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At least 433 records · Page 24
All sourcesNASA NTRSDOE OSTI
NASA NTRS1984

Ice-Soil Mixtures: Visual and Near-Infrared Remote Sensing Techniques

The spectral properties of ice soil mixtures are reviewed. Water ice has an absorption coefficient which varies by several orders of magnitude in the visual and near infrared, and has several prominent overtone absorptions. Different wavelengths can be used to probe to different depths in the surface as well as for different mineral impurity concentrations. Empirical and/or theoretical models might be used to derive characteristics grain size of the ice or of the impurity minerals and to derive abundance of the ice and rock or soil components. Quantitative analysis of remotely obtained reflectance spectra can only be performed by using absorption features in the spectra of ice and soil, and not by broadband response. It is suggested that this might be done by special selection of several narrow band filters in the near infrared which will adequately define the ice absorptions.

Clark, R. N.↗
NASA NTRS1981

Laboratory measurements of selected optical, physical, chemical, and remote-sensing properties of five water mixtures containing Calvert clay and a nonfluorescing dye

Total suspended solids concentrations ranged from 6.1 ppm to 24.3 ppm and sizes ranged between 1.5 micrometers and 10 micrometers with the most frequently occurring size less than 2 micrometers. Iron concentration was less than 1 percent of the total suspended solids. Nonfluorescing dye concentrations of the two mixtures were 20 ppm and 40 ppm. Attenuation coefficient for the five mixtures ranged from 4.8/m to 21.3/m. Variations in volume scattering function with phase angle were typical. Variations in attenuation and absorption coefficient with wavelength were similar for the mixtures without the dye. Attenuation coefficient of the mixtures with the dye increased for wavelengths less than 600 nm due to the dye's strong absorption peak near 500 nm. Reflectance increased as the concentration of Calvert clay increased and peaked near 600 nm. The nonfluorescent dye decreased the magnitude of the peak, but had practically no effect on the variation for wavelengths greater than 640 nm. At wavelengths less than 600 nm, the spectral variations of the mixtures with the dye were significantly different from those mixtures without the dye.

Usry, J. W.↗
DOE OSTI2017

Two-photon absorption measurements of deep UV transmissible materials at 213 nm

We report on two photon absorption measurements at 213nm of deep UV transmissible media including LiF, MgF 2 , CaF 2 , BaF 2 , Sapphire (Al 2 O 3 ) and high purity grades of fused-silica (SiO 2 ). A high stability 24ps Nd:YAG laser operating at the 5th harmonic (213nm) was used to generate a high intensity, long Rayleigh length Gaussian focus inside the samples. The measurements of the Fluoride crystals and Sapphire indicate two photon absorption coefficients between 0.004 and 0.82 cm/GW. We find that different grades of fused silica performed near identically for two photon absorption, however, there are differences in linear losses associated with purity. A low two photon absorption cross section is measured for MgF 2 making it an ideal material for the propagation of high intensity deep UV lasers.

36 MATERIALS SCIENCE↗
NASA NTRS1976

Complex refractive index of Martian dust - Mariner 9 ultraviolet observations

Mariner 9 ultraviolet spectrometer observations of the 1971 dust clouds obscuring the surface of Mars have been analyzed by matching the observed dust phase function with Mie scattering calculations for size distributions of homogeneous and isotropic material. Preliminary results indicate an effective particle radius of not less than 0.2. The real component of the index of refraction is not less than 1.8 at both 268 and 305 nm; corresponding values for the imagery component are 0.02 and 0.01. These values are consistent with those found by Mead (1970) for the visible and near-visible wavelengths. The refractive index and the absorption coefficient increase rapidly with decreasing wavelength in going from the visible to the ultraviolet, indicating the presence of an ultraviolet absorption band which may shield organisms from ultraviolet irradiation.

