Showing papers on "Radiative transfer published in 1971"

Thermal Radiation Heat Transfer

Abstract: A comprehensive discussion of heat transfer by thermal radiation is presented, including the radiative behavior of materials, radiation between surfaces, and gas radiation. Among the topics considered are property prediction by electromagnetic theory, the observed properties of solid materials, radiation in the presence of other modes of energy transfer, the equations of transfer for an absorbing-emitting gas, and radiative transfer in scattering and absorbing media. Also considered are radiation exchange between black isothermal surfaces, radiation exchange in enclosures composed of diffuse gray surfaces and in enclosures having some specularly reflecting surfaces, and radiation exchange between nondiffuse nongray surfaces. The use of the Monte Carlo technique in solving radiant-exchange problems and problems of radiative transfer through absorbing-emitting media is explained.

Theory of Radiative Heat Transfer between Closely Spaced Bodies

Abstract: A general formalism is developed by means of which the radiative heat transfer between macroscopic bodies of arbitrary dispersive and absorptive dielectric properties can be evaluated. The general formalism is applied to the heat transfer across a vacuum gap between two identical semi-infinite bodies at different temperatures. The peculiarities arising when the gap width is of the order of, or smaller than, the dominant thermal radiation wavelengths are studied and quantitatively evaluated for the case of two metal bodies. The predicted strong increase with diminishing gap width is in qualitative agreement with experimental results.

Radiative transition probabilities within the 4 f 3 ground configuration of Nd:YAG

Abstract: The absolute intensities of 20 absorption bands between 2.5 and 0.25 μ have been measured for Nd:YAG at room temperature. These bands are forced electric dipole transitions between the4 I_{9/2} ground manifold and 34 excited J manifolds of the Nd3+ion. The transition intensities have been accounted for in terms of three phenomenological parameters with an rms error of 10 percent. The intensity parameters are used to evaluate the line strengths for excited-state absorption from the metastable4 F_{3/2} J manifold. Significant line strengths are predicted at wavelengths near 1.06 and 1.35 μ, suggesting the possibility of radiative depumping of the4 F_{3/2} upper laser level via the stimulated emission field.

Multiple scattering calculations for technology.

Abstract: A many-flux (discrete ordinate) radiative transfer calculation procedure is described with the goal of making the mathematics easy to learn and use. The major approximation is the neglect of polarization. Emission within the scattering medium is not included, and the formulas are restricted to a scattering medium bounded by parallel planes. The boundary conditions allow for a variety of kinds of illumination, and the surface reflection coefficients at the boundaries of the scattering medium are accurately determined. A comparison is made with the two-flux (Kubelka-Munk) and four-flux calculation methods, and this leads to empirical expressions for the scattering and absorption coefficients in these simple theories, which make them give nearly the same results as exact theories. These empirical expressions provide a very simple method for estimating the absolute reflectance and transmittance of turbid media and greatly increase the utility of the two-flux and four-flux calculation methods. The two-flux equations give excellent results provided the absorption is small compared to scattering and the optical thickness is greater than 5. A comparison with experimental data taken with collimated illumination shows that the four-flux equations give good results at any optical thickness even if the absorption is strong.

Mariner 6 and 7 Ultraviolet Spectrometer Experiment: Analysis of hydrogen Lyman-alpha data

Abstract: Four Lyman alpha airglow measurements of the limb and disk of Mars, made by ultraviolet spectrometers on Mariner 6 and 7 in 1969 and Mariner 9 in 1971, are analyzed to determine the amount and distribution of atomic hydrogen above 80 km. The variation of atomic hydrogen with altitude is calculated by using time-independent chemical diffusion models from 80 to 250 km, and an exospheric model is used above 250 km. By employing radiative transfer theory that includes effects of pure absorption and accounts for temperature variations in the atmosphere, a spherical model of the airglow Lyman alpha emission is used to produce theoretical intensities for comparison with the data. It is found that (1) the exospheric temperature and distribution in 1971 are consistent with those determined in 1969, (2) the vertical optical depth above 80 km was 2.2 in 1969 and 5 in 1971, and (3) the derived atomic hydrogen distribution from 80 to 250 km requires a source of atomic hydrogen above 80 km. Comparison of observed profiles with chemical diffusion models implies a large downward flow of atomic hydrogen at 80 km coupled with a large upward flow of molecular hydrogen.-

