TE Spectra Research Articles

Discrete structure in the 4d photoabsorption spectrum of tellurium and its ions

The 4d inner-shell absorption spectra of Te, Te+, Te2+ and Te3+ have been recorded with the dual laser produced plasma technique. Discrete structure due to 4d5p transitions has been observed in the 37 - 45 eV region and identified using Hartree-Fock calculations within the superposition of configurations method. For comparisons of theoretical and experimental spectra, the method approximates a calculation of the resonant part of the photoionization cross section. It was also necessary, especially in the higher ion stages, to include absorption from excited states of the ground configurations. From these, synthetic spectra were produced which generally agree well with the experimental data.

Synthesis and characterization of dicyclohexyl telluride; Raman spectra and multinuclear NMR studies of dialkyl tellurides

Dicyclohexyl telluride, Te( cyclo-C 6H 11) 2, a somehow surprisingly not reported molecule, is synthesized and characterized by multinuclear NMR, IR and Raman spectroscopy. A temperature dependence of the NMR spectra ( 125Te{ 1H}, 13C{ 1H}) for Te( cyclo-C 6H 11) 2 is observed and studied in detail. The 125Te-NMR spectrum of Te( cyclo-C 6H 11) 2 at different temperatures is calculated and compared with the experimental spectra. The vibrational spectra of Te( cyclo-C 6H 11) 2 and values calculated ab initio at HF/3-21G* level are discussed. The Raman spectra of symmetric dialkyl tellurides TeR 2 with R 2=( n-C 3H 7) 2, ( i-C 3H 7) 2, ( n-C 4H 9) 2 and ( n-C 6H 13) 2 are reported and TeC stretching frequencies assigned. In addition, thorough NMR characterization of those derivatives ( 125Te{ 1H}, 13C{ 1H}, 1H) is given.

Homocarnosine and the measurement of neuronal pH in patients with epilepsy.

Homocarnosine is a dipeptide of gamma-aminobutyric acid (GABA) and histidine found uniquely in the brain, most likely in a subclass of GABAergic neurons. By comparison of spectra from the occipital lobe of patients receiving a homocarnosine elevation drug to normal subjects we have assigned two elevated resonances in the short TE 1H MRS spectrum to homocarnosine. These resonances are partially resolved at 7.05 and 8.02 ppm in a short TE spectrum at 2.1 T when macromolecule resonances are removed by subtraction of a spectrum in which the metabolite resonances are nulled by inversion recovery. The chemical shift of both of these resonances is sensitive to pHi. By comparison with a titration curve the pHi was calculated from the downfield resonance to be 7.06 in the patient group which is similar to values reported using the P(i) resonance. Based on the in vivo results and theoretical considerations the potential sensitivity for using nonelevated homocarnosine to measure pH is similar to that of P(i) under physiological conditions.

Investigation of Thin Film Waveguides Using Leaky Modes for Laser Applications

Leaky modes have been observed and investigated. A simple method is used to evaluate refractive index and thickness by solving the so-called mode equation. Application and tests of the assessments are made using a thin film of SiO x N y elaborated by the PECVD technique. From the measured TE and TM mode spectra, it is found that the films are isotropic with good confinement of the light wave. Results are in good agreement with those reported by many authors for such kind of materials.

Single-voxel proton brain spectroscopy exam (PROBE/SV) in patients with primary brain tumors.