Pang, K.↗
NASA NTRS1993

Near-surface temperature gradients and their effects on thermal-infrared emission spectra of particulate planetary surfaces

The infrared energy emitted from a planetary surface is generated within a finite depth determined by the material's absorption skin depth. This parameter varies significantly with wavelength in the infrared but has an average value of around 50 microns for most geologic materials. In solid rock, heat transfer is efficient enough so that this 50 micron zone of the near surface from which the radiation emanates will be more or less isothermal. In particulate materials, however, heat transfer is more complicated and occurs via a combination of mechanisms, including solid conduction within grains and across grain contacts, conduction through the interstitial gas, and thermal radiation within individual particles and across the void spaces in between grains. On planets with substantial atmospheres, the gas component dominates the heat transfer and tends to mitigate near-surface thermal gradients. However, on airless bodies, the gas component is absent and heat transfer occurs via solid conductions and radiation. If the particles are small relative to the average absorption skin depth, then the top 50-100 microns or so of the surface will be cooled by radiation to space allowing the creation of significant near-surface thermal gradients. In those regions of the spectrum where the absorption coefficient is low, the emission will come from the deeper, warmer parts of the medium, whereas in regions of high absorption, the emission will emanate from shallower, cooler parts of the medium. The resulting emission spectrum will show non-compositional features as a result of the thermal structure in the material. We have modeled the heat transfer in a particulate medium in order to determine the magnitude of near-surface thermal gradients for surfaces on airless bodies and on Mars. We use the calculated thermal structure to determine the effects it has on the infrared emission spectrum of the surface.

Henderson, B. G.↗
NASA NTRS2005

Bio-Optics of the Chesapeake Bay from Measurements and Radiative Transfer Calculations

We combined detailed bio-optical measurements and radiative transfer (RT) modeling to perform an optical closure experiment for optically complex and biologically productive Chesapeake Bay waters. We used this experiment to evaluate certain assumptions commonly used when modeling bio-optical processes, and to investigate the relative importance of several optical characteristics needed to accurately model and interpret remote sensing ocean-color observations in these Case 2 waters. Direct measurements were made of the magnitude, variability, and spectral characteristics of backscattering and absorption that are critical for accurate parameterizations in satellite bio-optical algorithms and underwater RT simulations. We found that the ratio of backscattering to total scattering in the mid-mesohaline Chesapeake Bay varied considerably depending on particulate loading, distance from land, and mixing processes, and had an average value of 0.0128 at 530 nm. Incorporating information on the magnitude, variability, and spectral characteristics of particulate backscattering into the RT model, rather than using a volume scattering function commonly assumed for turbid waters, was critical to obtaining agreement between RT calculations and measured radiometric quantities. In situ measurements of absorption coefficients need to be corrected for systematic overestimation due to scattering errors, and this correction commonly employs the assumption that absorption by particulate matter at near infrared wavelengths is zero.

Tzortziou, Maria↗
DOE OSTI2014

Electronic and Optical Properties of Nanoporous Silicon for Solar-Cell Applications

Bulk silicon is a poor light emitter and absorber, but its optical properties are significantly enhanced by nanostructuring. We use first-principles calculations to investigate the electronic and optical properties of nanoporous silicon. We determine and analyze the electronic structure, optical coefficients, and photovoltaic conversion efficiency as a function of pore size and spacing. Our results show that the visible-range absorption coefficient of nanoporous Si is greatly improved compared to the bulk for pore spacings and sizes on the scale of a few nanometers. Here, the photovoltaic efficiency is improved to up to 2.7 times compared to bulk Si, and the theoretical maximum conversion efficiency reaches up to 6.3% for a thin-film nanoporous material thickness of 135 nm. Our results show that nanoporous Si is a promising material for thin-film photovoltaic applications.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Non-resonant below-bandgap two-photon absorption in quantum dot solar cells

We study the optically nonlinear sub-bandgap photocurrent generation facilitated by an extended tailing distribution of states in an InAs/GaAs quantum dots (QDs) solar cell. The tailing states function as both the energy states for low energy photon absorption and the photocarriers extraction pathway. One of the biggest advantages of our method is that it can clearly differentiate the photocurrent due to one-photon absorption (1PA) process and two-photon absorption (2PA) process. Both 1PA and 2PA photocurrent generation efficiency in an InAs/GaAs QD device operated at 1550 nm have been quantitatively evaluated. A two-photon absorption coefficient β = 5.7 cm/GW is extracted.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
NASA NTRS1988

