Showing papers on "Absorption (electromagnetic radiation) published in 1975"

Theory of the extended x-ray absorption fine structure

Abstract: The extended x-ray absorption fine structure is a consequence of the modification of the photoelectron final state due to scattering by the surrounding atoms. We present a theory of the absorption fine structure starting from theoretically obtained electron-atom scattering phase shifts. The electron scattering is treated using a spherical wave expansion which takes into account the finite size of the atoms. Multiple-scattering effects are included by classifying multiple-scattering paths by their total path lengths. Their effects are quite large but appear to make quantitative but not qualitative changes on the single-scattering contribution. The exceptional case is the fourth shell in fcc or bcc structure, where it is shadowed by the first-shell atom and is profoundly affected by forward scattering due to the first shell. This may account for the anomaly observed experimentally at the fourth-shell radius in metals. A detailed numerical calculation is carried out for copper and is shown to agree quite well with experiment.

Estimation of sea surface temperatures from two infrared window measurements with different absorption

Abstract: Radiances measured at two different wavelengths or angles, with a resulting difference in absorption, can be used to determine the atmospheric attenuation of the surface radiance so that sea surface temperatures can be derived. Previous investigations used a correction equal to a constant times the difference in measured radiances. Some of these investigations were based on radiances calculated from models that underestimated absorption in moist atmospheres. When better transmittance models were used, the accuracy decreased. Radiances at 835 cm−1 are calculated for moist atmospheres at different zenith angles to test methods used to correct infrared measurements for atmospheric attenuation. Higher-order corrections are compared to first-order corrections and are shown to result in a significant increase in accuracy, reducing the rms error by one third, from 0.6 K to 0.4 K.

Shape of the 5 mm oxygen band in the atmosphere

Abstract: The problem of absorption of microwaves by molecular oxygen in the atmosphere is treated by means of a first-order approximation to the impact theory of overlapping spectral lines. By including only the coupling between adjacent rotational states in molecular collisions, we have devised a simple approximate method for computing the interference between lines from measurements on the resolved lines. The need for an empirically determined function describing the linewidth/ pressure ratio is eliminated. Comparisons with measurements at atmospheric pressures show that the first-order interference accounts for the low absorption at the band wings near 1 atm pressure. It also predicts the correct amount of asymmetry between high and low frequency wings. Improvement over previous models for the pressure broadening is obtained at frequencies \gsim 55 GHz. This approach is not specific to oxygen and could be adapted to other similar molecules.

Scattering and absorption of electromagnetic radiation by a semi-infinite medium in the presence of surface roughness

Abstract: In this paper, we present a theoretical description of the scattering and absorption of electromagnetic radiation induced by roughness on the surface of a semi-infinite medium. We approach the problem by the use of scattering theory applied to the classical Maxwell equations. We obtain formulas for the roughness-induced scattering from the surface of an isotropic dielectric for both $s$- and $p$-polarized waves incident on the surface at a general angle of incidence. When the real part of the dielectric constant of the material is negative and its imaginary part small (as in a simple nearly-free-electron metal), we extract from the expressions for the total absorption rate that portion which describes roughness-induced absorption by surface polaritons (surface plasmons). We compare our results with those recently published by Ritchie and collaborators for the case of normal incidence, and we present a series of numerical studies of the roughness-induced scattering and absorption rates in aluminum.

Dielectric function and plasma resonances of small metal particles

Abstract: Using the simple model of electrons in a box, a dielectric function is derived which should be appropriate for small metal particles. This dielectric function is used to examine quantum size effects in the optical absorption spectra. For very small particles of uniform size and shape, the plasma resonance absorption should shift and broaden and should show fine structure corresponding to transitions between discrete conduction band energy levels. The size dependence of the shift and broadening was measured and found to be in quantitative agreement with theory.

Theory and simulation of resonant absorption in a hot plasma

Abstract: A detailed theoretical and simulation study of resonant absorption in a hot plasma is presented which isolates the behavior of the plasma for times short compared to an ion response time. The extent to which an electron fluid model can describe the absorption process in the kinetic regime is discussed. At high intensities the absorbed energy is observed to be deposited in a suprathermal tail of electrons whose energy varies approximately as the square root of the incident power. The density profile modification due to the ion response to the pondermotive force is also discussed.

