Showing papers on "Absorption (electromagnetic radiation) published in 1984"

Reflectance spectroscopy: Quantitative analysis techniques for remote sensing applications

Abstract: The empirical methods and scattering theories that are important for solving remote sensing problems are among the methods for remotely sensed reflectance data analysis presently compared. In the case of the photon mean optical path length concept's implications for reflectance spectra modeling, it is shown that the mean optical path length in a particulate surface is in roughly inverse proportion to the square root of the absorption coefficient. Absorption bands, which are Gaussian in shape when plotted as true absorptance vs photon energy, are also Gaussians in apparent absorptance, although they have a smaller intensity. An apparent continuum in a reflectance spectrum is modeled as a mathematical function that is used to isolate a particular absorption feature for analysis, and it is noted that this continuum should be removed by dividing it into the reflectance spectrum.

Room temperature excitonic nonlinear absorption and refraction in GaAs/AlGaAs multiple quantum well structures

Abstract: We present detailed experimental studies and modeling of the nonlinear absorption and refraction of GaAs/AlGaAs multiple quantum well structures (MQWS) in the small signal regime. Nonlinear absorption and degenerate four-wave mixing in the vicinity of the room temperature exciton resonances are observed and analyzed. Spectra of the real and imaginary parts of the nonlinear cross section as a function of wavelength are obtained, and these are in excellent agreement with experimental data. A simple model for excitonic absorption saturation is proposed; it accounts qualitatively for the very low saturation intensities of room temperature excitons in MQWS.

An assessment of the impact of pollution on global cloud albedo

Abstract: Increased pollution leads to increasing particulate concentrations. Since some particles nucleatedrop formation, clouds will contain, with increasing pollution, more drops per unit volume, andhence will tend to be optically thicker and more reflecting. An opposite effect is also present, inthat increasing absorption also attends increasing pollution. Measurements suggest that theformer (brightening) effect is the dominant one for global climate and that the climatic effect isquite comparable to that of increased carbon dioxide, and acts in the opposite direction. DOI: 10.1111/j.1600-0889.1984.tb00254.x

Charge storage in doped poly(thiophene): Optical and electrochemical studies

Abstract: We present a new method of electrochemical polymerization of poly(thiophene) using dithiophene as the starting material, from which we obtain a high-quality film with a sharp interband absorption edge. An in situ study of the absorption spectrum during the electrochemical doping process has been carried out. In the dilute regime, the results are in detailed agreement with charge storage via bipolarons; weakly confined soliton pairs with confinement parameter $\ensuremath{\gamma}\ensuremath{\cong}0.1\ensuremath{-}0.2$. At the highest doping levels, the data are characteristic of the free-carrier absorption expected for a metal. From a parallel electrochemical voltage spectroscopy study, we find evidence of charge injection near the band edge and charge removal from the bipolaron gap states. In the dilute regime, the position of the chemical potential is consistent with charge storage in weakly confined bipolarons. The high Coulombic recovery over a charge-discharge cycle indicates that poly (thiophene) may be an excellent cathode-active material in battery applications.

Thermal detectors as X-ray spectrometers

Abstract: Sensitive thermal detectors should be useful for measuring very small energy pulses, such as those produced by the absorption of X-ray photons. The measurement uncertainty can be very small, making the technique promising for high resolution nondispersive X-ray spectroscopy. The limits to the energy resolution of such thermal detectors are derived and used to find the resolution to be expected for a detector suitable for X-ray spectroscopy in the 100 eV to 10,000 eV range. If there is no noise in the thermalization of the X-ray, resolution better than 1 eV full width at half maximum is possible for detectors operating at 0.1 K. Energy loss in the conversion of the photon energy to heat is a potential problem. The loss mechanisms may include emission of photons or electrons, or the trapping of energy in long lived metastable states. Fluctuations in the phonon spectrum could also limit the resolution if phonon relaxation times are very long. Conceptual solutions are given for each of these possible problems.

Dependence of relationship between inherent and apparent optical properties of water on solar altitude

Abstract: A computer simulation study, using the Monte Carlo calculation procedure, has been carried out to determine in what way the relationships between the apparent and the inherent optical properties of natural waters are affected by the angle of incidence of the photons on the surface. With the particular normalized volume scattering function used in these simulations, the vertical attenuation coefficient for downward irradiance at the midpoint of the euphotic zone, Kd(zm), can be expressed as a function of the absorption (a) and (b) coefficients, and the cosine of the incident photons just below the surface (µo), in accordance with Thus, as the direction of the incident light departs increasingly from the vertical, Kd(zm) increases but becomes progressively less responsive to increases in scattering at constant absorption. The irradiance reflectance just below the surface [R(0)] at all angles of incidence increases linearly with the ratio of the backscattering coefficient (bb) to the absorption coefficient, and at any value of bb/ a increases as the angle of the light departs further from the vertical, in approximate conformity with

