Intrinsic Absorption Coefficient Research Articles

A design for a stable and reproducible phantom for use in near infra-red imaging and spectroscopy

The note describes a stable, reproducible phantom which replicates the optical properties of tissue, for use in near infrared spectroscopy and imaging. The base material is a clear, unpolymerized liquid resin, which forms a polyester on addition of a catalyst. The polyester has a low intrinsic absorption coefficient for infra-red light and is non-scattering. Before the resin is set, measured quantities of scattering particle suspensions and absorbing dyes can be added. The scattering-phase function of these particles has been measured, and the mean cosine of the scattering angle, g, calculated. Knowing this, by varying the concentration of scattering particles a range of materials with known transport scattering coefficients can be produced. By varying the concentration and mix of dyes, the absorption coefficient of the resin can be easily controlled. The resin can be easily machined or cast into arbitrary shapes, with regions of differing optical properties. Once set the polyester is stable, in terms of both physical and optical properties.

Critical-point phonon frequencies of diamond.

The phonon-dispersion curves derived from neutron-scattering experiments performed on diamond [J. Warren et al., Phys. Rev. 158, 805 (1967)] are not accurate enough to yield the exact frequencies of critical-point (CP) phonons and, thus, to provide a satisfactory interpretation of second-order optical spectra. A self-consistent analysis of such spectra [S. Solin and A. Ramdas, Phys. Rev. B 1, 1687 (1970)] was not fully successful because of ambiguities that arise in assigning second-order infrared absorptions and features in the Raman spectra to specific two-phonon summations. A more effective method for obtaining accurate CP phonon frequencies involves investigating defect-activated one-phonon absorptions; in this paper, we take advantage of the availability of chemically vapor-deposited (CVD) diamond for the purpose of locating and assigning infrared (ir) absorption features in the one-phonon region to CP phonons at the Brillouin-zone boundary. Fourier-transform ir absorbance spectra of CVD diamond exhibit a complex structure at wave numbers below the 1332.5-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ lattice-mode cutoff, which is induced by impurity-associated defect centers and yields the exact positions of 16 zone-edge CP phonons. In conjunction with the triply degenerate zone-center mode, this set of phonons then provides the basis for predicting the positions of second-order optical features through simple summations. When the selection rules are taken into account, the procedure yields excellent results, not only in terms of CVD-diamond ir spectra, but also in regard to earlier measurements of the intrinsic two-phonon absorption coefficient of type-IIa natural diamond [J. Hardy and S. Smith, Philos. Mag. 6, 1163 (1961)] and the second-order polarization-dependent Raman spectra recorded by Solin and Ramdas.

Synthesis and characterization of a new series of BiOI 1−x−yBr xCl y pigments

New bismuth oxyhalides of the type BiOI 1− x Br x, form a complete solid solution series and crystallize in a layered tetragonal matlockite structure Color varies from yellow to red as x decreases from 0 75 to 0 and layer separation, c, increases from 8 45 to 9 16 Å The compounds have large intrinsic optical absorption coefficients ( α > 10 5 cm -1). Used as pigments, they are light-fast but show less color saturation than PbCrO 4 of the same hue.

Temperature considerations in metal-oxide-semiconductor solar cells

Calculations are presented for the performance of metal-oxide-semiconductor solar cells at various temperatures by incorporating the effects of the variations in band gap and effective mass of the charged carriers in addition to the variations intrinsic carrier concentration and absorption coefficient. A significant change in the device characteristics is obtained and the results presented here are in better agreement with the available experimental results.

Optical transitions in oriented polyacetylene

Calculations of the conductivity tensor for π-band transitions give new results on the anisotropy of the dielectric function. The intrinsic optical absorption coefficient for linear polarization has been measured on thin films consisting of highly aligned fibers and was found to be in qualitative agreement with the theory.

