Showing papers on "Absorption (electromagnetic radiation) published in 1976"

Concentration‐dependent absorption and spontaneous emission of heavily doped GaAs

Abstract: A model for the calculation of the absorption and emission spectra for GaAs at carrier concentrations in excess of 1×1018 cm−3 is described. This model utilizes a Gaussian fit to Halperin‐Lax band tails for the concentration‐dependent density of states and also includes an energy‐dependent matrix element. The calculated absorption and emission spectra are compared to previous experimental results. All results are for 297 K. For p‐type GaAs, the agreement is very good. The concentration dependence of the effective energy gap is obtained and can be expressed as Eg (eV) =1.424−1.6×10−8 [p (cm−3)]1/3. The concentration‐dependent thermal equilibrium electron‐hole density product n0p0 and the radiative lifetime τr are calculated for p‐type GaAs. The value of n0p0 increases from the low‐concentration value of 3.2×1012 cm−6 to 1.2×1013 cm−6 at p=1.6×1019 cm−3. This value of n0p0, together with the thermal generation rate obtained from the experimental absorption coefficient, gives τr as 0.37 nsec at p=1.6×1019 cm−3.

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

Abstract: SUMMARY A theoretical treatment of light attenuation within phytoplankton suspensions, developed in a previous paper, has now been extended to certain non-spherical cells and colonies of diatoms, blue-green and green algae. Procedures for calculating the mean absorption cross-section for randomly oriented cells or colonies with the shape of cylinders, prolate spheroids or oblate spheroids, are described. The mean absorption cross-sections at a series of wavelengths throughout the photosynthetic waveband are compared for model algae of various shapes, sizes and pigment compositions. The results clearly illustrate the principle that for a strongly absorbing algal cell/colony of a given volume, the more extended it is in space, the more effective it is as a light collector. The advantages of the more extended package are much less evident at wavelengths which are weakly absorbed. For algae of a compact, e.g. spherical, shape a large cell or colony captures much less light (of a wavelength which is strongly absorbed) than an equivalent volume of smaller cells. However, it is also true that a large particle of very extended shape, e.g. a long thin cylinder, can capture as much light as a greater number of smaller particles (of the same total volume) of more compact shape. This means that for purposes of light collection an alga is almost as well off when organized into long multicellular filaments as it would be if separated into a large number of single cells. Using the calculated values of mean absorption cross-section the relationship between phytoplankton canopy structure and attenuation of photosynthetically active radiation (PAR) has been further investigated. Calculated values for the spectral distribution of light intensity, the total transmitted quantum flux, the vertical attenuation coefficients, and the increments in vertical attenuation coefficient per unit algal concentration, are presented for various model non-scattering suspensions corresponding to common naturally-occurring algae. The possible effects of turbidity on light attenuation in such systems are discussed.

2.3. Absorption and Emission by Atmospheric Gases

Abstract: Publisher Summary This chapter focuses on absorption and emission by atmospheric gases. At centimeter and shorter wavelengths, absorption and emission by atmospheric gases can significantly affect the propagation of electromagnetic radiation through the atmosphere. At frequencies from 1 to 300 GHz, referred to as “microwave frequencies,” absorption by atmospheric gases is dominated by water vapor lines at 22 and 183 GHz, oxygen lines near 60 GHz and at 118 GHz, and relatively narrow and weaker ozone lines above 100 GHz. Non-resonant absorption by water vapor and oxygen has significant effects in the window regions away from the dominant lines. The radiative transfer expressions appropriate for calculating absorption and emission by atmospheric gases are formulated. The general expressions for the spectral-line absorption coefficient are provided, followed by specific expressions for calculating absorption by water vapor and oxygen. Problems associated with the calculation of absorption by these two molecules are discussed. The absorption by microwave lines of ozone and other minor constituents is considered. Finally, the results of calculations of atmospheric absorption and emission, as well as the measured values are presented in the chapter.

Thermal lens technique: a new method of absorption spectroscopy.

