Showing papers on "Absorption (electromagnetic radiation) published in 2001"

A review of absorption refrigeration technologies

Abstract: This paper provides a literature review on absorption refrigeration technology. A number of research options such as various types of absorption refrigeration systems, research on working fluids, and improvement of absorption processes are discussed.

High-Resolution X-ray Emission and X-ray Absorption Spectroscopy

Abstract: In this review, high-resolution X-ray emission and X-ray absorption spectroscopy will be discussed. The focus is on the 3d transition-metal systems. To understand high-resolution X-ray emission and reso-nant X-ray emission, it is first necessary to spend some time discussing the X-ray absorption process. Section II discusses 1s X-ray absorption, i.e., the K edges, and section III deals with 2p X-ray absorption, the L edges. X-ray emission is discussed in, respectively, the L edges. X-ray emission is discussed in, respec-tively, and section V on 2p3s and 2p3d X-ray emission. Section VI focuses on magnetic dichroism effects, and in section VII selective X-ray absorption experiments are discussed. To limit the scope of this review paper, many related topics (for example, EELS, XPS, and resonant photoemission, phonon-oriented inelastic X-ray scat-tering, and X-ray microscopy) will not be discussed. In addition, many aspects of X-ray absorption, such as reflection experiments, diffraction absorption fine structure, and related experiments, will remain untouched. EXAFS will be discussed very briefly, and its X-ray emission analogue EXEFS 69,71 will not be discussed.

Controlling photons using electromagnetically induced transparency

Abstract: It is well known that a dielectric medium can be used to manipulate properties of light pulses. However, optical absorption limits the extent of possible control: this is especially important for weak light pulses. Absorption in an opaque medium can be eliminated via quantum mechanical interference, an effect known as electromagnetically induced transparency. Theoretical and experimental work has demonstrated that this phenomenon can be used to slow down light pulses dramatically, or even bring them to a complete halt. Interactions between photons in such an atomic medium can be many orders of magnitude stronger than in conventional optical materials.

A view from the inside: Complexity in the atomic scale ordering of supported metal nanoparticles

Abstract: In this report, we describe the use of several analytical techniques, including X-ray absorption spectroscopy (XAS), electron microscopy, and electron diffraction, as tools for characterizing the structural dynamics of supported Pt nanoscale particles. We examined several carbon-supported samples. Electron microscopy shows that the particles in these samples (S1−S3) have average particle diameters of roughly 20, 40, and 60 A respectively, while electron microdiffraction data for these particles provided evidence of long-ranged ordering in the form of face centered cubic structures. This study highlights the use of advanced synchrotron X-ray absorption spectroscopies (XAS), in particular extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES), as powerful tools for studying the structural habits and dynamics of these prototypical nanoscale materials. Using state-of-the-art methods of measurement and computational modeling, we demonstrate that it is possible to deve...

Ultrasensitive absorption spectroscopy with a high-finesse optical cavity and off-axis alignment

Abstract: A simple and easy to use method that allows high-finesse optical cavities to be used as absorption cells for spectroscopic purposes is presented. This method introduces a single-mode continuous-wave laser into the cavity by use of an off-axis cavity alignment geometry to eliminate systematically the resonances commonly associated with optical cavities, while preserving the absorption signal amplifying properties of such cavities. This considerably reduces the complexity of the apparatus compared with other high-resolution cavity-based absorption methods. Application of this technique in conjunction with either cavity ringdown spectroscopy or integrated cavity output spectroscopy produced absorption sensitivities of 1.5 x 10(-9) cm(-1) Hz(-1/2) and 1.8 x 10(-10) cm(-1) Hz(-1/2), respectively.

Absorption of sunlight by dust as inferred from satellite and ground-based remote sensing

Abstract: Dust absorption of solar radiation is not well known due to limitations in the accuracy of in situ measurements. Here we report two new independent remote sensing techniques that provide sensitive measurements of dust absorption. One uses satellite spectral measurements, the second ground based sky measurements. Both techniques demonstrate that Saharan dust absorption of solar radiation is several times smaller than the current international standards. For example, at wavelength of 0.64 µm the dust single scattering albedo is reported here as 0.97±0.02 rather than 0.87±0.04 in recent review.

A Thermal Infrared Radiation Parameterization for Atmospheric Studies

Abstract: This technical memorandum documents the longwave radiation parameterization developed at the Climate and Radiation Branch, NASA Goddard Space Flight Center, for a wide variety of weather and climate applications. Based on the 1996-version of the Air Force Geophysical Laboratory HITRAN data, the parameterization includes the absorption due to major gaseous absorption (water vapor, CO2, O3) and most of the minor trace gases (N2O, CH4, CFCs), as well as clouds and aerosols. The thermal infrared spectrum is divided into nine bands. To achieve a high degree of accuracy and speed, various approaches of computing the transmission function are applied to different spectral bands and gases. The gaseous transmission function is computed either using the k-distribution method or the table look-up method. To include the effect of scattering due to clouds and aerosols, the optical thickness is scaled by the single-scattering albedo and asymmetry factor. The parameterization can accurately compute fluxes to within 1% of the high spectral-resolution line-by-line calculations. The cooling rate can be accurately computed in the region extending from the surface to the 0.01-hPa level.