The Single-Voxel Proton Brain Exam (PROBE/SV) is an automated MR spectroscopic technique. The purpose of this study was to investigate the use of PROBE/SV as a diagnostic tool in patients with primary brain tumors and to compare our findings with the known information obtained from conventional nonautomated MR spectroscopic techniques. We used PROBE/SV to image 10 normal adults and 46 patients with primary brain tumors: 29 glioblastoma multiformes (GBMs), five anaplastic astrocytomas, and 12 low-grade astrocytomas. All studied were performed on a 1.5-T Signa unit. Average voxel sizes were 6-8 cm3. A corresponding point-resolved spectroscopy spectrum was obtained from normal-appearing brain parenchyma in each patient for comparison with the spectra from known areas of pathology. In patients with low-grade gliomas (grades 1 and 2), we observed decreased N-acetylaspartate (12 of 12) and slightly increased choline (11 of 12) when we compared these metabolites with those in the spectra of patients' normal brains. This comparison in patients with GBM yielded markedly decreased N-acetylaspartate (29 of 29) and prominently increased choline (27 of 29). In the short TE spectra, we frequently saw lipid signal in high-grade tumors, especially in GBMs (12 of 20). We identified lactate peaks in high-grade tumors (anaplastic astrocytoma and GBM, 29 of 34) and also in low-grade tumors (four of 12). The creatine signal in all gliomas was slightly less than that of healthy brain tissue. The lowest N-acetylaspartate, choline, and creatinine levels in conjunction with the highest lactate levels were usually found in necrotic portions of high-grade tumors. PROBE/SV is a simplified MR spectroscopy technique that reduces setup time and provides automatic on-line data processing and display. The voxel location can be selected to focus on the area of interest and to minimize voxel contamination from unwanted tissue. The results from our experimentation with PROBE/SV in patients with brain tumors generally concur with published reports of tumor spectra obtained by conventional MR spectroscopic techniques. The ease and accuracy of this new technique make it a useful clinical tool in differentiating human brain tumor grades.

Polarization Insensitivity in Interdiffused, Strained InGaAs/InP Quantum Wells

ABSTRACTTheoretical results are presented showing how quantum well disordering affects the TE and TM absorption coefficient spectra of In0.53Ga0.47As/InP single quantum wells. An error function distribution is used to model the constituent atom composition after interdiffusion. Different interdiffusion rates on the group V and group III sublattices are considered resulting in a strained structure. With a suitable interdiffusion process the heavy hole and light hole ground state, excitonic transition energies merge and the absorption coefficient spectra near the fundamental absorption edge become polarization insensitive. The results also show that this polarization insensitivity can persist with the application of an electric field, which is of considerable interest in waveguide modulators.

Tailoring the heavy-hole and light-hole quantum-confined Stark effect using multistrain-stepped quantum wells

We show theoretically how the quantum-confined Stark effect (QCSE) shifts of the E1–HH1 and E1–LH1 energy transitions can be engineered to be closely matched over the entire range of applied fields up to ±100 kV/cm using structures containing more than one strained-layer composition within each well. In contrast, conventional quantum wells that contain a single uniform composition within each well can at best have the energy levels for the HH1 and LH1 states matched at only one field value. Furthermore, we show how the TE and TM absorption spectra are modified using multistrain-stepped structures leading to improved polarization-insensitive QCSE modulators.

The optical properties of tellurium under high pressure

Reflectivity spectra of Te have been measured in the energy range from 0.5 to 3.8 eV at pressures up to 96 kbar using a diamond anvil cell. Optical conductivity spectra sigma ( omega ) have been derived from the reflectivity by using an oscillator fitting analysis. The sigma ( omega ) spectrum for trigonal Te at 1 kbar has a main peak at 2.2 eV and a shoulder around 1.2 eV. These peaks shift to lower energy and grow with increasing pressure. When Te is transformed into a monoclinic phase around 40 kbar, a Drude term appears in the low-energy region of the sigma ( omega ) spectrum, which suggests that monoclinic Te has a metallic nature. In the orthorhombic phase at 73 kbar, a main peak at 0.5 eV and a shoulder around 1.2 eV are observed for the sigma ( omega ) spectrum. These changes in the sigma ( omega ) spectra of Te with pressure are discussed in comparison with the results of band calculations.

Transfer hyperfine interaction in Cd1-xMnxTe studied by NMR.

NMR measurements of $^{125}\mathrm{Te}$ and $^{113}\mathrm{Cd}$ have been performed on a series of samples of ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Te (x=0.01, 0.015, 0.02, 0.029, 0.055). The nuclear spin-lattice relaxation rates of the two nuclei are enhanced by two orders of magnitude compared to the relaxation rates of the same nuclei in the nonmagnetic II-VI semiconductor alloys. Well-resolved $^{113}\mathrm{Cd}$ and $^{125}\mathrm{Te}$ spectra were obtained using the magic-angle-spinning technique. The line shifts are temperature dependent and follow the Curie-Weiss law, indicating that the predominant interaction responsible for the NMR behavior is the transfer hyperfine interaction of the ${\mathrm{Mn}}^{2+}$ ion with the Cd or Te nuclei. All the lines in the Te spectra and all but two of the lines in the Cd spectra are shifted to higher frequencies with reference to the NMR line in CdTe. A full spectral assignment has been obtained based on two assumptions: (i) the Mn ions are randomly distributed in the lattice, and (ii) the transfer hyperfine interaction that causes the line shifts depends on the number of bonds between the Mn ion and the Cd (or Te) ion. According to the spectral assignment, the spin density transferred to the Cd sites in the lattice decreases tenfold from one Cd shell to the next.