Measurement of soft X-ray absorption spectra with a fluorescent ion chamber detector

Abstract: The problem of absorption of soft X-rays by thick Be windows in hard X-ray beam lines is well known. Although the signal at 2.4 keV was reduced by ∼ 103 we have routinely measured the absorption spectra of S (2472 eV) and elements at higher energies including Cl, Ar and K. These spectra were obtained on hard X-ray beam lines at Stanford Synchrotron Radiation Laboratory (SSRL) with Si(111) monochromator crystals and a fluorescent ion chamber detector [1]. Higher energy harmonics were minimized by detuning and the end station was enclosed in a helium bag to prevent absorption by air. Although the diminished X-ray flux and decreasing fluorescent yield were serious negative factors at these low X-ray energies the spectra from thick samples were of excellent quality with sufficient sensitivity to characterize 1% S in coal. Representative spectra are shown comparing data from focused and unfocused beam lines and with S data from JUMBO [2]. Comparison of Ar and KCl data to excellent data found in the older literature [3–5] allow a confirmation of the resolution function (energy bandpass) of the monochromator. A simple new beam line is suggested which would allow a substantial increase in low energy X-ray flux (measurements down to Al and Si) with the sample and detector in a He atmosphere.

Photo-Chemistry of Colloidal Metal Sulfides 8. Photo-Physics of Extremely Small CdS Particles: Q-State CdS and Magic Agglomeration Numbers

Abstract: Extremely small CdS particles were prepared in propanol−2 solution at − 78°C and in aqueous solution in the presence of sodium hexametaphosphate at room temperature. These colloids are colorless. Their UV absorption spectra exhibit several maxima. Aging of the colloids is accompanied by intensity variations in the absorption maxima and by a shift of the onset of absorption to longer wavelengths. These small CdS particles hardly possess semiconductor properties (Q-state CdS). A semi-classical treatment of the energies of an electron-hole pair in these particles yielded the wavelengths of their absorption spectra. At the small particle sizes used, the first excited state was reached by photon-absorption in the UV, and the second excited state was generally not reached at all. The various maxima in the absorption spectra are explained in terms of a size distribution of the colloids with preferential agglomeration numbers. Reasons for the formation of such a structured size distribution are given. The fluorescence and fluorescence excitation spectra of the small particles were also investigated. Particles below a certain size have only one broad fluorescence band at a much longer wavelengths than the onset of absorption. As the particle size increases, this band is shifted towards longer wavelengths, and finally an additional rather sharp band appears at the threshold of absorption. CdS colloids in the Q-state can be made which fluoresce as desired anywhere between the red and the blue. Also reported are the first experiments in which the preparation of Q-type CdS in the solid state is achieved by evaporating the solvent from the colloidal solutions.

Low-temperature exciton trapping on interface defects in semiconductor quantum wells

Abstract: We present the results of low-temperature photoluminescence experiments performed on GaAs single quantum wells grown by metal organic chemical-vapor deposition. The luminescence line is down shifted by a few milli-electron-volts below the $n=1$ heavy-hole exciton absorption peak. This behavior is interpreted in terms of exciton trapping on interface defects. A simple model provides reasonable values for the exciton binding energy on these defects as well as insights on the lack of thermalization which characterizes the trapped exciton photoluminescence.

Absorption of metals by natural polymers generated from seafood processing wastes

Abstract: La chitine et la chitosane sont des materiaux aptes a eliminer les metaux (Zn, Cr, Cd et Pb) des eaux usees

Low-energy electron diffraction from Cu(111): Subthreshold effect and energy-dependent inner potential; surface relaxation and metric distances between spectra

Abstract: In the present low-energy electron-diffraction (LEED) study of the copper (111) surface the inner potential and the surface relaxation are determined independently from the subthreshold effect and from Bragg-type diffraction. A "subthreshold effect" is a narrow LEED intensity structure occurring at a setting where new beams have an emergence threshold in the metal: a "subthreshold." We reconsider the absorption of electrons with regard to its spatial distribution in the crystal and design a phenomenological model comprising two parameters which are adjusted to the absorptive scattering cross section of the ion cores and that of the interstitial region. The two-parameter model for the interlayer attenuation indicates the existence of a transparent scattering channel "pseudoparallel" to the surface for beams emerging in the crystal. The channeling extends over all layers penetrated by the LEED electrons, giving the subthreshold effect a peak width of about 2 eV. Each observable subthreshold effect fixes a point on the energy-dependent inner potential; for the copper (111) surface we are able to measure the inner potential at 19.5-, 73.6-, and 109-eV incidence energy. A local excited-state potential of the Hedin-Lundqvist type produces for the copper (111) surface an inner-potential curve that agrees well with the measured points. From LEED spectra for the 00, 10, and 01 beams from the copper (111) surface in the energy range 16-190 eV we infer a top-layer spacing contracted (0.7 \ifmmode\pm\else\textpm\fi{} 0.5)% relative to the layer spacing in the bulk. The theoretical and experimental spectra are compared by means of metric distances, which are stable with respect to noise in the data and give a linear response to small variations of the structural parameters.

