Infrared Reflectivity Measurements Research Articles

Par infrared spectra op ferroelectric RbHSeO4

Abstract Far infrared reflectivity measurements of the new hydrogen bonded ferroelectric RbHSeO4 in both paraelectric and ferroelectric phases are reported. It is found that the dielectric anomaly must lie below 5cm-1 and that the critical dynamics of the phase transition is probably of classical order-disorder type.

Long‐wavelength optical phonons in ZrSe3

AbstractZone‐center optical phonons in ZrSe3, the prototype of the IVB‐trichalcogenides family, are investigated by Raman scattering for several incident and scattered light polarizations, and by infrared reflectance measurements. These layered compounds are characterized by isolated chains along the b‐monoclinic axis formed by the linear stacking of distorted trigonal prisms with Se atoms at the corners and Zr atoms in the center. Nine modes are found among the twenty‐ones theoretically predicted. Phonons assignment is made by a group theoretical analysis of the crystal structure, using the correlation method which relates single chain modes to crystal modes as the interchain coupling is truned on.

Intrinsic surface phonons in amorphous silica

We have detected intrinsic surface phonons in amorphous silica by performing Raman scattering and infrared-reflectivity measurements on samples of porous Vycor glass. Qualitative agreement of our experimental spectra with theoretical calculations provides an identification of the surface features in terms of microscopic atomic motions. We find a sharp Raman-active peak at 980 ${\mathrm{cm}}^{\ensuremath{-}1}$ associated with the stretching vibration of a hydroxyl group against the surface silicon atom to which it is bonded. The corresponding wagging motion of the hydroxyl group appears in the infrared spectrum as a sharp feature at 380 ${\mathrm{cm}}^{\ensuremath{-}1}$. Both the appearance of these surface features as sharp peaks in the spectra and their relative amplitudes in the Raman and infrared spectra can be explained only if the sympathetic vibrations of underlying substrate layers are taken into account. Experimental confirmation of our assignment is provided by the effects of deuteration and adsorbed ammonia molecules on the Raman-active surface modes. Our experiment is the first in which the perturbations of these surface modes due to adsorbed molecules are studied. In our examination of the effects of water adsorption on the Raman spectrum of porous Vycor, we have observed a decrease in the intensity of the bulk defect mode near 600 ${\mathrm{cm}}^{\ensuremath{-}1}$ with increasing water coverage. We also present evidence that the intense scattering background observed in Raman experiments on silica and other oxide surfaces is primarily an intrinsic electronic surface effect.

Infrared reflectivity spectra and Raman spectra of Ga1−xAlxAs mixed crystals

Thick layers of Ga1−xAlxAs with uniform composition were grown by an isothermal liquid-phase epitaxial technique. Infrared reflectivity and Raman-scattering spectra were measured with the same samples. The clean ’’two mode’’ behavior was confirmed in the reststrahlen band region, and the infrared reflectivity spectra were analyzed by using a Kramers-Kronig dispersion relation and by curve fitting to two forms for the classical dielectric function, one an additive form and the other a factorized form. The experimental data are in good agreement with the calculated TO and LO frequencies, when the Chang-Mitra model has been modified with the assumption of a nonlinear dependence of the oscillator strengths on composition. In the Raman spectra, the disorder-activated LA mode near 200 cm−1, a mode near 250 cm−1, and a background-level change were observed and attributed to atomic disorder. In the infrared reflectivity measurements the major disorder-induced effect was the increase in the TO and LO damping constants.

Infrared and electrical characterization of multilayered n-type GaAs wafers

We have characterized GaAs epitaxial wafers (N active/N+ buffer/N+ substrate) intended for varactors and mixer diodes. I-V and C-V measurements demonstrated that the active layers were excellent. Transmission-loss measurements suggested that the buffer-layer carrier concentrations were too low and/or the wafers had an additional high-resistivity interface layer. The layer thicknesses were measured using a spherical drilling technique followed by chemical delineation. Analysis of infrared reflectance measurements provided the carrier concentrations of the layers as well as their thicknesses. Both the drilling-delineation measurements and the reflectance measurements confirmed the transmission-loss indications.

A model for TTF‐TCNQ conductivity

AbstractThe dc and optical conductivity of TTF‐TCNQ is described by a modd including a collective mode and small polaron hopping. The optical conductivity is expressed in terms of the one phonon density of state. Satisfactory agreement with far infrared reflectivity measurements is obtained.