Spectral dependence of visible light absorption by carbonaceous particles emitted from coal combustion

Abstract: Optical characteristics of particles that absorb visible light are needed to model their effects on atmospheric radiation. Light absorption by particles emitted from low-technology coal combustion has exhibited a strong spectral dependence. I investigate various explanations for this phenomenon and conclude that a spectrally dependent imaginary refractive index is the most plausible. Following previous work on the structure of amorphous carbon, I propose that both the magnitude and spectral dependence of light absorption are controlled by the size of graphitic clusters within the material, and can be described using the optical band-gap theory. This hypothesis is an alternative to the current measurement divisions of light-absorbing “black carbon” and non-absorbing “organic carbon,” and offers an explanation for preferential absorption at blue wavelengths that may extend to ultraviolet wavelengths.

Water- and Oxygen-Induced Decay Kinetics of Photogenerated Electrons in TiO2 and Pt/TiO2: A Time-Resolved Infrared Absorption Study

Abstract: Electrons photogenerated in TiO2 and Pt/TiO2 catalysts were observed by time-resolved IR absorption spectroscopy. Transient IR absorption of an identical spectrum appeared on TiO2 and Pt/TiO2 irradiated by a 355 nm pump pulse. The absorption was attributed to optical transition of photogenerated electrons trapped in shallow midgap states. Electron- and hole-consuming reactions of adsorbates controlled the decay kinetics of the electrons competing with the electron−hole recombination. On TiO2, O2 from the gas-phase captured the electrons and accelerated the decay rate at a delay time of 10−100 μs. In water vapor, holes reacted with surface hydroxyls within 2 μs, and the recombination decay of the electrons was obstructed. When Pt/TiO2 was exposed to water, the oxidation and reduction steps of the water splitting reaction affected the decay kinetics in different time domains. Photogenerated holes oxidized water within 2 μs, whereas electrons reduced water at 10−900 μs.

Determining changes in NIR absorption using a layered model of the human head.

Abstract: A theoretical approach is presented to determine absorption changes in different compartments of a layered structure from distributions of times of flight of photons. In addition resulting changes in spatial profiles of time-integrated intensity and mean time of flight are calculated. The capability of a single-distance, time-domain method to determine absorption changes with depth resolution is tested on a layered phantom. We apply this method to in vivo measurements on the human head (motor stimulation, Valsalva manoeuvre) and introduce a small-sized time-domain experimental set-up suitable for bedside monitoring.

Intrinsic fluorescence spectroscopy in turbid media: disentangling effects of scattering and absorption.

Abstract: The fluorescence from a turbid medium such as biologic tissue contains information about scattering and absorption, as well as the intrinsic fluorescence, i.e., the fluorescence from an optically thin sample of pure fluorophores. The interplay of scattering and absorption can result in severe distortion of the intrinsic spectral features. These distortions can be removed by use of a photon-migration-based picture and information from simultaneously acquired fluorescence and reflectance spectra. We present experimental evidence demonstrating the validity of such an approach for extracting the intrinsic fluorescence for a wide range of scatterer and absorber concentrations in tissue models, ex vivo and in vivo tissues. We show that variations in line shape and intensity in intrinsic tissue fluorescence are significantly reduced compared with the corresponding measured fluorescence.

Transform-Limited Pulses Are Not Optimal for Resonant Multiphoton Transitions

Abstract: Maximizing nonlinear light-matter interactions is a primary motive for compressing laser pulses to achieve ultrashort transform limited pulses. Here we show how, by appropriately shaping the pulses, resonant multiphoton transitions can be enhanced significantly beyond the level achieved by maximizing the pulse's peak intensity. We demonstrate the counterintuitive nature of this effect with an experiment in a resonant two-photon absorption, in which, by selectively removing certain spectral bands, the peak intensity of the pulse is reduced by a factor of 40, yet the absorption rate is doubled. Furthermore, by suitably designing the spectral phase of the pulse, we increase the absorption rate by a factor of 7.