Investigation of the shape of the imaginary part of the optical-model potential for electron scattering by rare gases

Abstract: We present a comparative study of several empirical and nonempirical models for the absorption potential, which is the imaginary part of an optical-model potential, for electron scattering by rare gases. We show that the elastic differential cross section is most sensitive to the absorption potential for high-impact energy and large scattering angles. We compare differential cross sections calculated by several models for the absorption potential and by several arbitrary modifications of these model potentials. We are able to associate the effect of the absorption potential on the elastic differential cross section with its form at small electron-atom distances r, and we are able to deduce various qualitative features that the absorption potential must possess at small and large r in order to predict both accurate differential cross sections and accurate absorption cross sections. Based on these observations, the Pauli blocking conditions of the quasifree scattering model for the absorption potential are modified empirically, thus producing a more accurate model that may be applied to other systems; e.g., electron-molecule scattering, with no adjustable parameters.

A study of the electrical and optical properties of the GeO2-TeO2 glass system

Abstract: Results of measurements of the dc electrical conductivity, optical absorption edge and infrared optical absorption of GeO2-TeO2 glasses are reported Conduction in these glasses is found to be electronic and the hopping of polarons seems to be the dominant process in the transport mechanism The electrical activation energy decreases with the increase of tellurium content and this decrease corresponds to a decrease in the optical energy gap The optical gap is of the order of 274 eV, somewhat lower than for many other oxide based glasses Most of the sharp absorption bands characteristic of the basic materials GeO2 and TeO2 are modified with the formation of broad and strong absorption bands in the process of going from the crystalline to the amorphous state Density measurements show the glasses to have a compact structure

A Shortwave Parameterization Revised to Improve Cloud Absorption

Abstract: We present a modification to the parameterization scheme of Stephens which improves on the estimation of shortwave absorption by cloud. In particular, the variation of cloud absorption with solar elevation angle is improved with the modified scheme.

Catalysis of photochemical reactions by colloidal semiconductors

Abstract: Illumination of semiconductor colloids such as ZnS , CdS and Ti02, generates electrons and positive holes which may react with the aqueous solvent or dissolved substances In ZnS and CdS , recombination of the charge carriers is often accompanied by emission of light The absorption spectra of very small CdS or ZnS particles (1 3 nm) show changes which indicate a transition from semiconductor to polymolecular material The conduction band energy is shifted, and the exciton transition band becomes more pronounced The fluorescence spectra of small CdS particles contain a band at the onset of absorption and two bands at longer wavelengths They are attributed to emission from the short-lived exciton state and from longer-lived states of trapped electrons and holes The chemical reactions described include the photo-anodic and the photocathodic dissolution of colloidal sulfides, the reduction of water and carbon dioxide , and the oxidation of alcohols and sulfite anions Two-electron (two-hole) and one-electron (one-hole) transfer mechanisms are discussed Flash photolysis studies of colloidal Ti02 gave the absorption spectra of long-lived electrons and positive holes which are trapped in surface states of different energies Chemical reactions of these charge carriers are also described, including the reduction of tetranitromethane, the oxidation of SCN and OHanions, and of various organic compounds The electrochemical nature of all these photo-reactions is emphasized

On the importance of permanent moments in multiphoton absorption using perturbation theory

Abstract: Rates for multiphoton absorption by molecules in the regime where perturbation theory is valid are computed The importance of the contributions to the rates of permanent moments is shown The results are expressed in terms of generalised Einstein B coefficients which are shown to arise both from quantum electrodynamical and semiclassical arguments; the authors remain within the electric dipole approximation throughout Explicit and detailed results for two- and three-photon absorption are given for plane-polarised and circularly-polarised light These are found both for molecules rigidly fixed relative to the light beams and for random orientation of the molecules In the latter case the rates are presented in terms of the transition and permanent moments in the molecular-body coordinate frame of reference

Determination of Absorption and Fluorescence Excitation Spectra for Phytoplankton