Simple theory of microwave absorption in alkali halides

The intrinsic microwave absorption coefficient $\ensuremath{\beta}$ of alkali halides is well explained by lifetime-broadened two-phonon difference processes, in contrast to energy-conserving two- and three-phonon processes as previously thought. A simple closed-form expression for $\ensuremath{\beta}$ with no adjustable parameters gives excellent agreement with the magnitude, frequency dependence, and temperature dependence of $\ensuremath{\beta}$ in LiF, NaCl, KBr, and KI. As $\ensuremath{\omega}$ increases from $\ensuremath{\omega}<\frac{{\ensuremath{\omega}}_{c}}{3}$ to $\ensuremath{\omega}={\ensuremath{\omega}}_{c}+{\ensuremath{\gamma}}_{293}$, the frequency and temperature dependence of $\ensuremath{\beta}$ at 293 K (or 1000 K) change from the low-frequency limit $\ensuremath{\beta}\ensuremath{\sim}{T}^{l}{\ensuremath{\omega}}^{2}$ to $\ensuremath{\beta}\ensuremath{\sim}{T}^{h}{\ensuremath{\omega}}^{2+}$, where $2.0\ensuremath{\le}l\ensuremath{\le}2.6$ (or $2.2\ensuremath{\le}l\ensuremath{\le}4.0$) and $0.9\ensuremath{\le}h\ensuremath{\le}1.2$ (or $0.6\ensuremath{\le}h\ensuremath{\le}1.6$) for the four materials. Here ${\ensuremath{\omega}}_{c}=20$ ${\mathrm{cm}}^{\ensuremath{-}1}$ for NaCl is the frequency difference between the transverse-acoustical and transverse-optical phonon modes at the Brillouin-zone edge along the [111] direction, and $\ensuremath{\gamma}$ is the combined-phonon inverse lifetime at 293 K. The temperature dependence of the phonon frequencies and other parameters must be included in order to explain the temperature dependence of the experimental results. The present lifetime-broadened results reduce to previous energy-conservation results (apart from smoothing, which explains the lack of sharp structure in $\ensuremath{\beta}$, but otherwise has little effect in general) and agree with experimental results in the region $\ensuremath{\omega}>{\ensuremath{\omega}}_{c}+\ensuremath{\gamma}$. Previous energy-conserving results disagree with the $T$ and $\ensuremath{\omega}$ dependence and magnitude of the experimental results in the region $\ensuremath{\omega}<{\ensuremath{\omega}}_{c}$.

Polarization fluctuations and optical absorption edge of ferroelectrics

Abstract An intrinsic optical absorption coefficient of ferroelectric crystal is calculated using the Green functions method with taking into account the thermal fluctuations of a crystal electrical polarization. The calculations are made for the uniaxial and three-axis ferroelectrics having the standard parabolic energy bands and for the three-axis ferroelectrics having the many-valley band structure of a BaTiO3 type. The dependence between band gap and polazrization was assumed quadratic. The spectral dependences of the absorption coefficient are found to be different in the para- and ferroelectric phases. In the ferroelectric phase the edge shape is exponential.

Optical Properties of Bismuth Tri-Iodide Single Crystals. I Interband Transitions

The reflection spectra of BiI s single crystals were measured between 1.5 and 5.5 eV at 6, 77 and 286 K. A Kramers-Kronig dispersion analysis of the data was made, and the intrinsic absorption coefficient and the real and imaginary parts of the dielectric constants were calculated. The large peak at 2.0 eV is ascribed to direct excitonic transition near the fundamental absorption edge. The main features observed in the absorption spectra are discussed in terms of two proposed models for anionic and cationic excitons in heavy metal halides. Measurements have also been made on the ordinary refractive index near the absorption edge. The data are well described by a two-oscillator model.

Characteristics of Glassy PbO-GeO2 Binary System by X-Ray Diffraction, Infrared Absorption, Dielectric Constant, and Intrinsic Absorption

Four characteristics of the glassy PbO-GeO 2 system were examined. X-ray diffraction pattern of the sample of 50 mol % PbO showed the maximum intensity at d -value 2.885 A which was in accordance with that of the compound PbO·GeO 2 (relative intensity 60 %). Infrared absorption band of the glassy GeO 2 at 11.5 µ, which would be assigned as ν 3 mode of tetrahedral GeO 4 unit, shifted toward longer wavelength side with increasing PbO content and the shift showed a plateau at 47-50 mol % PbO. Dielectric constant also kept neatly constant value at 47-50 mol % PbO The wavelengths corresponding to the intrinsic absorption coefficients per PbO mole fraction ( e. g. 70 cm -1 /m.f.) increase linearly with increasing PbO content, having the inflection point at 45 and 55 mol % PbO. The ionic character of the bond GeO 4 -Pb is discussed on the basis of the results.

