Showing papers on "Absorption (electromagnetic radiation) published in 1973"

Optical and photoelectric properties and colour centres in thin films of tungsten oxide

Abstract: Thin films of Wo3 deposited on quartz substrates at room temperature have been shown to be amorphous in structure. The optical absorption spectra of the amorphous and crystalline films have been measured in the temperature range 110° to 500°K. The fundamental absorption edge of an amorphous film occurs at 3800 A which on crystallization moves to 4500 A. On the high-energy side of the absorption edge several absorption peaks are resolvable in both types of film. The frequency dependence of the absorption coefficient below 104 cm−1 is described by an expression of the form K (v, T) = K 0 exp[− (β/kT) (E 0 − hv)] and above 104 cm−1 it follows a square law dependency. The temperature coefficient of the band edges was found to be − 5.0 × 10−4 eV/°K and the estimated band gaps at 0°K were found to be 3.65 and 3.27 eV for the amorphous and crystalline films, respectively. The electrical conductivity of a thin film has been measured in the temperature range 298–573°K and the activation energy was found t...

Multiple absorption of laser photons by atoms

Abstract: Applying a space translation operation, the Schrodinger equation for an atom in an electromagnetic field is solved with sufficient accuracy to obtain probabilities for multiple absorption of photons from a monochromatic laser beam of arbitrary intensity or frequency. It is shown that the derived expression for the N-photon T-matrix contains the usual single photon matrix elements given by the perturbation theory and that the perturbative result is obtained in the limit of low intensity. Other explicit examples are considered. The conditions of applicability of the method are specified.

Absorption and noise in x‐ray phosphors

Abstract: When the signal in an x‐ray image system is formed by integrating the scintillation pulses rather than by counting them, the signal‐to‐noise ratio is reduced by a factor which depends on the shape of the pulse‐height distribution. The signal‐to‐noise ratio cannot be related directly to either quantum absorption or energy absorption, and a new quantity called noise‐equivalent absorption is defined which bears a simple relationship to the signal‐to‐noise ratio. Quantum, energy, and noise‐equivalent absorption are calculated as a function of thickness and x‐ray energy for CsI, Gd2O2S, LaOBr, Zn0.6Cd0.4S, and CaWO4.

Absorption coefficient of atmospheric aerosol: a method for measurement.

Abstract: Recent models predict that the effect of atmospheric aerosol particles on global temperature depends in part on b(abs,)or on n(2), where b(abs) is the aerosol absorption coefficient, and n(2) is the imaginary part of the aerosol refractive index for radiation in visible wavelengths Satisfactory techniques for measuring b(abs) have yet to be developed A method for measurement of b(abs) is described and discussed The apparatus uses a piece of opal glass to integrate light scattered by particles collected on Nuclepore filters It is quite mobile, inexpensive, and can be calibrated Analysis of the errors inherent in the measurement shows it is accurate to well within an order of magnitude for particles of radius larger than 01 microm

New thermooptical measurement method and a comparison with other methods

Abstract: A simple and sensitive method of measuring the thermally induced index changes arising from absorption of a laser beam in a low-loss material is described. The sample is placed outside the laser cavity, but at the position of minimum radius of curvature of the wavefront, which is a confocal distance behind the waist. It is estimated that the method is sensitive enough to measure absorption coefficients of the order of 5 x 10(_6) cm(_1) and it is shown experimentally to have good accuracy on low-loss materials. A detailed comparison of sensitivity and accuracy estimates is given for the various published thermal-lens methods for measuring low absorption coefficients.Although the method is illustrated for the thermal effect, it is applicable to other nonlinear index changes induced by laser beams.