Ice absorption features in the 5-8 μm region toward embedded protostars

Abstract: We have obtained 5{8 m spectra towards 10 embedded protostars using the Short Wavelength Spectrometer on board the Infrared Space Observatory (ISO-SWS) with the aim of studying the composition of interstellar ices. The spectra are dominated by absorption bands at 6.0 m and 6.85 m. The observed peak positions, widths and relative intensities of these bands vary dramatically along the dierent lines of sight. On the basis of comparison with laboratory spectra, the bulk of the 6.0 m absorption band is assigned to amorphous H2O ice. Additional absorption, in this band, is seen toward 5 sources on the short wavelength wing, near 5.8 m, and the long wavelength side near 6.2 m. We attribute the short wavelength absorption to a combination of formic acid (HCOOH) and formaldehyde (H2CO), while the long wavelength absorption has been assigned to the C{C stretching mode of aromatic structures. From an analysis of the 6.85 m band, we conclude that this band is composed of two components: a volatile component centered near 6.75 m and a more refractory component at 6.95 m. From a comparison with various temperature tracers of the thermal history of interstellar ices, we conclude that the two 6.85 m components are related through thermal processing. We explore several possible carriers of the 6.85 m absorption band, but no satisfactory identication can be made at present. Finally, we discuss the possible implications for the origin and evolution of interstellar ices that arise from these new results.

A Module for Radiation Hydrodynamic Calculations with ZEUS-2D Using Flux-limited Diffusion

Abstract: A module for the ZEUS-2D code is described that may be used to solve the equations of radiation hydrodynamics to order unity in v/c, in the flux-limited diffusion (FLD) approximation. In this approximation, the factor Eddington tensor f, which closes the radiation moment equations, is chosen to be an empirical function of the radiation energy density. This is easier to implement and faster than full-transport techniques, in which f is computed by solving the transfer equation. However, FLD is less accurate when the flux has a component perpendicular to the gradient in radiation energy density and in optically thin regions when the radiation field depends strongly on angle. The material component of the fluid is here assumed to be in local thermodynamic equilibrium. The energy equations are operator split, with transport terms, radiation diffusion term, and other source terms evolved separately. Transport terms are applied using the same consistent transport algorithm as in ZEUS-2D. The radiation diffusion term is updated using an alternating direction-implicit method with convergence checking. Remaining source terms are advanced together implicitly using numerical root finding. However, when absorption opacity is zero, accuracy is improved by instead treating the compression and expansion source terms using a time-centered differencing scheme. Results are discussed for test problems including radiation-damped linear waves, radiation fronts propagating in optically thin media, subcritical and supercritical radiating shocks, and an optically thick shock in which radiation dominates downstream pressure.

Photochemical incorporation of silver quantum dots in monodisperse silica colloids for photonic crystal applications.

Abstract: We developed a novel method to fabricate nanocomposite monodisperse SiO2 spheres (approximately 100 nm) containing homogeneously dispersed Ag quantum dots (approximately 2 to 5 nm). The inclusion morphology is controlled through the timing of the photochemical reduction of silver ions during hydrolysis of tetraethoxysilane in a microemulsion. Depending on the timing, Ag quantum dots can be directed to different annuli within the SiO2 spheres, as well as onto the SiO2 sphere surfaces. The embedded Ag quantum dots show a plasmon resonance absorption band at 438 nm. These Ag@SiO2 particles have significant surface charge and readily self-assemble into crystalline colloidal array (CCA) photonic crystals which Bragg-diffract light in the visible region. The magnitude of the plasmon resonance absorption depends on the CCA Bragg diffraction condition. The negative dielectric constant of the silver nanoparticles may be decreasing the silica-silver nanodot composite refractive index below that of the water medium. We may be observing an analogue of the Borrmann effect previously observed in X-ray scattering, where the incident and diffracted electric field standing wave becomes localized in regions of small CCA crystal absorption.

The temperature dependence (203-293 K) of the absorption cross sections of O3 in the 230-850 nm region measured by Fourier-transform spectroscopy

Abstract: Absolute absorption cross sections of O3 were measured in the 230–850 nm (11765–43478 cm−1) region at five different temperatures (203–293 K) using a Fourier-transform spectrometer, at a spectral resolution of 5.0 cm−1 (corresponding to about 0.027 nm at 230 nm and to about 0.36 nm at 850 nm). The spectral accuracy of the data is better than 0.1 cm−1 — about 0.5 pm at 230 nm and about 7.2 pm at 850 nm — validated by recording of I2 absorption spectra in the visible using the same experimental set-up. O3 absorption spectra at different concentrations were recorded at five different sample temperatures in the range 203–293 K, and at each temperature at two total pressures (100 and 1000 mbar) using O2/N2 mixtures as buffer gas. Within the limits of experimental uncertainties, no influence of total pressure on the O3 spectrum was observed in the entire spectral region, as expected from the short lifetimes of the upper electronic states of O3. The temperature dependence of the O3 absorption cross sections is particularly strong in the Huggins bands between 310 and 380 nm, as observed in previous studies. An empirical formula is used to model the temperature dependence of the O3 absorption cross sections between 236 and 362 nm, a spectral region that is particularly important for atmospheric remote-sensing and for photochemical modelling.