EXAFS Measurements for Dense Vapors of Chalcogens and Chalcogenides

We have measured the EXAFS spectra of dense Se vapor in the temperature-pressure range up to 1600°C and 150 bar. For the measurements we have developed a high-pressure vessel and a polycrystalline sapphire cell of our own design. We have found that dense Se vapor under the present experimental condition consists mainly of Se2. The bond length is much shorter than that of liquid Se, and nearly the same as the data for Se2 in low-density vapor Se obtained by the recent spectroscopic study. When the saturated vapor-pressure curve is approached, the bond is elongated. We have also measured the EXAFS spectra of vapor Te and AsS. We have found the bond length of the molecule in vapor Te is nearly equal to that for Te2 obtained from the spectroscopic data. The structural parameters of the molecules in vapor AsS is similar to those of cagelike AS4S4 molecules in the crystal with a monoclinic form.

TE and TM optical gains in AlGaAs/GaAs single-quantum-well lasers

TE and TM optical gains were measured in AlGaAs/GaAs single-quantum-well (SQW) diode lasers at 300 K. Both the TE and TM gain spectra were step-like. The long and short wavelength steps in the experimental spectra were attributed to the onset of transitions from the first (e1) and second (e2) electronic subbands respectively. With increasing pumping current, the growth of the gain amplitude in the e1 transition slows down, the peak gain being approximately the same for TE and TM polarizations. On the contrary, the TE gain dominates over the TM gain in the e2 transition, which shows up at higher injection. Most of the polarization features of the observed gain spectra are easily interpreted within a model of direct optical transitions with the band mixing, relaxation broadening and bandgap shrinkage effects taken into account. At the same time, the gain saturation in the e1 transition observed in experiment can hardly be explained by the theory.

X-ray photoelectron spectra for the oxidation state of TeO2-MoO3 catalyst in the vapor-phase selective oxidation of ethyl lactate to pyruvate

X-ray photoelectron spectra of Te 3d5/2 and Mo 3d5/2 core electrons of TeO2-MoO3 provided an evidence for reduction of Te to the metallic state even in the presence of oxygen in the feed, while the component Mo was rather stable under reductive environment, in the vaporphase oxidation of ethyl lactate at 300 °C. Lattice oxygen was supplied to make up for the oxygen-deficit at the surface, and the catalyst should be used under oxidative, oxygen-rich conditions.

Vibrational spectra of Te(OH)6.2NH4H2AsO4 and Te(OH)6.2(NH4)2HAsO4

Abstract IR and Raman spectra of Te(OH)6.2NH4H2AsO4.(NH4)2HAsO4 (compound I) and Te(OH)6.2(NH4)2HAsO4 (compound II) are recorded and analysed. The symmetry of different groups and the vibrational interaction between them are discussed. The observed spectra suggest the existence of HAsO2−4 in II and coexistence of HAsO2−4 and H2AsO−4 in I. The ammonium ion is found to execute hindered rotation in the lattice in both the compounds.

Wide-band Bragg reflectors made with silica on silicon waveguides

We report the fabrication of strong Bragg reflectors embedded into fiber matched silica on silicon waveguides. First order gratings are dry etched into the waveguide core and are covered with a thin Si3N4 layer prior to cladding deposition. Reflection bands 225 Å wide for TE and 193 Å wide for TM polarizations are obtained with 200 Å Si3N4 cover layers. The TE and TM spectra are overlapping, resulting in nearly polarization independent Bragg reflectors.