A model of the photosynthetically available and usable irradiance in the sea

A theoretical model describing the depth dependence of the solar irradiance available to phytoplankton for photosynthesis is developed for waters classified as Case I by Jerlov (1976). The techniques used to account for the effects of incident solar irradiance, pigment concentration, and the spectral diffuse attentuation coefficient are described; an expression for the photosynthetically usable irradiance is derived; and particular attention is given to the specific diffuse absorption coefficient for chlorophyll (Morel, 1978). The relationships among the primary model parameters are shown in graphs.

Collins, Donald J.↗
NASA NTRS1994

An improved quasistatic line-shape theory: The effects of molecular motion on the line wings

A theory is presented for the modification of the line-shape functions and absorption coefficient due to the breakdown of the quasistatic approximation. This breakdown arises from the effects of molecular motion and increases the absorption in the near wings. Numerical calculations for the high-frequency wing of the nu(sub 3) band of CO2 broadened by Ar are reported and it is shown that these effects are significant near the bandhead. The importance of such corrections in other spectral regions and for other systems is discussed briefly.

Ma, Q.↗
NASA NTRS2005

Scattering Properties and Brightness Temperatures Associated with Solid Precipitation

In the past few years, early solid precipitation detection and retrieval algorithms have been developed and shown to be applicable for snowing clouds and blizzards. NOAA has an operational snow versus rain classifier based on AMSU-B observations. Solid precipitation retrieval algorithms reported in the literature over the past two years include those that rely on neural nets, statistics, or physical relationships. All of the algorithms require the use of millimeter-wave radiometer observations. The millimeter-wave frequencies are especially sensitive to the scattering and emission properties of frozen particles due to the ice particle refractive index. Passive radiometric channels respond to both the integrated particle mass throughout the volume and field of view, and to the amount, location, and size distribution of the frozen (and liquid) particles with the sensitivity varying for different frequencies and hydrometeor types. This investigation probes the sensitivity of scattering and absorption coefficients, and hence computed brightness temperatures, resulting from variations in solid precipitation cloud profiles. The first study compares the single scattering, absorption, and asymmetry parameters associated with snow particles in clouds. Several methodologies are used to convert the physical characteristics (e.g., shape, size distributions, ice-air-water ratios) of ice particles to electromagnetic properties (e.g., absorption, scattering, and asymmetry factors). These methodologies include: conversion to solid ice particles, homogeneous dielectric mixing, or discrete dipole approximation. Changes in the conversion methodology can produce computed brightness temperature differences greater than 50 Kelvin.

Skofronick-Jackson, Gail M.↗
DOE OSTI2022

Intercomparison of airborne and surface-based measurements during the CLARIFY, ORACLES and LASIC field experiments

Data are presented from intercomparisons between two research aircraft, the FAAM BAe-146 and the NASA Lockheed P3, and between the BAe-146 and the surface-based DOE (Department of Energy) ARM (Atmospheric Radiation Monitoring) Mobile Facility at Ascension Island (8 S, 14.5W, a remote island in the mid-Atlantic). These took place from 17 August to 5 September 2017, during the African biomass burning season. The primary motivation was to give confidence in the use of data from multiple platforms with which to evaluate numerical climate models. The three platforms were involved in the CLouds-Aerosol-Radiation Interaction and Forcing for Year 2017 (CLARIFY-2017), ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES), and Layered Atlantic Smoke and Interactions with Clouds (LASIC) field experiments. Comparisons from flight segments on six days where the BAe-146 flew alongside the ARM facility on Ascension Island are presented, along with comparisons from wing-tip to wing-tip flight of the P3 and BAe-146 on 18 th August 2017. The intercomparison flight sampled a relatively clean atmosphere overlying a moderately polluted boundary layer, while the 6 fly-bys of the ARM site sampled both clean and polluted conditions 2–4 km upwind. We compare and validate characterisations of aerosol physical, chemical, and optical properties, atmospheric radiation, and cloud microphysics between platforms. We assess the performance of measurement instrumentation in the field, under conditions where sampling conditions are not tightly controlled as in laboratory measurements where calibrations are performed. Solar radiation measurements compared well between airborne platforms. Optical absorption coefficient measurements compared well across all three platforms, even though absolute magnitudes were often low (< 10 Mm -1 ) and close to the sensitivity limits of measurement instrumentation thereby confounding assessments of the comparability of absorption Ångström exponent characterisations. Aerosol absorption measurements from airborne platforms were more comparable than aircraft-to-ground observations. Scattering coefficient observations compared well between airborne platforms, but agreement with ground-based measurements was worse, potentially caused by small differences in sampling conditions or actual aerosol population differences. Chemical composition measurements followed a similar pattern, with better comparisons between the airborne platforms. Thermodynamics, aerosol, and cloud microphysical properties generally compared well.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2018