Two‐dimensional relativistic simulations of resonance absorption

Abstract: Resonant absorption has been simulated for radiation of energy density E20/4πnT ranging from much less than to somewhat greater than unity. Characteristic features of the absorption process are an absorption efficiency of approximately 50%, generation of suprathermal particles, and strong modification of the density profile in the vicinity of the critical density. The latter effect, the appearance of a finite density variation over the distance of a few Debye lengths, is forced by strong gradients in the plasma wave intensity and electron temperature. Such a density discontinuity greatly enhances the range of incidence angles for which resonance absorption is effective and decreases the effects of the oscillating two‐stream and ion‐acoustic decay instabilities.

Empirical relation between interstellar x-ray absorption and optical extinction

Abstract: An empirical relation between interstellar X-ray absorption and optical extinction is derived from the correlation of measurements made on objects of large intrinsic diameter. The result is Av = 4.5 times 10 to the -22nd power N(H) mag, with the principal error being largely systematic in origin, where N(H) represents the column density of interstellar matter in the Brown and Gould model for the X-ray absorption coefficient. Applying this ratio to optically identified compact sources, it is concluded that sources in binary systems showing pronounced X-ray occultations have an intrinsic absorption equivalent to about 10 to the 22-nd power atoms per sq cm of interstellar matter and that there are a few compact sources where the absorption seems to be primarily interstellar in origin. The interstellar absorption expected in Cyg X-1 from the extinction of its optical counterpart is much greater than that suggested by X-ray spectra, which may be due to a soft X-ray component greater than that predicted by the power law fitted to higher energy data.

The absorption and scattering of light in bovine and human dental enamel

Abstract: The reflectance and transmission of thin slabs of dental enamel has been measured at all wavelengths between 220 and 700 nm by means of an integrating sphere. From the results the true scattering and absorption coefficients have been computed. The theoretical model used is an extended two-flux model, which is presented and discussed. The absorption spectrum of the dissolved organic component of enamel was also determined. An absorption peak at 270 nm is common to all the spectra. This peak in the bovine enamel spectrum is about three times as high as in the spectrum of human enamel. The peak of the dissolved material is about as high as the peak of the corresponding enamel. Hence it is concluded that the organic component, presumably aromatic amino acids, is responsible for most or all of the observed optical absorption.

A theoretical analysis of the contribution of algal cells to the attenuation of light within natural waters ii. spherical cells

Abstract: SUMMARY A theoretical treatment of light attenuation within natural phytoplankton suspensions, developed in a previous paper, has now been applied to model suspensions of green and blue-green algal cells and colonies, in order to obtain a quantitative assessment of the effects of changes in the phytoplankton canopy structure on light attenuation within the system. It is shown that for green algae suspended in water in which effectively all the blue light is removed by dissolved yellow substances (gelbstoff) the light attenuation properties of the suspension are rather similar to those of an equivalent solution of algal pigments: this is because the individual cells have relatively low absorption values in the spectral region (545–655 nm) in which most of the transmitted light occurs. With blue-green algae, which have a strong absorption in this region, there can be marked differences between the light attenuation properties of suspensions and of equivalent pigment solutions. The suspensions transmit substantially more photosynthetically active radiation than the solutions: the effect increases with cell/colony size, with algal concentration and with depth, and can amount to a difference of several-fold. Values of vertical attenuation coefficients, and increments in attenuation coefficients per unit algal concentration, for the photosynthetic waveband have been calculated, and their dependence on pigment composition and cell/colony size is demonstrated. Calculations have also been carried out to show the relationship between the phytoplankton canopy structure, and the euphotic depth and maximum sustainable standing crop, of the suspension.

Absorption and dispersion of ultrasound in biological tissue

Abstract: Absorption of ultrasound is the process of conversion of vibrational energy into heat. In biological soft tissues, the absorption coefficients are roughly proportional to the frequency; typically α = 1 dB cm −1 MHz −1 . The velocities in soft tissues are similar, being about 1,500 msec − . Classical viscosity theory cannot explain this form of absorption. A relaxation process is associated with a range of frequency over which there is dispersion in velocity—from a low value at low frequencies to a high value at high frequencies—and a maximum in the absorption per wavelength. Experimental data, particularly for haemoglobin solutions, indicate that absorption and dispersion in biological materials are due to relaxation processes distributed over a range of frequencies. The dispersion is small, and usually negligible in relation to variations and uncertainties of measurement. The natures of the relaxation processes which are involved have yet to be resolved; possibilities include solvent-solute interactions and disturbances in H-bonding equilibria. Lung has a lower velocity than that of solid tissues, whereas that of bone is higher; both have higher values of absorption.