On the Theory of Optical Absorption Spectra of Disordered Semiconductors

AbstractThe low frequency tails of the intrinsic and impurity optical absorption coefficients of nondegenerate semiconductors are calculated by the Green's functions method. The tails are due to the random force fields present in the samples. The fields are supposed to be generally anisotropic, Gaussian, and slowly varying in space. The intrinsic absorption tail has a Gaussian shape and a classical origin if the random fields acting upon electrons and holes are different. If the field is independent of the band index the absorption tail has a purely quantum origin; its shape depends somewhat upon the type of the field anisotropy and is close to exponential. As an example a special problem is considered in which the random field is provided by an intense acoustic noise flux propagating through the sample (both the deformation and the piezoelectrical interactions are considered). The results obtained are applied to interpret some experiments in which a modulation of an intrinsic absorption edge of piezocrystals by acoustoelectric domains was studied.

Optical Absorption of Natural Garnets from 1000 to 30 000 Wavenumbers

The optical absorption coefficient of five different natural garnet crystals has been measured at 300 K over the wavenumber range from 1000 to 30 000 cm−1. Peaks in the absorption spectrum associated with OH, Fe2+, Fe3+, Mn2+, and Cr3+ ions have been identified. Pure Mg3Al2Si3O12 and Ca3Al2Si3O12 appear to have an intrinsic absorption coefficient below 1 cm−1 in the wavenumber range 3000–30 000 cm−1. There are intrinsic multiphonon lattice peaks at wavenumbers of about 1790 and 2600 cm−1 in all five garnets.

Interionic Vibrational Spectra of Electrolytes in Nonaqueous Solvents

Far-infrared spectroscopy is shown to provide a powerful new method for studying ion aggregates in solution. The interionic vibration frequency of Na+BPh4− is at 175 cm−1 in pyridine, 1,4-dioxane, piperidine, and tetrahydrofuran, indicating that bare, rather than solvated ions, produce the absorption. The isotope shift of the 198-cm−1 band in NH4+BPh4− to 183 cm−1 in ND4+BPh4− also fits a bare-ion model. The Li+BPh4− salt gives a band at 410 cm−1 in THF, which is probably due to a Li+ THF vibration, while the K+BPh4− band at 133 cm−1 arises from interionic vibration. The Na+, NH4+, ND4+, and K+ salt stretching force constants, based on a bare ion-pair model, are nearly identical, 3.85 ± 0.11 × 104 dyn cm−1. These force constants differ markedly from those predicted from the Pettit and Bruckenstein ion-pair potential used to interpret conductivity data of strong electrolyte solutions. Present observations indicate that the dominating terms in the ion-pair potential are −(e2 / R)+(B / Rn), where n≫7 (Pettit and Bruckenstein), and may approach 30 for a pair of polyatomic ions. The dependence of the band intensities on solvent, concentration, and the nature of the cation appear to be primarily due to variations in the proportion of absorbing systems, rather than in the intrinsic absorption coefficient, and support the assignment of the bands to ion pairs, rather than higher aggregates. Intensity measurements in a mixed benzene–pyridine solvent demonstrate the phenomenon of preferential solvation by pyridine.