On the theory of the light curves of supernovae

Abstract: An account of the theory of the light curves of supernovae is presented, based on certain assumptions concerning the passage through the stellar atmosphere of powerful shock waves. The investigation is based on numerical integration of appropriate equations of gas dynamics and radiative heat-conductivity. The calculations substantially involve the ionization and recombination of hydrogen in the envelope of a supernova. Changes are traced in the curves arising from the transition from compact stars with small radius (∼10R ⊙), to stars with very extensive envelopes (∼10000R ⊙). The light curves for compact stars agree well with observations of the peculiar supernovae in NGC 5457, NGC 6946 and NGC 5236. The characteristics of the light curves with the passage of shock waves through the extended atmosphere coincide within an order of magnitude with observations of the supernovae of type II and type I near their maximum brightness. A powerful heat-wave propagates before the shock-front in the extensive atmosphere which gives rise to a detached supernova envelope in the form of a thin spherical layer. We investigated the condition in an ascending wave of cooling and recombination in the supernova envelope. It is shown that part of the hydrogen may recombine to attain full transparency for radiation passing through it. The observations are compared with the results of the theory of radioactive decay of the elements. This explanation of the light curves by the passage of shock waves requires energies of 1050 to 1052 ergs, which are in agreement with mechanisms of thermonuclear explosions.

Optical Spectra and Intensities of Nd3+ in YAlO3

Abstract: The optical absorption and emission spectra, fluorescence kinetics, and radiative and nonradiative transition probabilities of Nd3+ ions in YAlO3 are investigated. Measurements are presented of the energy levels, absorption and emission cross sections, linewidths as a function of temperature and Nd concentration, thermal line shifts, and fluorescence branching ratios. Judd‐Ofelt intensity parameters for electric‐dipole transition probabilities are derived from measurements of the absorption spectrum. Stimulated emission cross sections, absorption‐band intensities, fluorescence quantum efficiency, possible excited‐state absorption, and other properties of importance for YAlO3:Nd3+ laser action are discussed.

Radiative Properties of Carbonaceous Aerosols

Abstract: The complex index of refraction for various carbonaceous materials is presented for wavelengths from 0.3μ≲λ≲20μ. An ensemble of spherical carbon dust particles represented by two size distributions typical of urban aerosols is utilized in conjunction with Mic theory to determine the extinction coefficient, the albedo for single scattering, the asymmetry factor, 〈cosθ〉, the average phase function, and the polarization for wavelengths 0.3μ≲λ≲15μ.

Excited Singlet State Radiative and Nonradiative Transition Probabilities for Acetone, Acetone‐d6, and Hexafluoroacetone in the Gas Phase, in Solution, and in the Neat Liquid

Abstract: Fluorescence lifetime and fluorescence quantum yield measurements have been made on acetone and acetone‐d6 in the gas phase, in solution, and in the neat liquid. The variation of the radiative and nonradiative transition probabilities with deuteration, and the effect of solvent environment are discussed. Radiative transition probabilities are compared with those calculated from the equation of Strickler and Berg. The discrepancies between the observed and calculated lifetimes are discussed in terms of vibronically induced transitions.