Vibrational spectra of crystals with the A-type rare earth oxide structure—I. La 2O 3 and Nd 2O 3

Infrared reflectance measurements of Nd 2O 3 and single crystal Raman measurements of La 2O 3 and Nd 2O 3 were obtained. The A-type RE oxides belong to space group P 3 m1 and should have (2 A 1 g + 2 E g ) Raman + (2 A 2 u + 2 E u ) i.r. modes. The Raman spectra are in exact agreement with the predictions. The i.r. spectra of Nd 2O 3 are also in agreement with the factor group analysis with two rather strong bands for E| c and E⊥ c, but in addition has weaker bands, possibly multiphonon bands, adjacent to the high-frequency bands.

Spin orientation effects of shallow electrons induced by magnetization in transition metal surfaces

From infrared reflection measurements of transition metal surfaces it has been found that the profile is sensitively changed by both magnetization and demagnetization. In particular, the longer wavelength side of the reflection edge induced at the 7 – 15 μm region by magnetization with about 300 G is attributed to shallow electrons which are strongly spin oriented. The shorter wavelength side of the edge is due to mixed electrons which are rather weakly spin oriented. The effect of the three types of edge distribution indicates that the interface between the continuum and the mixed level is sensitive to the degree of magnetization; the degree of variation of the magnetic oscillator is estimated from the Nf-R curve to be within the range 10 21 cm -3 - 10 24cm -3.

Infrared lattice vibration spectra of CuInSe 2 and CuGaTe 2

The lattice vibration spectra of CuInSe 2 and CuGaTe 2 were investigated by infrared reflectivity measurements in the wavenumber range from 55 to 5000 cm −1. The mode energies were determined by a Kramers-Kronig analysis of the spectra. The results are compared with existing measurements and some general trends in the phonon energies of the I-III-VI 2 compounds are discussed.

Infrared study of superlattice formation in Ti 1+xSe 2

Infrared reflectivity measurements have been made on two Ti 1+xSe 2 crystals of different stoichiometry over the range 200 cm −1 to 4000 cm −1 before and after the formation of a 2a o × 2c o superlattice. In both crystals at room temperature a heavily damped Drude edge is observed at about 1000 cm −1. At liquid helium temperatures, below the phase transition, the Drude minimum of a non-stoichiometric crystal ( x ⋍ 0.02, T c = 155 K ) is at 750 cm −1 whereas the low temperature minimum of a more stoichiometric sample (T c = 198 K) is below 350 cm −1. Also, new absorption peaks are seen to develop below T c at about 3700 cm −1 and 3300 cm −1 for the non- stoichiometric and stoichiometric-crystals respectively. The observed phenomena are related to the energy gaps which open up in the band structure below the phase transition temperature.

Ice releasing block-copolymer coatings

The ice releasing characteristics of a series of polydimethylsiloxane-polycarbonate block-copolymers films have been investigated. The abhesion is not only a function of the hydrophobic nature of the surface of the films but is also dpendent of the polysiloxane content, its block chain length and the glass temperature of the polymer, i. e. it is dependent on the rheological properties of the films telative to ice. Addition of some silicone oil to the films only improves the abhesion of the optimum block-copolymers which have a suitable polydimethylsiloxane composition. The oil acts as plasticizer and lubricant. Attenuated infrared reflectance measurements show that the polydimethylsiloxane blocks lie preferentially in the polymer/air interface, i. e. the methyl groups are located uppermost.

Raman scattering in SnSe

Zone center optical phonons in SnSe were investigated using Raman scattering data obtained here and previously reported far infrared reflectivity measurements. The values of the ratio of the pairs of interlayer force constants were calculated te be between 3·3 and 4·5. Thus it can be concluded that SnS is a layer like semiconductor but less layer like than GeS, GeSe or SnS.

Intrinsic Surface Phonons in Porous Glass

Raman scattering and infrared reflectivity measurements have been performed on samples of porous Vycor glass. Three features in the spectra are due to intrinsic surface phonons. This identification is supported by a simple new theory for the surface.

Pb0.8Sn0.2Te Infrared photodiodes by indium implantation

Indium ions were implanted in p-type Pb0.8Sn0.2Te films to produce shallow p-n-junction infrared photodetectors. The films were grown epitaxially on BaF2 (111) cleavage planes and showed hole concentrations of the order of 10 17 cm-3. The indium implantation was carried out using doses between 2 x 1013 and 1 x 10-15 cm-2 and an energy of 700 keV with the films either at room temperature or 450 K. The indium concentration profiles were determined by SIMS before and after annealing of the implanted films at temperatures up to 670 K. The implanted films were covered with silicon nitride layers to avoid evaporation during the subsequent annealing. The electrical properties caused by the indium implantation were examined by far infrared reflectivity measurements in the plasma resonance region. The carrier type of the implanted layers was determined by a conventional thermoelectric method. To convert the implanted layers into n-type the films had to be annealed at a temperature of at least 570 K for 30 min. regardless of the implanted indium dose. Mesa diode were fabricated by standard photolithographic procedures. Zero bias resistance area products as high as 0.4 Ω cm2 were obtained.