Preparation and semiconducting properties of platinum antimonide

Single crystals of PtSb 2, having purities greater than any previously reported were prepared by horizontal zone refining and Czochralski methods. Extrinsic carrier concentrations near 10 6 cm 3 and hole and electron mobilities at 50–60°K as high as 7,500 cm 2 Vsec and 6,000 cm 2 Vsec , respectively, were achieved. The thermal energy band gap, the intrinsic carrier concentration, the electron and hole mobilities and the mobility ratio of intrinsic PtSb 2, were derived from Hall coefficient and resistivity measurements in the mixed conduction range (77–300°K). The data give E 0 g = 0.11 eV, n i = 1·32 × 10 16 T 3 2 exp (0·110 eV 2k T ) , μ p = 6·55 × 10 6 T -1·57 and μ n μ p = 0·50. Scattering by acoustic mode phonons limits the mobilities of electrons and holes in intrinsic PtSb 2 in the temperature range 77–300°K Infrared absorption measurements at ≈10°K revealed that the band-to-band absorption threshold is ≈0·11 eV. The intrinsic absorption coefficient increases slowly above the threshold α = 19 cm -1 at λ = 8μ), suggesting that the optical transition is either indirect or direct but electric-dipole forbidden. Free carrier absorption data at 77°K show that the conductivity masses are about 0·16 m 0 g 1 2 for holes and 0·31 m 0 g 1 2 for electrons (where m 0 is the free electron mass and g 1 2 is an undetermined transport coefficient with a value between 1 and 1.8). The index of refraction at λ = 16 μ was found to be 5·5 ± 0·2 from transmission interference fringe measurements.

Infrared Reflectance from Mat Surfaces

A general theory is presented which segregates the rays composing the total reflectance from a mat surface according to their dependence on the intrinsic absorption coefficient. It is tested against experimental data obtained from samples of two materials (calcite and gypsum) which display a range of absorption band intensities over the 4-14-micro region. The ability of the theory to predict the different reflection behavior of bands of various intensities is demonstrated and considered important for explaining remote sensing data.

Separation of amylose from amylopectin of starch by an extraction‐sedimentation procedure

AbstractCorn, wheat, and potato starch were fractionated into their amylose and amylopectin components by selective extraction of amylose in hot water after the starch had been pretreated by heating in aqueous mixtures of glycerol, n‐butanol, Pentasol, Cellosolve, or dioxane. About three‐fourths of the amylose was leached in relatively pure form from starch in the first extraction at 98°C. The granule residue, sedimented by centrifugation, was separated as relatively pure amylopectin after two or more additional extractions. Retention of the coherent structure of the granule in the extracted residue emphasizes the importance of amylopectin to granule properties. Amylose fractions isolated from successive extractions had progressively lower intrinsic viscosities and absorption coefficients of their iodine complexes indicating that this component was subfractionated according to molecular size and/or structure. X‐ray diffraction patterns of the pretreated starches indicated that a complex of amylose and organic solvent was formed which may account for the increased solubility of amylose after pretreatment. Evidence is presented which shows that little, if any, amylose occurs in the crystalline regions of granular starches.

Contribution à l'analyse quantitative par les spectres d'absorption infrarouge des poudres: I. Examen théorique de la question

In a previous paper [5] we have given some preliminary quantitative experimental results about the influence, on the absorption of infrared radiations, of the particlesize of a powder dispersed in a potassium bromide pellet, and we showed that those results were in agreement with a theoretical formula we proposed.In this part, we give the theoretical considerations which lead to this formula: (1) In case of a quantity of substance insufficient to give more than one layer, the measured transmission is given by the formula: ▪(I0,I: intensities of the incident and transmitted light, Θ: transmission of one particle; XXX : fraction of the surface covered by the powder.)It follows from (I) that it is the absorption ▪ and not the extinction log ▪ which is proportional to the quantity of substance dispersed in the sample. (2) In case of a quantity of substance sufficient to give a number n of layers, it has been shown that, for cubic particles, the absorption of infrared radiations is given by the following law: ▪(m: mass of substance; ϱ: specific weight; s: surface of a section of the pellet normal to the beam; d: cube's edge; K: fraction of the surface covered by a layer.)In the case of spheres, the relation (2) has to be multiplied by a factor 32. The consequences which derive from this relation (2) are: (a) when kd is large, the measured absorption is independent of the intrinsic absorption coefficient of the substance (k). (b) when kd < 0.3, the particle dispersion behaves like a molecular one from the point of view of the absorption of radiations. (c) the measured extinction is proportional to the quantity of substance (equivalent to Beer's Law).The whole theory was established in the case of the dispersion of a powder in a transparent medium, where all particles are of the same size.The experimental results obtained in order to test this theory will be given in a second part.