On the ultimate lower limit of attenuation in glass optical waveguides

Abstract: The fabrication of an extremely‐low‐loss glass optical waveguide, having as little as 4 dB/km total attenuation, has allowed interpretation of the absorption spectrum to a much greater degree than previously possible It is shown that, beyond about 700 nm, intrinsic ultraviolet absorption will have no effect Between 700 and 1100 nm, all absorption can be accounted for on the basis of OH to within ±07 dB/km Marked reduction of the water content will leave the scattering as the major loss mechanism, thereby permitting total attenuations of about 2 dB/km

Resolution of the visible‐infrared absorption spectrum of I2 into three contributing transitions

Abstract: New extinction coefficient measurements are reported for I2 in the region 4200–8000 A. An argon discharge source is used to estimate the underlying continuum absorption in the banded region. Together with other work these results locate the maximum in the 1 u(1Π) ← X[O+g(1Σ)] absorption at 4985 A with a peak extinction of 200 liter mole−1 · cm−1. From the shape of the spectrum the 1u (1Π) potential curve behaves as r−9 in the region of strong absorption. The A[1u(3Π)]← X absorption is found to be a factor of 2 stronger than previous estimates with a peak extinction of 41 liter mole−1 · cm−1 at 6730 A. Earlier indications of a linear dependence of the dipole strength on r centroid for the B [O+u(3Π)]↔ X system are supported in this study. Spectroscopic data for the B↔ X transition are reviewed and new spectroscopic constants recommended for the B state.

Correct values for high-frequency power absorption by inverse bremsstrahlung in plasmas

Abstract: Numerical corrections are made to previous values for the power absorption coefficient and some common errors noted.

Thermal diffusion measurements using spatially periodic temperature distributions induced by laser light

Abstract: Two light waves with different directions of propagation derived from a pulsed Nd:YAG laser are superimposed in an absorbing sample and generate an interference field. Due to absorption, a spatially periodic temperature distribution occurs, producing a spatial modulation of the refractive index which can be considered a thermal phase grating. The light of an argon laser simultaneously incident on the sample is diffracted by the thermal grating. When excitation is over, the decay time of the diffracted light is measured. From this decay time the thermal diffusivity of the sample is determined. Measurements on colored methanol and glycerin as well as on ruby compare favorably with the results of other authors. The possibility of exciting and detecting temperature waves (second sound) in solids by the method of light‐induced gratings is discussed.

Photoconductivity and absorption in amorphous Si

Abstract: The paper deals with photoconductivity and absorption in aSi specimens, prepared mainly by the decomposition of silane in a glow discharge. Substrate temperatures, Td, between 300 K and 650 K were used during deposition. The normalised photoresponse was measured at room temperature as a function of photon energy and Td in a spectral range from 0.5 eV to 3 eV. The absorption coefficient was determined for evaporated, sputtered and glow discharge specimens. The main features of the results are in agreement with conclusions drawn from previous electrical transport and field effect measurements and can be interpreted on the basis of the proposed model for the localised state distribution. It is confirmed that ϵc−ϵv is 1.5 to 1.6 eV, and that there is a local density of state maximum at about 1.2 eV below ϵc. At room temperature the steady photocurrent is carried predominantly by electrons in states above ϵc, whether excitation is from localized or extended states. Specimens prepared at Td > 500 K are highly photosensitive, with electron recombination lifetimes, τ, of up to 10−5s. Rise and decay times of the signal lie in the millisecond range. For Td < 500 K there is a drastic decrease in τ, which falls to 10−11 s at Td ⋍ 300 K and is even less for evaporated specimens. These results are discussed in some detail.

Effect of Aerosols on the Transfer of Solar Energy Through Realistic Model Atmospheres. Part I: Non-Absorbing Aerosols

Abstract: Extensive calculations aimed at determining the effect of aerosols on the solar energy absorbed, reflected and transmitted by cloudless, nonhomogeneous, plane-parallel atmospheric models were recently carried out with the object of treating the radiation transfer in as comprehensive a manner as possible consistent with reasonable computing time. The concentration of aerosol (spherical particles with size distribution and refractive index independent of height), ozone and water vapor were specified for 160 layers of varying thickness from the surface to 45 km. The solar spectrum (0.285–2.5 μm) was divided into 83 intervals with appropriate functions representing the scattering and absorption of gases and aerosol assigned to each, the index of refraction of the aerosol taken to be wavelength-independent. Upward and downward fluxes for each spectral interval at each level were computed taking into account all orders of scattering. Results will be presented for four model atmospheres to show the abso...

The absorption cross sections of N2, O2, CO, NO, CO2, N2O, CH4, C2H4, C2H6 and C4H10 from 180 to 700Å

Abstract: The absorption cross sections of N2, O2, CO, NO, CO2, N2O, CH4, C2H4, C2H6, C4H10 have been measured photoelectrically in the 180–700 A region using synchrotron radiation. The absorption cross sections in the region λ ≥ 500 A was found to be structureless and to increase monotonically with wavelength for all gases. The positions of the structure observed in the 520–720 A region for N2, O2, CO2 and N2O are consistent with the various Rydberg series reported by previous authors.

