In the wonderland of ultra-parallel neutron beams

Bragg reflections from single crystals yield angular widths of a few arcsec for thermal neutron beams. The Bonse-Hart proposal to attain a sharp, nearly rectangular profile by Bragg reflecting neutrons multiply from a channel-cut single crystal, was realized in its totality three and a half decades later by achieving the corresponding Darwin reflection curves for 5.23 Å neutrons. This facilitated SUSANS (Super USANS) measurements in the Q ～ 10^?5 Å^?1 range. The polarized neutron option was introduced into the SUSANS set-up by separating the up- and down-spin neutron beams by ～10 arcsec with a magnetic (air) prism. The neutron angular width has recently been reduced further by an order of magnitude to ～0.6 arcsec by diffracting 5.3 Å neutrons from a judiciously optimized Bragg prism. This constitutes the most parallel monochromatic neutron beam produced to date. I present the first SUSANS spectra probing the Q ～ 10^?6 Å^?1 domain, recorded with this beam.     

Bragg prism monochromator and analyser for super ultra-small angle neutron scattering studies

We have designed, fabricated and operated a novel Bragg prism monochromator-analyser combination. With a judicious choice of the Bragg reflection, its asymmetry and the apex angle of the silicon single crystal prism, the monochromator has produced a neutron beam with sub-arcsec collimation. A Bragg prism analyser with the opposite asymmetry has been tailored to accept a still sharper angular profile. With this optimized monochromator-analyser pair, we have attained the narrowest and sharpest neutron angular profile to date. At this facility, we have recorded the first SUSANS spectra spanning wave vector transfers Q ～ 10^?6 Å^?1 to characterize samples containing agglomerates up to tens of micrometres in size.     

Electron-hole plasma in pulse photoexcited single quantum wells

We report photoluminescence studies of MOCVD grown, GaAsAl_xGa_1?xAs single quantum wells which were intensly excited with a pulse dye laser at T=2K. For a well width of d～40Å, the spectra are interpreted as due to the radiative recombination of a hot electron-hole plasma confined to the well. The density of charge carriers and their temperature depend upon the excitation intensity, and vary in the range of 10¹¹–10¹³ cm^?2 and 100–500K for an absorbed photon flux of 10¹³–10¹⁶ photons-cm^?2 per pulse, respectively. The observed spectral features are identified as the e1-hh1 and e1-lh1 transitions and two additional bands which are tentatively assigned to transitions involving virtual bound states of either the electron or the hole. The electron-hole plasma spectra of the d～40Å sample are strongly polarized perpendicular to the well quantization axis. For wider wells (d～80 and 150Å) smaller photoexcited carrier densities were observed for the same absorbed photon flux. It is thus concluded that the capture efficiency of the well is small.     

A High Resolution Study of the Shock Excited Spectrum of Yttrium Oxide

Abstract

The blue-green (B²? X²?) and orange (A²? –X²?) systems of yttrium oxide have been shock excited at temperatures of about 4000°K and photographed at high dispersion (～ 1.2Å/mm) in the first order of a 21 ft. grating spectrograph. The high dispersion shock excited spectra were recorded using multiple exposures and latensification of extremely high speed film. Over 50 new bands have been observed and assigned to specific vibrational transitions for the systems. New molecular constants have been derived from the A, B and X states. Provisional calculations have been made of Franck-Condon factors and r-centroids for the systems appropriate to new constants.     

Electronic Absorption Spectra of Some Fluoroaminotoluenes

Abstract

The near ultraviolet absorption spectra of 2-fluoro-5-amino-; 3-fluoro-4-amino-, 3-fluoro-6-amino-and 4-fluoro-2-aminotoluene have been investigated in vapour phase. The strongest band appearing at 2887.5 Å (34621 cm^?1), 2966.1 Å (33704 cm^?1), 3026. 7 Å (33029 cm^?1) and 2891.4 Å (34575 cm^?1) in the respective molecules has been identified as the 0,0 band. All the bands have been analysed in terms of some ground and excited state fundamentals. The assignment of the fundamental frequencies to the probable modes of vibration have also been discussed.     