Abstract: Both a single beam and a dual beam (with synchronous detection) thermal lens technique have been employed in the measurement of "colorless" organic compounds in the range 15,700 to 17,400 cm-1 Combination overtones of C-H stretching vibrations in benzene have been identified and agree with previous results obtained by conventional spectroscopy with a long optical path Extinction coefficients as low as 1 X 10(-6) liter mole-1 cm-1 have been accurately determined The sensitivity of the technique has been further demonstrated by measuring the So leads to T1 absorption of anthracene; the spectrum compares favorably with results obtained by conventional techniques

A Method for Inferring Total Ozone Content from the Spectral Variation of Total Optical Depth Obtained with a Solar Radiometer

Abstract: A solar radiometer has been used to monitor solar irradiance at eight discrete wavelengths. From these monochromatic measurements at varying zenith angles the total optical depth has been deduced by a computerized curve-fitting method. A unique technique will be described whereby the ozone absorption optical depths, and hence total ozone content of the atmosphere, can be inferred directly from the spectral variation of total optical depth. This procedure permits a systematic determination of total ozone content on a daily basis when other measurements are not available. Using the ozone absorption optical depths determined in this manner, the values of aerosol optical depth may be obtained more accurately by subtracting the molecular scattering and estimated ozone absorption contributions from the total optical depth. A technique is also described for estimating the absorption optical depths at wavelengths where additional molecular absorption other than ozone occurs. Results are presented as 1) d...

Attenuation measurement of cylindrical dielectric-rod waveguide

Abstract: Measurements have been made of the attenuation coefficient of cylindrical dielectric-rod waveguide by a transmission method at 100 GHz. An 8 dB/m attenuation coefficient was obtained, caused half and half by the absorption due to losses in the dielectric and by the scattering due to irregularities and inhomogeneities in the dielectric.

Infrared absorption spectra of carbon monoxide in rare gas matrices

Abstract: Medium and high resolution infrared spectra of CO trapped in solid Ne, Ar, Kr and Xe are presented. Spectra of low optical density samples are identical to those previously observed but a different assignment is proposed. Monomeric absorption is clearly identified by double doping experiments and by sample deposition through a nitrogen cold trap. In addition other broad weak bands are observed on the spectra of high optical density samples. These bands are assigned to combinations between internal vibration, libration and lattice vibrations. The barrier hindering rotation due to site distortion is estimated to 30–50 cm −1 . Librational motion is strongly perturbed by coupling with lattice motion. Carbon monoxide is also an efficient probe to study the dynamical properties of rare gas lattices.

Surface plasmons in small Au-Ag alloy particles

Abstract: The optical absorption spectra in 2-butanol have been studied. The position of the absorption peak which is attributed to surface plasmons of small alloy particles depends on the composition of the alloy. The spectra obtained are qualitatively in agreement with those calculated by Mie's equation.

High power laser propagation.

Abstract: High power laser beams propagating in the atmosphere are subjected to a variety of effects, the most important of which are absorption, scattering, turbulence induced beam spreading and wander, thermal blooming, and gas breakdown. In this paper simplified models are used to show how the various atmospheric effects interrelate and impact on the best laser choice for high power applications through their dependence on the laser wavelength and temporal mode (e.g., cw or pulsed) of operation. Results for sea level propagation at seven common laser wavelengths varying from 0.34 microm to 10.6 microm. are presented that show the mid-ir wavelengths to be favored for typical turbulence and aerosol scattering conditions. At the longer 10.6-microm CO(2) laser wavelength thermal blooming is dominant due to stronger molecular absorption, while at the shorter wavelengths turbulence induced beam spreading and aerosol absorption and scattering effects become important and tend to limit the increase in irradiance expected on the basis of diffraction effects alone.

The electronic structure of NO2. II. The ? 2B2← ? 2A1 and ? 2B1←? 2A1 absorption systems

Abstract: The results of an accurate ab initio study of the electronic structure of NO2 have been applied to an analysis of the two important visible and near infrared absorption systems of this molecule. The long wavelength absorption (λ≳6000 A) arises from an ? 2B2←? 2A1 transition. A theoretical absorption spectrum that is generated from the C2V ab initio potential surfaces of these two states qualitatively reproduces most of the features of an experimental low resolution absorption spectrum between 9000 and 6000 A. The (0–0) band of the transition is predicted to be several times less intense than nearby hot bands even at temperatures as low as 300 K. The computed ? 2B2 spectroscopic parameters are Te=1.18 eV, Re=1.26 A, ϑe=102°, ω1=1461 cm−1, ω2=739 cm−1, and μ=0.46 D. There is a marked difference between experimentally determined ? 2B2 rotational constants and those deduced from the ab initio equilibrium geometry; this datum adds to the rapidly increasing evidence for strong vibronic coupling of the ? 2B2 sta...