Intrinsic Photoconduction and Photoemission in Polyethylene

Abstract: Intrinsic photoemission and photoconduction of polyethylene in the photon energy range 7

Pollutant Detection by Absorption Using Mie Scattering and Topographic Targets as Retroreflectors

Abstract: Remote pollutant measurement by absorption using topographical reflectors or atmospheric Mie scattering as a distributed reflector offers increased range and sensitivity compared to that achieved by Raman or resonance backscattering methods. The use of topographical reflectors offers the advantage of a single-ended absorption measurement for ranges up to 10 km and sensitivities to less than 0.01 ppm for a 10-mJ, 100-nsec transmitted pulse. The distributed Mie reflector permits absorption measurements over a depth cτ/2, determined by the pulse length τ, and allows ranging by time-of-flight measurement. For a 100-mJ, 100-nsec pulse sensitivities to 0.3 ppm at a 15-m depth resolution to ranges of 1–4 km are possible. This sensitivity is 104 to 105 times better than that achieved by the Raman method.

Low resolution optical spectroscopy of retinyl polyenes: low lying electronic levels and spectral broadness*

Abstract: — Optical absorption and emission spectra of retinal, axerophtene, anhydrovitamin A, and 2,10-dimethylundecapentaene in rigid glass matrixes at 77°K are presented. An interpretation of the observed spectra which provides a rationalization on the basis of electronic structure for the diffuse nature of the retinal spectrum is discussed. For the undecapentaene a weakly absorbing 1Ag state was found 3250 cm-1 lower in energy than the level responsible for the strong visible absorption.

Comments on the evidence for sharp and gradual optical absorption edges in amorphous germanium

Abstract: Several investigators have analyzed optical data on amorphous Ge films prepared by evaporation on room-temperature substrates to derive a sharp absorption edge near 0.5 eV. A re-examination of their work shows that the sharp edges, found hitherto, may be artifacts resulting from either experimental or calculational errors and that the preponderance of the current evidence favors a gradual absorption edge. Von einigen Autoren wurden die optischen Daten von amorphen Ge-Schichten, die durch Aufdampfen bei Zimmertemperatur hergestellt wurden, analysiert und daraus eine scharfe Absorptionskante bei 0,5 eV abgeleitet. Eine wiederholte Untersuchung ihrer Arbeiten zeigt, das die scharfen Kanten, die bisher gefunden wurden, Fehleffekte sein konnen, die entweder von experimentellen oder rechnerischen Fehlern herruhren und das die Mehrzahl der gegen wartigen Hinweise fur eine allmahlich ansteigende Absorptionskante spricht.

A Radiative Ignition Model of a Solid Fuel

Abstract: A theoretical model describing radiative ignition of a solid fuel is constructed and is numerically analyzed. The model includes the effects of gas phase reaction and a finite value of the absorption coefficient of the solid (in-depth absorption of incident radiation). It is found that the gas phase reaction must be included in the model in order to understand radiative ignition of a solid fuel and to find its ignition boundary. The in-depth absorption of the incident radiation by a solid fuel significantly affects the ignition delay time. The results indicate that there is a finite range of values for pyrolysis or gas phase reaction activation energy for which ignition will occur. This finding has a direct bearing on efforts to reduce material ignitability.

Observations of stimulated emission from high‐pressure krypton and argon/xenon mixtures

Abstract: Experimental data demonstrating coherent oscillation in high‐pressure krypton at 1457 ± 1 A with an 8‐A linewidth are reported. Observations of stimulated emission from argon/xenon mixtures arising from the operation of efficient energy transfer processes and resulting in a twofold enhancement of the xenon laser output at ∼ 1720 A are also given. The influence of collisional‐induced absorption on the oscillator output is indicated by the measurement of a blue shift in the spectrum of the stimulated output for argon/xenon mixtures relative to the case of pure xenon.