Remaining methacrylate groups in composite restorative materials

The quantity of the remaining unreacted methacrylate groups in polymerized composite materials has been determined. Six proprietary composites were investigated by infrared multiple internal reflection spectroscopy. Infrared reflectance measurements were made before polymerization and repeated after the composites were subjected to polymerization at 37 degrees C for 24 hours. The quantities of remaining unreacted methacrylate groups were determined and the data expressed as percentages of the total amount of methacrylate groups in the unpolymerized materials. The specimens were specially prepared to ensure that the surface properties simulated the bulk properties of the polymerized composites. The quantities of remaining methacrylate groups in the six composites determined by this surface measuring technique ranged from 25 to 48%. The results demonstrate that commercially available composite materials exhibit different degrees of conversion 24 hours after the start of polymerization. These differences can be correlated to the different monomer compositions of the composite restorative resins.

5. Asteroid Infrared Reflectances and Compositional Implications

This paper assesses the state of asteroid infrared reflectance measurements and discusses what they have contributed to our understanding of asteroidal surfaces. The data suggest that space weathering has not significantly affected the optical properties of the asteroids. Thus comparisons of asteroid reflectance spectra with unweathered meteorite and rock data is a valid procedure. The high 1.6 and 2.2 μm reflectances of a number of asteroids strongly suggest the presence of a metallic phase.

Infrared and Raman spectra of the IV-VI compounds SnS and SnSe

The results of Raman scattering and infrared reflectivity measurements on the IV-VI layer-type semiconductors SnS and SnSe are presented. The infrared-active TO, the associated LO-phonon frequencies, and the dielectric constants for all three principal polarizations have been determined from a Kramers-Kronig analysis of the reflectivity data. The symmetries of the zone-center phonons observed in the different polarization configurations are in agreement with the group-theoretical analysis of the ${D}_{2h}^{16}$ space group of these compounds. Despite the center of inversion symmetry in this structure, some infrared- and Raman-active modes are found to be nearly degenerate, suggesting the importance of the layerlike character in these compounds as in the isomorphic GeS and GeSe. A comparison of the phonon frequencies of the corresponding modes in the spectra of SnS and SnSe, or GeS and GeSe, indicates that the frequencies vary as a power (-2.2) of the lattice constant.

Optical phonons in AgGaSe2

AbstractPolarised infrared reflectivity measurements are made on a single crystal of the chalcopyrite compound, AgGaSe2. This is the only selenium I—III—VI2 compound so far investigated for zone centre phonon properties. They are compared with previous studies of lattice dynamics in the sulphide family and its binary zinc blende analogues. Four modes having E irreducible representation are observed between 35 and 75 μm with the light polarised perpendicular to the c‐axis, while two B2 modes are observed in the range 35 to 65 μm with the light polarised parallel to the c‐axis. A classical dispersion harmonic oscillator computer fit is made to the data to give the oscillator parameters.

Infrared reflectivity of GeS

Infrared reflectivity measurements at room temperature and 77K are reported for all three principal polarisations. The hitherto missing B1u phonon is shown to have a frequency of 315 cm-1. A new order of pairing with the known Raman active modes is suggested and the resulting splitting factors and force constant ratios calculated. The work suggests that GeS is even less anisotropic than has been thought up to the present.

Polar optical phonons and dielectric dispersion of GeSe crystals

The polar optical phonons at k = 0 of GeSe have been investigated by polarized infrared reflection measurements in the range 20–4000 cm -1. All the four, expected from group theory, infrared optically active phonons have been observed and identified with respect to their symmetry types. The phonon frequencies appear at 155 cm -1 ( B 2 u ) for E‖ b and at 88 cm -1 ( B 3 u ), 173 cm -1 ( B 3 u ) and 181 cm -1 ( B 3 u for E ‖ a in good agreement with an earlier determination from the optical absorption spectrum. Dielectric and optical dispersion properties due to the lattice vibrations and TO and LO frequencies have been derived from Kramers-Kronig integration and oscillator analysis of the reflectivity spectra. The effective ionic charge is estimated as e ∗/ e ≅ 0.3 by the Szigeti formula.