A new concept for solar energy thermal conversion

Abstract: A material has been developed which allows a new approach to be made to the conversion of solar energy to heat. It consists of a dense array of metal whiskers grown with spacings of a few wavelengths of visible light. The material selected has low emissivity, and achieves significant optical absorption by trapping the light by a geometric maze effect. We have deomonstrated that absorption of normal incidence light is greater than 98% from 0.5 to 40‐μm wavelengths, and hemispherical emissivity at 550 °C can be made less than 0.26. Since surfaces can be made of a single refractory element, such as W, high‐temperature solar conversion (550 °C) should be maintained with good surface stability.

Long-Path Monitoring of Atmospheric Carbon Monoxide with a Tunable Diode Laser System

Abstract: Long-path measurements of carbon monoxide in the atmosphere are described. The technique of resonance absorption was used in which the wavelength of radiation from a PbS0.82Se0.18 semiconductor diode laser was tuned into coincidence with an absorption line of CO in its fundamental band around 4.7 μm. By employing rapid frequency modulation of the laser emission to overcome atmospheric turbulence effects, it was possible to achieve a minimum detectable concentration of 5 parts per billion over a 0.61-km path. Continuous around-the-clock monitoring was also performed and permitted increases in the ambient CO level due to commuter traffic to be observed.

The temperature-dependent absorption spectrum of the v3 band of SF6 at 10·6 μm

Abstract: Mixtures of SF 6 diluted by argon were heated behind an incident shock front in the range 400 to 1500K. Absorption measurements at 11 wavelengths near 10·6 μm were made on the post-shock, equilibrated gas using a CO 2 laser. A band contour model, which makes use of known spectroscopic constantd and the experimental results, satisfactorily accounts for the observed temperature dependence of the absorption coefficient. We are able to estimate the identity of the transitions responsible for absorption of the laser radiation.

Absorption Characteristics of Multilayered Sphere Models Exposed to UHF/Microwave Radiation

Abstract: The interaction of electromagnetic plane waves with multilayered spherical models composed of lossy dielectric media that represent different biological tissues found in human and animal heads is examined in the frequency range 0.1 to 10 GHz. The model ranges in size from 2 to 12.5 cms outer radius and consists of a core of brain-like material surrounded by five outer layers of CSF, bone, fat and skin-dura tissues. The absorption properties of the model have been analyzed throughout this range of size and frequency. The distribution of internally deposited energy has also been investigated in detail for three basic spheres of 3.3, 6 and 10 cms radii, with emphasis on the creation of localized regions of strong heating (hot-spots). Based on these results, some generalized conclusions are presented on the interaction of microwaves with different sized biological objects.

Direct Observation of Atomic Energy Level Shifts in Two-Photon Absorption

Abstract: We report direct observation of optically induced shifts of atomic energy levels in sodium vapor that occur in two-photon absorption. These shifts must be considered in high-resolution two-photon spectroscopy. Two cw dye laser beams of different frequencies and propagating in opposite directions are utilized to obtain high-resolution spectra. The intensity and frequency dependence of the level shifts are examined.

Attenuation of Modified Mercalli intensity with distance from the epicenter

Abstract: Thirteen simple formulas describing the attenuation of Modified Mercalli (MM) intensity are compared to determine which best predicts observed values in North America. The equation (19) ln I = ln I o + a 2 − b 2 ln Δ − c 2 Δ based on the assumption that intensity is related to energy by the equation (12) I = k E P appears to be the most useful, although it is not the most precise. P was found to be approximately 1/6, indicating that MM intensity is proportional to the sixth root of energy or the cube root of amplitude. The United States and southern Canada are divided into three attenuation provinces: a San Andreas province with an energy absorption coefficient of 0.015/km, an Eastern province with an absorption coefficient of 0.0031/km and a Cordilleran province with an absorption coefficient of 0.0063/km. The different rates of absorption can be explained as the result of slightly greater average focal depths in the East than in the West, although the data are too scattered to prove that this is the cause. Greater depth of focus, especially if low-angle faulting is involved, would explain the lack of surface displacement with eastern earthquakes.