Low hygroscopicity of ambient fresh carbonaceous aerosols from pyrotechnics smoke

Pyrotechnics (fireworks) displays are common for many cultures worldwide, with Independence Day celebrations occurring annually on July 4th as the most notable in the U.S. Given an episodic nature, fireworks aerosol properties are poorly characterized. Here we report observations of optical properties of fresh smoke emissions from Independence Day fireworks smoke sampled at Los Alamos National Laboratory, New Mexico U.S.A. on 4–5 July 2016. Aerosol optical properties were measured with a photoacoustic extinctiometer (PAX, DMT, Inc., Model 870 nm) at low RH < 30% and a humidity controlled nephelometry system (Ecotech, Inc., 450 nm Aurora). ‘Dry’ light scattering coefficient (σsp) increased from background < 15 Mm –1 reaching 120 Mm –1 (450 nm) as a 2-min event peak, while the absorption coefficient increased from background of 0.5–4.4 Mm –1 (870 nm). The event peak occurred at 00:35 on 5 July 2016, ~3 h after local fireworks events, and decreased to background by 04:00 on 5 July 2016, showing well mixed aerosol properties. A notable result is that the aerosol hygroscopic response, as characterized by the ratio of wet to dry light scattering or f (RH = 85%), declined to 1.02 at the peak fireworks influence from a background ~1.7. Strong wavelength dependence of light scattering with Ångström exponent ~2.2 throughout the event showed a size distribution dominated by sub-micrometer particles. Likewise, single scattering albedo at 870 nm remained constant throughout the event with ω = 0.86 ± 0.03, indicating light absorbing carbon, though not dominant, was mixed with organic carbon. Subsequent laboratory testing with ground-level sparklers showed that pyrotechnics smoke can generate a strong hygroscopic response, however. As confirmed with chemical analysis, the chemistry of the fireworks was key to defining the hygroscopic response. As a result, sparkler smoke was dominated by salt species such as hygroscopic potassium chloride while it lacked the black powder explosives in aerial fireworks that contribute organic and elemental carbon to its non-hygroscopic smoke.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2016