Effect of MeV electron irradiation on the free volume of polyimide

The free volume of the microvoids in the polyimide samples, irradiated with 6 MeV electrons, was measured by the positron annihilation technique. The free volume initially decreased the virgin value from ～13.70 to ～10.98 Å³ and then increased to ～18.11 Å³ with increasing the electron fluence, over the range of 5?×?10¹⁴ – 5?×?10¹⁵ e/cm². The evolution of gaseous species from the polyimide during electron irradiation was confirmed by the residual gas analysis technique. The polyimide samples irradiated with 6 MeV electrons in AgNO₃ solution were studied with the Rutherford back scattering technique. The diffusion of silver in these polyimide samples was observed for fluences >2?×?10¹⁵ e/cm², at which microvoids of size ≥3 Å are produced. Silver atoms did not diffuse in the polyimide samples, which were first irradiated with electrons and then immersed in AgNO₃ solution. These results indicate that during electron irradiation, the microvoids with size ≥3 Å were retained in the surface region through which silver atoms of size ～2.88 Å could diffuse into the polyimide. The average depth of diffusion of silver atoms in the polyimide was ～2.5 μm.     

Characterization of vacuum evaporated In - Se thin films

The InSe films of different thicknesses (290–730 mm) were deposited onto glass substrates under a pressure of 3×10^?5 Torr by vacuum evaporation method. The composition (In=53.50%, Se=46.50%) of this film was confirmed using Auger Electron Spectroscopy (AES). Thicknesses of the deposited films have been measured using a Multiple Beam Interferometry. The amorphous nature of the film is confirmed with X-ray diffractogram. From the transmittance spectra in the range of 500 nm-1200 nm, it is observed that the film showed direct allowed transition. Effect of thickness on the optical parameters such as the fundamental band gap, absorption constant, refractive index of InSe thin films are reported. Under low electric field (～ 1.5×10⁵ Vcm^?1), the results of DC conductivity measurements revealed that the variable range hopping is the dominant conduction mechanism. The values of localized states density, localization radius and hopping energy of this film are estimated as 5.57×10²⁰ cm^?3eV^?1, 0.84 Å and 0.247 eV, respectively.     

Investigation of local lattice structure around Cr3+ ions at the trigonal and tetragonal sites in RbCdF3: Cr3+ crystal

The local lattice structure and EPR, optical spectra for Cr³⁺ doped in RbCdF₃ crystal have been studied by diagonalizing the complete energy matrices. The results show that the local structure of the Cr³⁺ ions in RbCdF₃ exhibits a compressed distortion at the trigonal and tetragonal sites. The compressed distortion can be ascribed to the fact that the radius of Cr³⁺ ion is smaller than that of Cd²⁺ ion, and therefore Cr³⁺ ion will draw the fluorin ligands inwards. The variational ranges of the local structural parameters for Cr³⁺ doped in RbCdF₃ crystal R =?1.9491 Å ～?1.9814 Å, θ?= 55.234° ～?55.286° at the trigonal site and R ₁ =?1.8617 Å ～?1.8928 Å, R ₂ =?1.9527 Å ～?1.9851 Å at tetragonal site are obtained respectively, and the EPR and optical spectra agree well with the experimental results.     

Spin-wave resonance in Co/Pd magnetic multilayers and NiFe/Cu/NiFe three-layered films

The spectrum of standing spin waves has been detected by the ferromagnetic resonance method in NiFe(740 Å)/Cu/NiFe(740 Å) three-layered film structure in the perpendicular configuration for the copper thickness d _Cu ≤ 30 Å. At thicknesses d _Cu > 30 Å, the resonance absorption curve is a superposition of two spinwave resonance spectra from individual ferromagnetic NiFe layers. For Co/Pd multilayer films, united spinwave responance spectra have also been observed at thicknesses of the paramagnetic palladium layer up to d _Pd < 30 Å. The partial exchange stiffness has been calculated for a spin wave propagating across the Pd layer (A _Pd = 0.1 × 10^?6 erg/cm). This value is always positive (up to the critical thickness of the palladium interlayer d _Pd < d _c) or equal to zero (d _Pd > d _c).     