Line shape and strength of two-photon absorption in an atomic vapor with a resonant or nearly resonant intermediate state

Abstract: The line shape and strength of two-photon absorption in an atomic vapor are studied both theoretically and experimentally for the case of absorption of unequal-frequency photons. Special attention is given to the situation in which the intermediate state is resonant or nearly resonant. It is shown that for the resonant case, maximum absorption rates and Doppler-free spectra are simultaneously obtained. Either counterpropagating or copropagating beams can be used. With counterpropagating beams the two-photon linewidths are quite insensitive to power broadening of the intermediate state. With copropagating beams the two-photon spectra can also reveal the intermediate-state structure with a resolution determined mainly by the intermediate-state lifetime (nearly as well as can be done using any spectroscopic technique). The theoretical analysis provides a closed-form solution for the line shape and strength of the two-photon spectrum. In general the line shape is composed of two features. One is Doppler broadened and predominates when the intermediate state is nonresonant. The second feature is Doppler free, and it predominates with a resonant intermediate state. Detailed comparisons of experimental measurements of the line shape and strength of two-photon absorption in Na vapor are made with the theory and found to be in excellent agreement. Signal strengths for the onresonance case are 6 \ifmmode\times\else\texttimes\fi{} ${10}^{9}$ larger than the case of equal-frequency photons with no loss in resolution.

The absorption spectrum of NH2 between 6250 and 9500 Å

Abstract: The earlier analysis by Dressier and Ramsay of the absorption system of NH2 has been considerably extended at the long wavelength end of the spectrum. All the low-lying vibronic levels of the excit...

Infrared observations of ices and silicates in molecular clouds

Abstract: Spectrophotometric observations from 2 to 4 microns and from 8 to 13 microns of several infrared sources associated with molecular clouds are reported. Narrow absorption features at 3.08 microns, attributed to interstellar ices, appear in all sources with a molecular cloud in the intervening line of sight. All sources showing ice absorptions also show broad absorption features, attributed to cold silicates, from 8 to 13 microns. The observed ice absorption profiles are all quite similar; however, they do not fit in detail Mie theory predictions of extinction for pure H2O or NH3 ices. The ratio of ice-to-silicate optical depths is found to vary, with most sources showing a ratio in the range 0.1-0.4. The ratio of visual extinction to ice absorption is found to increase rapidly from inside to outside the molecular cloud in NGC 2024.

A simple method for background and matrix correction of spectral peaks in trace element determination by X‐ray fluorescence spectrometry

Abstract: Between adjacent major element absorption edges, background intensities at differing wavelength positions were found to be related linearly. Also, background intensity at any position is linearly related to the reciprocal of the mass absorption coefficient for any wavelength between the absorption edges. The method presented here allows for the determination of background, as well as mass absorption coefficient, by a single measurement at an interference-free background position (which can be the Compton scatter ‘peak’). If the mass absorption coefficient is already known, backgrounds may be calculated directly. Tests on geological materials show that, while not as accurate as conventional methods of background determination, relative accuracies of two to five percent are obtainable with a very considerable saving in time, since intensity at only one background position need be measured. Use of low dispersion high reflectivity analysing crystals, e.g. LiF(200), is possible because it is no longer necessary to attain interference-free background positions between peaks. Furthermore, the method is ideally suited to the determination of background intensities in energy dispersive systems where spectral resolution is a problem and in multi-channel units where only one channel need be used for the determination of both mass absorption coefficient and backgrounds for a group of trace elements. The method has been used in the low cost determination of trace elements in large numbers of geochemical prospecting samples.

Water vapor absorption of carbon dioxide laser radiation

Abstract: An optoacoustic detector or spectrophone has been used to perform detailed measurements of the absorptivity of mixtures of water vapor in air. A C(12) O(2)(16) laser was used as the source, and measurements were made. at forty-nine different wavelengths from 9.2 microm to 10.7 microm. The details of the optoacoustic detector and its calibration are presented, along with a discussion of its performance characteristics. The results of the measurements of water vapor absorption show that the continuum absorption in the wavelength range covered is 5-10% lower than previous measurements.

Size and Shape Effects on the Broadening of the Plasma Resonance Absorption in Metals

Abstract: Size and shape effects on the broadening of the plasma resonance absorption of light in metallic samples are investigated. The relation between three different quantum mechanical methods which have been used in the past to calculate this broadening, is clarified. The general theory is applied to various specific geometries, and some physical implications are discussed. Es wird der Einflus der Form und Grose der Proben auf die Verbreiterung der Plasmaresonanzabsorption von Licht in Metallproben untersucht. Die Beziehung zwischen drei unterschiedlichen quantenmechanischen Methoden, die bereits zur Berechnung dieser Verbreiterung angewendet wurden, wird geklart. Die allgemeine Theorie wird dann auf unterschiedliche Probengeometrien angewendet. Es werden einige physikalische Auswirkungen besprochen.