Absorption Cross Sections of O2(a1Δg) and O2(X3Σg−) in the Region from 1087 to 1700 Å

Abstract: An attempt has been made to measure the absorption cross sections of O2 in the metastable state a1Δg from 1087 to 1700 A. The absorption is measurable (σa ≥ 3 × 10−19 cm2) from around 1500 A toward...

High−sensitivity optical recording in KTN by two−photon absorption

Abstract: Birefringence changes are induced in a cubic potassium tantalate niobate crystal with an external electric field by two−photon absorption of picosecond optical pulses. The recording sensitivity of 100 μJ/cm2 for elementary holograms with a few percent reconstruction efficiency is comparable to the sensitivity of holographic silver halide emulsions. The high sensitivity of KTN is shown to be due to efficient charge transport of photoexcited carriers.

The low-energy absorption edge in 2H-MoS2 and 2H-MoSe2

Abstract: The optical transmission of thick samples of 2H-MoS2 and 2H-MoSe2 has been measured below the A exciton energy for each material. The absorption coefficients have been calculated, and the low absorption starting at 1·1–1·2 eV has been attributed to an indirect d-d transition for both materials. There appears to be considerable d-p valence band overlap giving rise to further higher absorption before the strong A, B excitons, which have been associated with direct transitions from a p-like valence band. An energy band scheme has been sketched on the basis of these interpretations.

Design of a reflection–absorption experiment for studying the ir spectrum of molecules adsorbed on a metal surface

Abstract: This paper discusses the experimental requirements for maximizing the infrared reflection–absorption bands from a thin layer on a metal surface Calculated curves are shown for 19 different metals for two spectral regions (2100 and 500 cm−1) From these curves, one can determine the performance of a particular experimental system (eg, using only one reflection or using two reflections) relative to the theoretical best performance of the optimum system In many cases, one reflection may yield a band intensity acceptably close to the maximum theoretical intensity

Absorption profile at surfaces

Abstract: Attenuation of the elastic component of an electron beam outside a surface is calculated. Significant attenuation is found in the case of RHEED experiments, explaining an earlier disagreement between theory and experiment observed by Menadue and Colella (see abstr. A9238 of 1972).

An experimental method of identifying and quantifying the active transfer electrogenic component from the diffusive component during sugar absorption measured in vivo.

Abstract: 1. The kinetics of absorption of glucose, galactose and alpha-methyl glucoside have been measured in rat jejunum in vivo using a chemical method and a new electrical technique. 2. Sugar absorption estimated by chemical methods exhibited two components. One component was phlorrhizin-sensitive, saturable and generated electrical potential differences (electrogenic active component) while the other was phlorrhizin-insensitive, non-saturable and did not generate electrical potentials (diffusive component). 3. The diffusive component of the actively transported sugars was indentical to the absorption behaviour of sorbose, a hexose that is not actively transferred. 4. A method for correcting the data obtained from chemical absorption studies for the diffusive component was developed. The corrected, operational kinetic constants for ‘apparent Km’ obtained by this method were not significantly different to values obtained electrically. The identity between the values obtained by both methods supports the concept that they represent a measure of the same rate-limiting step in the absorption process. 5. The application and significance of the techniques is discussed in relation to the clinical assessment of intestinal sugar absorption.

Intrinsic and impurity infrared absorption in As2Se3 glass

Abstract: A quantitative study of infrared absorption in the 250–4000 cm −1 region of As 2 Se 3 glasses doped with small amounts of As 2 O 3 or purified by various procedures has been carried out with particular attention to absorption in the wavelength regions of the CO 2 and CO lasers. The dependence of the relative intensities of the oxide impurity bands in the 650–1340 cm −1 region on the total amount of As 2 O 3 added to the glass indicates the existence of three distinct oxide-impurity species. A number of higher-frequency impurity bands which are due to the presence of hydrogen in the glass and whose intensities are highly dependent on the glass-melting conditions have been observed and classified. Intrinsic multiphonon absorption in the 400–1100 cm −1 region has been interpreted in terms of combination and overtone bands of the two highest-frequency fundamental vibrational modes. Absorption coefficients of As 2 Se 3 glass in the 920–1090 cm −1 CO 2 laser region are limited by intrinsic multiphonon absorption to values of around 10 −2 cm −1 . The lowest absorption coefficients measured in the 1700–2000 cm −1 CO laser region were around 2 × 10 −3 cm −1 and may contain contributions from hydrogen-impurity bands.