Magnetic anisotropy in multilayers

Magnetic anisotropy between in-plane and out of plane magnetic alignments is studied in a variety of multilayer systems using Mössbauer spectrosopy to observe the (Fe) magnetic orientation. The surface anisotropy in Fe/Au (1 1 1) multilayers is measured as K _s = 0.9 × 10^?3 Jm^?2. In Fe/Ni multilayers the dependence of magnetic orientation on external field applied normal to the layers enables volume and interface anisotropies K _v = (?5 ± 1) × 10⁴ Jm^?3 and K _s = (?0.6 ± 0.4)× 10^?3 Jm^?2 to be evaluated. In similar applied field experiments coherent rotation of the magnetic Fe and NiFe layers in Fe/Cu/NiFe/Cu multilayers was observed for intervening Cu layer thickness x = 5 Å but independent rotation for x = 50 Å. Out of plane magnetic components are observed for DyFe₂, YFe₂ thin films and DyFe₂/YFe₂ multilayers. In fields of up to 0.25 T applied inplane only the moments of the YFe₂ film showed significant rotation.     

Rotation of the polarization plane in a gas near resonance

Spectroscopic investigation of the Xe line at λ=5823.89 Å in a gas discharge by measuring the angle of rotation of the polarization plane of the laser radiation passing through a gas-discharge cell in a longitudinal magnetic field is discussed in the paper. An optical scheme of the experiment is described. It is predicted that the sensitivity of this method which implies the measurement of an absorption coefficient is characterized by a value ～10^?9 cm^?1     

Ionenlaser hoher Leistung

Investigations on ion lasers with large bored tubes (7 ... 15 mm I.D.) without additional axial magnetic field are performed. An axial magnetic field is shown to be not necessary to achieve high laser power. By absence of additional magnetic fields the laser construction is considerably simplified. Experimental criterions for maximum laser power are derived by means of a previously published theoretical paper. 120 W total power summed up over the 4p-4s Argon II transitions in the visible part of the spectrum, and 1.5 W ultraviolet power in Argon (3638 Å, 3511 Å) and Krypton (3507 Å) are obtained in continuous mode operation. High inversion densities of 7 · 10⁹ cm^?3 give rise to non-resonant laser oscillations. By multipass amplification the spontaneous emission is amplified up to 20 W/cm², having a beam divergence of about 10^?4 rad. All results are pointing out the influence of radiation trapping effects on the laser power to be smaller than estimated and measured by other authors using conditions deviating from our optimum conditions for maximum laser power.     

Linear expansion, phase separation, and magnetic inhomogeneities in La0.92Ca0.08MnO3

A relation of the thermal expansion with magnetic and magnetotransport properties has been revealed in La_0.92Ca_0.08MnO₃ single crystals in the paramagnetic state. The magnetotransport and lattice properties and the specific features in the neutron scattering characteristics of the La_0.92Ca_0.08MnO₃ single crystals have been explained by the phase separation in the paramagnetic state into magnetic inhomogeneities (clusters) with short-range (～10 Å) and long-range (>10² Å) orders. The performed investigations have demonstrated that the clusters are closely related to the crystal lattice and that the magnetic inhomogeneities in the paramagnetic region are correlated to T ～ 250–300 K ? TC.     

15N Isotopic Effects on the Raman Spectra of Tribromoacetamide

Abstract

¹⁵N-Tribromoacetamide has been synthesized with an isotopic content of 99, 4%, its Raman spectra have been recorded in the range 4.000–50 cm^?1. The isotopic shifts arising from ¹⁵N have been determined and interpreted. We have assigned the vibrational spectra of Br₃CCONH₂ and some overtones, combinations and difference bands. The molecular structure of tribromoacetamide has been studied employing the Ab Initio teoretical calculations and the Teller - Redlich isotopic product rule has been applied by assuming these geometrical parameters:

rCN = 1.4623 Å, rCC = 1.6014 Å, rBrC = 1.9468 Å, rCO = 1.2144 Å, rNH = 1.0292 Å, C-C-Br = 108.83320, C-C-0:118.2440, C-C-N:120.4137, C-N-H:110.45930     

Quantum properties of strong accumulation layers on ZnO surfaces

Extremely strong accumulation layers with surface electron densities ΔN approaching 10¹⁴ cm^?1 have been achieved on ZnO surfaces in contact with an electrolyte. Quantization effects, which are very pronounced in such narrow (?10 Å) layers, are studied by measurements of ΔN versus surface barrier height V_s. Comparison of the results with self-consistent calculations shows very good agreement up to ΔN = 2 × 10¹³ cm^?2. Deviations observed at higher ΔN are probably associated with the huge electric fields (～10⁷ V/cm) experienced by the surface electrons.     