Multiple backscattering and depolarization from water clouds for a pulsed lidar system

Abstract: Linearly polarized lidar light scattering in spherically symmetrical uniformly distributed cloud water drops, investigating multiple backscattering effects on depolarization

Mean Lives of Excited Levels in O i–O vi*

Abstract: Ion beam-foil produced oxygen spectra in wavelength range between 450 and 2200 A, determining mean radiative lives of O I - O VI excitation levels

Studies on Triplet Radiative Lifetimes, Phosphorescence, and Delayed Fluorescence Yields of Aromatic Hydrocarbons in Liquid Solutions

Abstract: The phosphorescence of phenanthrene, pyrene, and naphthalene in fluid solutions has been studied. The radiative lifetimes are calculated from the quantum yields and actual lifetimes and vary only slightly or not at all with temperature within the experimental error. For phenanthrene, pyrene, naphthalene, and anthracene the radiative lifetimes at 77°K in ethanol are 25, 30, 75, and 180 sec, respectively. The temperature dependence and the absolute value of the triplet–triplet annihilation efficiency vary from molecule to molecule. The results are discussed in terms of a re‐encounter association of the annihilation partners to form an excimer.

CO(a 3Π), Its Production, Detection, Deactivation, and Radiative Lifetime

Abstract: This paper describes the first reported observation of the CO Cameron bands (a 3Π→X 1Σ+) produced from CO through photon absorption processes The technique depends on quasiresonance fluorescence processes involving d 3Δ↔X 1Σ+ and a′ 3Σ+↔X 1Σ+ transitions and subsequent radiation from these triplet levels to the a 3Π state From experiments in which the exciting lamp is pulsed, time decay measurements permit evaluation of the quenching rate constants for CO(a 3Π) by CO, NO, O2, H2, and CO2 The radiative lifetime of CO(a 3Π) has been obtained by use of the quenching rate constants, the intensities of the a 3Π populating transitions, and the Cameron band intensities, and it has an average value of 44±11 msec Transfer of electronic energy takes place from CO(a 3Π) to NO, resulting in production of two radiating states, NO(A 2Σ) and NO (B 2Π), with approximate efficiencies of 23% and 10%, respectively Either of these may be used as a monitor of the CO(a 3Π) state Quenching of CO(a 3Π) by N2 leads to pro

The absorption of radiation near 600 Å by helium

Abstract: Free-bound and free-free radiative absorptions by a pair of atoms are discussed and explicit formulas are derived for the free-bound and free-free absorption cross sections. The theory is applied to the interpretation of helium absorption near 600 a and it is argued that the first two bands observed arise from transitions into quasibound resonating vibrational levels of the A 1Σ u + state. The shape of the A 1Σ u + potential energy curve is derived for internuclear distances between 2 and 3 a. The absorption data analysis lead to a potential barrier of magnitude 0·049 ev. The effects on the absorption structure of replacing 4He by 3He are predicted. An iterative procedure is described that simultaneously locates the energy and evaluates the width of a quasibound-shape resonance state.

Lifetimes of Excited Levels in PI-PV

Abstract: We have studied the beam-foil spectra of phosphorus between 600 and 2 200 A and measured radiative lifetimes for 21 excited terms in P I-P V. We discuss the merits of several methods for evaluating the decay constants and compare the results with theoretical transition probabilities as well as with recent experimental studies of oscillator strengths in the P I, Si I, Al I, Mg I, and Na I isoelectronic sequences.

Charge-Transfer Interaction and Fluorescence in Some Tetracyanobenzene Complexes

Abstract: Fluorescence lifetimes and quantum yields were measured for some tetracyanobenzene complexes with benzene and its methyl derivatives, and the radiative and nonradiative rate constants were determined under various conditions The results show that the low fluorescence intensities of the complexes in nonpolar fluid media are due to small radiative rate constants and that nonradiative processes become important for the complexes in polar fluid media From the theoretical consideration of the observed radiative transition probability, it is found that the geometry and the charge distribution in the excited equilibrium state are greatly different from those in the excited Franck-Condon state for the complexes in fluid media at room temperature and that the degree of the charge-transfer in the former state amounts to 96–100%, while it is only 12–45% in the latter state The stabilization energy due to the reorientation was obtained from the temperature dependence of the fluorescence spectrum