting at room temperature in the spectral range of 4.8 µm are presented. Power characteristics and turn-on...     

Magnetic properties of nano-sized 5 at.%Fe–Al systems

High energy ball milling is a promising materials processing technique that is widely used to produce nanocrystalline structures. However, when stainless steel or hardened steel containers and balls are used for milling, contamination from the milling medium can influence the material properties of the final nanostructured products due to intercalation of iron (Fe) as an impurity. This study reports the effect of iron contamination on nanocrystalline aluminum (Al) powder. ⁵⁷Fe Mössbauer spectroscopy and bulk magnetization studies using a vibrating sample magnetometer show that pure Al powder milled in hard steel media is strongly ferromagnetic at room temperature due to Fe contamination of about 5 at.% from the milling medium. TEM studies indicate that the system consists mainly of nano-sized Fe interspersed in Al with average crystallite sizes of ~2 and ~5 nm for Fe and Al, respectively. A comparative study of this system made with a mechanically alloyed Fe–Al system with the same percentage of Fe mixed with pure Al and mechanically alloyed using tungsten carbide vials and balls shows that the saturation magnetization, coercivity, Curie temperature, and low temperature behavior (field cooled–zero field cooled) are very different in the two cases. The different magnetic properties of the two systems can be attributed to the presence of magnetic and non-magnetic phases present.