Experimental Effects on IR Reflectance Spectra: Particle Size and Morphology

For geologic and extraterrestrial samples it is known that both particle size and morphology can have strong effects on the species’ infrared reflectance spectra. Due to such effects, the reflectance spectra cannot be predicted from the absorption coefficients alone. This is because reflectance is both a surface as well as a bulk phenomenon, incorporating both dispersion as well as absorption effects. The same spectral features can even be observed as either a maximum or minimum. The complex effects depend on particle size and preparation, as well as the relative amplitudes of the optical constants n and k, i.e. the real and imaginary components of the complex refractive index. While somewhat oversimplified, upward-going amplitude in the reflectance spectrum usually result from surface scattering, i.e. rays that have been reflected from the surface without penetration, whereas downward-going peaks are due to either absorption or volume scattering, i.e. rays that have penetrated or refracted into the sample interior and are not reflected. While the effects are well known, we report seminal measurements of reflectance along with quantified particle size of the samples, the sizing obtained from optical microscopy measurements. The size measurements are correlated with the reflectance spectra in the 1.3 – 16 micron range for various bulk materials that have a combination of strong and weak absorption bands in order to understand the effects on the spectral features as a function of the mean grain size of the sample. We report results for both sodium sulfate Na2SO4 as well as ammonium sulfate (NH4)2SO4; the optical constants have been measured for (NH4)2SO4. To go a step further from the field to the laboratory we explore our understanding of particle size effects on reflectance spectra in the field using standoff detection. This has helped identify weaknesses and strengths in detection using standoff distances of up 160 meters away from the Target. The studies have shown that particle size has an enormous influence on the measured reflectance spectra of such materials; successful identification requires sufficient, representative reflectance data to include the particle sizes of interest.

Beiswenger, Toya N.↗
NASA NTRS1979

Liquid content of the lower clouds of Venus as determined from Mariner 10 radio occultation

S- and X-band radio occulation data obtained during the Mariner 10 flyby of Venus are analyzed to determine the effects of signal attenuation due to the clouds of Venus. The signal-amplitude data are corrected for errors introduced by spacecraft motion and antenna steering during the occultation, absorption-coefficient profiles are computed, and the liquid content of the Venusian clouds is estimated. Excess attenuation effects are found and ascribed to absorption by small liquid particles, consisting of a water-sulfuric acid mixture containing 75% + or - 25% sulfuric acid, in the clouds. Three absorbing cloud layers at respective heights of 68, 60, and 48 km are suggested by the X-band absorptivity profile, but only the 68- and 48-km layers are clearly indicated by the S-band profile. The relatively slow S-band absorption at 60 km is tentatively attributed to an unknown constituent that is much more absorbing in the X-band than in the S-band. The results are compared with nephelometer measurements performed at 0.9 micron by the Venera 9 and 10 probes.

Kliore, A. J.↗
DOE OSTI2018

Computational analysis of metamaterial–aluminum–silicon solar cell model

In this paper, the optical parameters of an improved waveguide structure for a more efficient silicon solar cell are studied. Despite its favorable electronic, physical, and chemical properties, silicon remains a poor absorber of light. The optical losses due to the reflection at the air/glass interface of the cell and the transmission at its back are other factors, which limit the cell conversion efficiency. Consequently, several mechanisms for light trapping capable to increase the collection of the incident photons as electrical current and to decrease the transmission loss, have been developed. In this context, we propose a multilayer waveguide structure in which the sunlight is guided by a metamaterial layer and the transmission loss is eliminated by an aluminum back reflector. The reflection and transmission coefficients are derived by using the Generalized Transfer Matrix Method. The application of the law of conservation of energy allowed the determination of the absorption coefficient. These optical parameters are examined for several angles of incidence for s-polarized light, p-polarized light and unpolarized light. Simulation results show a significant reduction of reflection and a complete suppression of transmission.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

A Density Functional Theory Investigations of Half-Heusler Compounds RhVZ (Z = P, As, Sb)

We have studied the half-Heusler compounds RhVZ (Z = P, As, Sb) using Density Functional Theory (DFT). The method of the Full Potential Linearly Augmented Plane Wave (FP-LAPW) is employed in the Wien2k package for calculation of structural, elastic, mechanical, electronic, magnetic, and optical properties. Lattice constants are found in the range 5.67 − 5.80 Å. Analysis of elastic properties shows that two of the compounds namely RhVP and RhVAs are ductile while RhVSb is brittle in nature. All three compounds are half-metals as revealed from the band structures and Density of States (DoS) calculations. In all of them, spin-down channels have the small band gaps, while spin-up channels are conducting. These compounds follow the Slater-Pauling 18 (M{sub Tot} = Z{sub Tot} − 18) electron rule with the total magnetic moments in the range 1 − 2 μ{sub B}. Optical properties like dielectric function, refractive index, reflectivity, conductivity, and absorption coefficient are calculated and discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