The temperature dependence of the electrical resistivity and the negative magnetoresistance of carbon nanoparticles

Temperature dependences of the electrical resistivity of samples of carbon nanoparticles obtained from nanodiamonds by annealing at 1800, 1900, and 2140 K were studied. The magnetoresistance of these samples was measured at 4.5 K. Data on the positive magnetoresistance obtained in fields above 3 T were used to estimate the mean free path l of carriers at liquid-helium temperature, l～12 Å for a sample annealed at 1800 K, l～80 Å for a sample annealed at 1900 K, and l～18 Å for the case of annealing at 2140 K. The samples annealed at 1800 and 2140 K exhibit a negative magnetoresistance in fields below 2 T. The carrier concentrations n in the samples annealed at 1800 and 2140 K were estimated as n～8×10²¹ and 3×10²¹ cm^?3, respectively.     

2D-Conductivity of thin Pd films condensed at low temperatures

Resistance measurements have been made on quenched condensed Pd films with thicknesses between 25 Å and 350Å. The films are prepared under different evaporation conditions by varying the system pressure between 10^?8 and 10^?5 mbar. Resistance minima with a logarithmic increase of the sheet resistance are observed for thick films (d<350Å) condensed under intentionally “bad” (10^?5 mbar) vacuum conditions, as well as for thin films (d<50Å) condensed at pressures around 10^?8 mbar. Structure investigations show that the thick films are granular. For these films the relation of granularity to 2D localization is discussed. The thin films are continuous and the logarithmic resistance increase is in agreement with predictions of 2D-theories.     

Raman-brillouin light scattering determination of the structural correlation range in ges2 glass

Polarized Brillouin and low frequency (3 cm^?1 ? ω ? 40 cm^?1) Raman spectra of GeS₂ glass have been measured. Parameters determined from the Brillouin spectra have been used in the theory of Martin and Brenig to obtain from a fit of the Raman spectrum the structural correlation range, 2σ. We find 2σ = 8 ± 1 Å.     

50 MeV Li3+ ion irradiation effect on structural, electrical, and dielectric properties of PVA-based nanocomposite polymer electrolytes

Nanocomposite polymer electrolyte thin films of polyvinyl alcohol (PVA)-orthophosphoric acid (H₃PO₄)-Al₂O₃ have been prepared by solution cast technique. Films are irradiated with 50 MeV Li³⁺ ions having four different fluences viz. 5?×?10¹⁰, 1?×?10¹¹, 5?×?10¹¹, and 1?×?10¹² ions/cm². The effect of irradiation on polymeric samples has been studied and characterized. X-ray diffraction spectra reveal that percent degree of crystallinity of samples decrease with ion fluences. Glass transition and melting temperatures have been also decreased as observed in differential scanning calorimetry. A possible complexation/interaction has been shown by Fourier transform infrared spectroscopy. Temperature-dependent ionic conductivity shows an Arrhenius behavior before and after glass transition temperature. It is observed that ionic conductivity increases with ion fluences and after a critical fluence, it starts to decrease. Maximum ionic conductivity of ～2.3?×?10^?5 S/cm owing to minimum activation energy of ～0.012 eV has been observed for irradiated electrolyte sample at fluence of 5?×?10¹¹ ions/cm². The dielectric constant and dielectric loss also increase with ion fluences while they decrease with frequency. Transference number of ions shows that the samples are of purely ionic in nature before and after ion irradiation.     

Isothermal desorption of indium from √31−In and √3−In on silicon (111) surfaces

Using RHEED (reflection high energy electron diffraction) techniques, superstructures of submonolayer indium films deposited on silicon (111) 7 × 7 surfaces have been investigated. Combination of the deposition experiments at a constant deposition rate and the desorption experiments under isothermal condition has yielded desorption energies and apparent vibrational periods of indium adatoms (66 kcalmol^?1, 6 × 10^?17 s for √31 phase and 63 kcalmol^?1, 3 × 10^?15 s for √3 phase). A pattern overlapping √31 + √31, observed in the desorption process, is interpreted as a fluctuation phenomenon caused by a finite interaction range of indium adatoms on silicon (111) surfaces. The range is determined to be ～24 Å.