EXAFS measurements of liquid Se-Te mixtures

The authors have measured EXAFS spectra of liquid(1-)Se, Te, Se-Te mixtures and vapour (v-)Se for the first time. It is clearly observed that the two-fold coordinated chain structure is largely preserved in l-Se and that the intrachain covalent bond contracts on melting. The EXAFS spectra of l-Te can be reproduced well by assuming a two-fold coordinated chain structure with shorter and longer bonds within the chain. In order to obtain partial structure-parameters in liquid Se-Te mixtures, spectra are taken on both Se and Te K edges. The Se-Se bond elongates with increasing Te concentration. The number of the longer Te-Te bonds increases with increasing temperature or increasing Te concentration; this may be associated with the semiconductor-to-metal transition.

Study of CdTe(1̄1̄1̄) surface reconstructions by RHEED and XPS

The surface and bulk contributions have been distinguished in the angle-resolved X-ray photoemission spectra of Te(3d52) electronic state from the CdTe(1̄1̄1̄) surface with the binding energies Eb1 = 574.0 eV (±0.1 eV) and Eb2 = 572.8 eV (± 0.1 eV), respectively. Several reconstructions, namely (1 × 1), (2 × 2), (2√3 × 2√3)R30° and c(8 × 4) have been observed for the CdTe(1̄1̄1̄) MBE-grown surfaces for different tellurium fluxes as a function of the substrate temperature. The growth of subsequent CdTe(1̄1̄1̄) layers on the reconstructed surfaces was attempted under different conditions such as different tellurium pressure and sample temperature. A correlation between the reconstruction type and the phase of the corresponding RHEED oscillations observed during the growth was found. Bilayer surface representations derived from the “hexagonal ring” model recently proposed for the √19 structure observed on the GaAs(1̄1̄1̄) surface [1] are suggested for the (2 × 2), c(8 × 4) and (2√3 × 2√3)R30° CdTe(1̄1̄1̄) structure

Raman and IR spectra of Te(OH)6·2Na3P3O9·6H2O

AbstractRaman and IR spectra of Te(OH)6·2Na3P3O9·6H2O were recorded and analysed. The vibrational bands due to P3O, TeO6 octahedra and water molecules were identified. It was found that P3O has a ring structure in the compound. The TeO6 group and P3O were found to coexist independently in the crystal. Further, the water molecules are non‐equivalent in the unit cell.

119Sn-Emission Mössbauer Studies on 119mTe and 119Sb in S, Se, and Te

Abstract The valence states of 119Sn produced by the EC decay of diluted 119Sb and 119mTe chemically doped in S, Se, and Te were studied by emission Mössbauer spectroscopy. The Mössbauer spectra of S(119mTe) and Se(119mTe) consist of peaks ascribable to Sn(IV) and to Sn(II) surrounded by S or Se, respectively. The Mössbauer spectra of Te(119Sb) samples show that, though a greater part of the 119Sb atoms are dispersed in the Te matrix after coprecipitation and sublimation, upon slow recrystallization after fusion most of the dispersed 119Sb atoms aggregate to form microcrystals of 119Sb2Te3.

Triple Langmuir probe measurements in the Phaedrus-T tokamak

Triple Langmuir probe data have been obtained in the initial phase of Phaedrus-T operation. The data were obtained during ohmic discharges of roughly 60 kA, 〈n〉=5×1018 m−3, Te(0)=0.5 keV, BT=0.7 T, q(a)=4 and 80 ms duration. Measurements of Te, φF, and n were made in a radial scan in the scrape-off layer of the plasma. The toroidal and poloidal positions were fixed throughout the scan. The spectra of Te, φF, and n fluctuations up to 100 kHz were also measured. One of the interesting features of these data is that m=2 oscillations detected by Mirnov coils were very pronounced in the φF and Te fluctuation spectra, but are not as evident in the n fluctuations. These data are presented and the techniques employed to get these data are discussed. Future plans to extend the maximum frequency resolution of the fluctuation measurements up to and beyond the frequency range of the ICRF power (13–18 MHz) that will be injected into Phaedrus-T are also discussed.

IR and single‐crystal Raman spectra of Te(OH)6 · 2CO(NH2)2

AbstractIR and single‐crystal Raman spectra of Te(OH)6 · 2CO(NH2)2 were recorded and analysed. Vibrational bands due to OH bonds and TeO6 and (NH2)2CO groups are identified. From the spectra it is concluded that the TeO6 groups occupy C2 sites and the urea molecules vibrate independently in the crystal.