Optical properties of discontinuous gold films

Abstract: The optical absorption of films having a variety of structures was studied: in no case was a second resonance peak observed. However, calculations based on an ellipsoidal particle model taking into account the absorption characteristics of bulk gold suggest that, for gold films, only one peak would be resolvable. The wavelength of this absorption peak was correlated with a structure factor for the film which was then used to compare the predictions of particle shapes by several theories.

A study of absorption currents in polypropylene

Abstract: Charging and discharging current transients in polypropylene films were measured over a wide range of temperatures and electric fields. Some limited studies of the behaviour of these current transients were also made with different electrode materials and sample thicknesses. The experimental evidence suggests that the absorption current in the temperature range 83-270K may be caused mainly by dipolar processes associated with structural motions. At temperatures above approximately 270K the absorption current may be attributed to a carrier hopping process.

Determination of Power Absorption in Man Exposed to High Frequency Electromagnetic Fields by Thermographic Measurements on Scale Models

Abstract: When the body of man, small compared to a wavelength, is exposed to high frequency (HF) electromagnetic (EM) fields, the absorbed power density patterns and total absorbed power may be approximated by the simple superposition of the internal electric fields obtained from the quasistatic coupling characteristics of the electric and magnetic field components determined independently. These characteristics were obtained for full scale man by thermographic studies of power absorption in scale models of man exposed to fields at frequencies scaled up inversely proportional to the model size. A VHF resonant cavity was used to provide the necessary field strengths for producing measurable power absorption patterns under simulated HF exposure conditions. The results indicate that peak power absorption densities as high as 5.63 W/kg can be produced in man exposed to 10 mW/cm 2 31 MHz radiation fields. The results show that the absorption decreases as the square of the frequency as predicted by theory for frequencies below 31 MHz.

On the Absorption, Reflection and Transmission of Solar Radiation in Cloudy Atmospheres

Abstract: Band-by-band calculations have been carried out to evaluate the reflection, absorption and transmission of solar radiation by cloud layer and model cloudy atmospheres in the entire solar spectrum. The radiation transfer program is based on the discrete-ordinate method with applications to inhomogencous atmospheres. The gaseous absorption in scattering atmospheres is taken into account by means of exponential fits to the total band absorption based on laboratory measurements. Thick clouds such as nimbostratus. and cumulonimbus reflect 80–90% and absorb 10–20% of the solar radiation incident upon them. The reflection and absorption of a fair weather cumulus with a thickness of 0.45 km are about 68–85% and 4–9%7p, respectively. A thin stratus, whose thickness is 0.1 km, reflects about 45–72% and absorbs about 1–6% of the solar flux incident on the cloud top. The reflection of a 0.6 km thick altostratus is about 57–77%, with a larger absorption of 8–15%. A number of aircraft observations reveal that ...

Shift and broadening of saturated absorption resonances due to curvature of the laser wave fronts

Abstract: Using frequency offset‐locked spectroscopy of methane at 3.39 μm we show that the saturated absorption peaks can be significantly blue or red shifted if the wave fronts of the laser beam are spherical in the gas cell. Good agreement with theoretical prediction is found. We discuss suitable conditions to avoid this geometry‐induced shift in optical frequency standards.

Analysis of coal

Abstract: A method of analysing coal or coke wherein the concentration of ash or mineral matter in coal or coke is determined from (i) the result of a measurement of transmission or scatter of X-rays or γ-rays of a first energy chosen such that there is significant difference in absorption of radiation per unit weight in coal matter and mineral matter excluding iron, combined with (ii) the result of at least one further measurement of transmission or scatter of X-rays or γ-rays at different energy/energies so chosen that there is significant difference in absorption of radiation per unit weight of coal matter and mineral matter and that the relative absorption per unit weights by said coal matter and said mineral matter at any one energy is significantly different from the relative absorption at each other energy including said first energy, and/or (iii) the result of a measurement of iron concentration by neutron capture γ-ray techniques.

A reversible optical transition in SeGe and PSeGe glasses

Abstract: A reversible optical transition has been found in thin films of SeGe and PSeGe glasses. This optical transition is similar to that found in AsSeGe and AsSeSGe glass films; heating causes the shift of the absorption edge to shorter wavelengths, and illumination moves it to longer wavelengths without crystallization. It was found that the presence of arsenic is not always necessary. Fundamental optical properties of these glasses, i.e. absorption constant, optical energy gap, change of the optical energy gap, and the refractive index by heating and illumination are reported. Change of the optical energy gap and the refractive index is small in SeGe and PSeGe glasses, compared with that in AsSeGe and AsSeSGe glasses.