Optical Properties of a Plasma Sphere

Abstract: The classical theory for the scattering and absorption of electromagnetic radiation by a homogeneous isotropic sphere is extended so as to apply to a sphere in which dispersive plasma oscillations exist. Computations performed for small metallic spheres predict the appearance of an absorption structure in the frequency region just above the plasma frequency.

Dynamics of solvation of an excess electron

Abstract: We have utilized the techniques of picosecond spectroscopy to study the localization of an excess electron in water. Quasifree electrons were generated by the photoionization of the ferrocyanide anion in aqueous solutions at 2650 A. Time resolved absorption of the localized electron was monitored continuously from 1.06 μ to 5300 A, while time and energy resolved absorption was monitored in the range 3100–9000 A by a broad continuum of picosecond duration. Optical absorption of the initially localized electron in the infrared (1.06 μ) is observed within 2 psec after the generation of the quasifree electron. The optical absorption band evolves in time, shifting from lower to higher energies, and the ``normal'' absorption of the hydrated electron is developed in ∼4 psec.

Spectral reflectance and emittance of particulate materials. 1: theory.

Abstract: The infrared spectral reflectance of a semi-infinite medium composed of irregular particles of different materials is calculated in terms of the sizes, shapes, and complex refractive indices of the particles. For particles larger than the wavelength, the scattering and absorption are computed mainly by geometrical optics but with important wave-optical corrections for the additional absorption caused by edges and asperities, which are represented by dipoles distributed over the surface of the particle. For particles smaller than the wavelength, a Lorentz-Lorenz model is used to derive the average complex index of the medium, the particles being treated as ellipsoids with a wide range of shapes. The average scattering of an individual ellipsoidal particle is then found from the relative refractive index of the particle with respect to the Lorentz-Lorenz medium. For both large and small particles the single-particle scattering is represented by six discrete beams. Calculation of the reflectance is then facilitated by a radiative transfer method that also involves six beams. For particles of intermediate size a suitable formula bridging the results for large and small particles is found to be satisfactory.

Radiation profiles in an empty annular photoreactor with a source of finite spatial dimensions

Abstract: A model of radiant energy emission for tridimensional sources has been developed and used to predict radiation flux density profiles in an annular photochemical reactor. This paper presents results for a reactor without dispersion or absorption effects; that is, reflexions and refractions have been neglected and the energy has been assumed to propagate in a transparent medium. Computed radiation profiles agree well with published experimental data in a similar lamp-reactor set up. Also, the formulation does not introduce, as it is the case with all line models, any form of singularities in the prediction of radiation flux density values. Finally, a parametric study of the effects of lamp dimensions on the shape of the radiation profiles was also made.

Two‐phonon infrared absorption spectra in crystalline carbon dioxide

Abstract: A theory of two‐phonon states appropriate to vibrational excitations in molecular crystals is presented. ``Phonon'' is taken to mean any vibrational excitation, external or internal to the molecules of the crystal. The theory includes general values of the wave vector. Possible mechanisms for the interaction of the two‐phonon states with radiation are considered and expressions for the intensity of infrared absorption developed. Experimental results are presented for infrared absorption measurements on thin and thick films of polycrystalline carbon dioxide at low temperature. Transitions involving molecular fundamentals of the major constituent (12CO2) and of isotopic impurities and involving two‐ or multiphonon absorption are observed. The latter include transitions to states involving simultaneous excitation of two internal vibrations as well as excitation of one internal and one lattice vibration. Dispersion curves and densities of states are obtained for two internal molecular fundamentals coupled by ...

Detection of interstellar thioformaldehyde

Abstract: The 211 +-212 transition of thioformaldehyde (HCHS) has been observed in absorption in the direction of SagittariusB2. For a rest frequency of 3139· 38 MHz the peak absorption occurs at a radial velocity of 60 ± 4 km s -1. The half-width of the absorption profile is equivalent to 20 kms-1 and the column density of HCHS is greater than 1016 molecules cm - 2. Comparison with the 211 +-212 absorption of formaldehyde (HCHO) at 15 GHz allows the relative abundance of the two molecular species to be computed as a function of the rotational excitation temperature. For thermal equilibrium the strength of the 110-111 absorption in SgrB2 by thioformaldehyde predicted from the 211+-212 HCHS observations is considerably greater than the limits set by a number of observers for the 110-111 transition at 1046 MHz.