Abstract: Investigations of photosynthetic processes in both aquatic ecosys tems and in the laboratory have sought to couple measurements of the rates of production with quantitative assessment of the ambient light field and the absorption capacity of the cells. Kiefer etal. (1979) and Bricaud etal. (1983) have described techniques for determination of ab sorption coefficients for cultures. For field studies, however, where the cell concentration is very dilute, direct measurement of absorption is not possible. Most field studies have assumed a constant absorption per chlorophyll, and have estimated the amount of absorbed light from measurements of the chlorophyll concentration and the available light for photosynthesis (Rhode,1965; Tyler,1975; Dubinsky and Berman,1976; Morel, 1978). We present here a technique for measuring the absorption coeffi cient (ap) for cultures or field samples. Yentsch (1957) first applied the technique of direct measurement of absorption on filters for algal cultures, and for qualitative analysis of suspended marine particulates (1962). Faust and Norris (1982) have used derivative absorption spectros copy of cultured phytoplankton on glass fiber filters in order to assess pigment concentration.

A Review of Numerical Models for Predicting the Energy Deposition and Resultant Thermal Response of Humans Exposed to Electromagnetic Fields

Abstract: For humans exposed to electromagnetic (EM) radiation, the resulting thermophysiologic response is not well understood. Because it is unlikely that this information will be determined from quantitative experimentation, it is necessary to develop theoretical models which predict the resultant thermal response after exposure to EM fields. These calculations are difficult and involved because the human thermoregulatory system is very complex. In this paper, the important numerical models are reviewed and possibilities for future development are discussed.

Infrared absorption enhancement of monolayer species on thin evaporated Ag films by use of a Kretschmann configuration: evidence for two types of enhanced surface electric fields

Abstract: Infrared absorption enhancement of m- and p-nitrobenzoic acid deposited on thin-evaporated silver films has been investigated using the Kretschmann's ATR coupling method. The absorption spectra provide direct evidence that enhancement is prominent only for vibrations of the first monolayer adsorbed on the Ag surface. It is shown that all of the vibrational modes observed obey the normal dipole selection rule. Moreover, it was found that there exist two types of absorption enhancement; the first is enhanced (∼300) by both p- and s-polarized radiation, and the second is enhanced (∼60) by p-polarized radiation alone. The Ag film thickness optimum is about 50 A in the former case and about 200 A in the latter. The enhancement insensitive to the polarization state of radiation can well be explained by the excitation of the transverse collective electron resonance of the Ag islands, whereas that obtained by p-polarized radiation may be due to the excitation of delocalized surface plasmons modified by surface roughness. The image-dipole effect may also be significant in the mechanisms.

Temperature distributions produced by scanning Gaussian laser beams.

Abstract: A general solution is presented for the temperature rise produced by the absorption of a scanning Gaussian laser beam in a solid target. In normalized coordinates, the temperature rise is found to depend only on the ratio of the scan speed to the rate of heat diffusion in the solid and the ratio of the beam radius to the absorption depth. For slow scan speeds the solution simplifies to the steady-state approximation in which the power input is balanced by heat conduction into the solid. For fast scan speeds the solution approaches the energy density limit in which the temperature rise is proportional to the integrated beam intensity. For highly absorbing materials the solution simplifies to the surface absorption approximation. The general solution demonstrates the conditions under which each approximation can be used. Similar solutions are found for the related case of pulsed exposure by a stationary beam. The solution is demonstrated experimentally by exposing thermal paper with a CO2 laser.

Experimental tests of a single‐photon calorimeter for x‐ray spectroscopy

Abstract: Tests have been made of a nondispersive spectroscopic x‐ray detector which operates by measuring the temperature rise following absorption of a single photon. We have observed thermal pulses from 6‐keV x‐rays, and have resolved the different amplitudes resulting from Mn Kα and K β events. This device was assembled to make quantitative tests of theoretical calculations of the properties of such detectors, and its high heat capacity does not allow it to attain the very high resolution predicted for detectors made by more sophisticated, but still straightforward, techniques. Both the measured resolution of 270‐eV full width at half maximum (FWHM) and the absolute amplitude of the response are consistent with predictions. Nonthermal effects in the thermistor limit the precision of this comparison to about 30%.

Power/energy limiter using reverse saturable absorption

Abstract: In materials with an excited‐state absorption cross section larger than the ground‐state absorption cross section, increasing the incident light intensity (thus populating the excited state) increases the absorption. We show that such a reverse saturable absorber can function as a power limiter and pulse smoother for long pulses and as an energy limiter and pulse shortener for short pulses. The necessary properties for such a material are described. Reverse saturable absorption is demonstrated at 488 nm in alexandrite. A simple model describing these effects is presented.