System and process for heating semiconductor wafers by optimizing absorption of electromagnetic energy

Abstract: An apparatus for heat treating semiconductor wafers is disclosed. The apparatus includes a heating device which contains an assembly linear lamps for emitting light energy onto a wafer. The linear lamps can be placed in various configurations. In accordance with the present invention, tuning devices which are used to adjust the overall irradiance distribution of the light energy sources are included in the heating device. The tuning devices can be, for instance, are lamps or lasers.

Near-infrared Absorption Property of Biological Soft Tissue Constituents

Abstract: Near infrared (NIR) can penetrate relatively deep into biological soft tissues. The NIR absorption property of tissue varies with tissue constituents especially water, fat, collagen, and their combination ratio. Therefore, combination ratio of tissue constituents can be evaluated by decomposing the absorption spectrum to determine the light path length in each constituent. Standardized absorption spectra of tissue constituents are required in order to carry out decomposition. Since water, fat, and protein are the major contributors at NIR spectral region. This study is to measure their absorption spectra from standardized samples as reference for quantifying tissue constituents. Five kinds of major fatty acid found in human fat were mixed in proper ratio as a standard reference. Absorption spectrum of bovine skin gelatin and elastin in hog eye lens were used as references of protein. NIR absorption spectra were measured using a Shimadzu 3101-PC spectrophotometer. The results show that temperature has a strong effect on the absorption property of water but not on fatty acid mixture. Absorption spectrum of elastin is similar to that of dry bovine gelatin. NIR spectroscopy also can be used to characterize or identify different types of soft tissue based on their major chemical composition, such as detecting a fat plaque in a muscular tissue or a tumor in a high fat content tissue.

X-ray Spectroscopy of QSOs with Broad Ultraviolet Absorption Lines

Abstract: For the population of QSOs with broad ultraviolet absorption lines, we are just beginning to accumulate X-ray observations with enough counts for spectral analysis at CCD resolution From a sample of eight QSOs [including four Broad Absorption Line (BAL) QSOs and three mini-BAL QSOs] with ASCA or Chandra spectra with more than 200 counts, general patterns are emerging Their power-law X-ray continua are typical of normal QSOs with Gamma~20, and the signatures of a significant column density [N_H~(01-4)x10^{23} cm^{-2}] of intrinsic, absorbing gas are clear Correcting the X-ray spectra for intrinsic absorption recovers a normal ultraviolet-to-X-ray flux ratio, indicating that the spectral energy distributions of this population are not inherently anomalous In addition, a large fraction of our sample shows significant evidence for complexity in the absorption The subset of BAL QSOs with broad MgII absorption apparently suffers from Compton-thick absorption completely obscuring the direct continuum in the 2-10 keV X-ray band, complicating any measurement of their intrinsic X-ray spectral shapes

Synthesis of extremely small InP quantum dots and electronic coupling in their disordered solid films

Abstract: Extremely small colloidal InP quantum dots (QDs) with diameters ranging from 15 to 23 A were synthesized, and the optical properties of close-packed arrays of these dots were studied. The isolated QDs in dilute colloidal solution exhibit pronounced discrete absorption spectra, indicating a narrow size distribution. The absorption spectra of close-packed solids of ∼18 A diameter QDs with interdot spacings of 9 and 18 A show that the absorption onsets and excitonic peaks are, respectively, redshifted and broadened in going from dilute solution to close-packed solids. These results can be explained by electron delocalization in disordered close-packed solids; the spacing of electronic levels in the QDs is reduced and produces a redshift in the absorption spectra.

Diode-Laser Absorption Sensor for Line-of-Sight Gas Temperature Distributions.

Abstract: Line-of-sight diode-laser absorption techniques have been extended to enable temperature measurements in nonuniform-property flows. The sensing strategy for such flows exploits the broad wavelength-scanning abilities (>1.7 nm approximately 30 cm(-1)) of a vertical cavity surface-emitting laser (VCSEL) to interrogate multiple absorption transitions along a single line of sight. To demonstrate the strategy, a VCSEL-based sensor for oxygen gas temperature distributions was developed. A VCSEL beam was directed through paths containing atmospheric-pressure air with known (and relatively simple) temperature distributions in the 200-700 K range. The VCSEL was scanned over ten transitions in the R branch of the oxygen A band near 760 nm and optionally over six transitions in the P branch. Temperature distribution information can be inferred from these scans because the line strength of each probed transition has a unique temperature dependence; the measurement accuracy and resolution depend on the details of this temperature dependence and on the total number of lines scanned. The performance of the sensing strategy can be optimized and predicted theoretically. Because the sensor exhibits a fast time response (~30 ms) and can be adapted to probe a variety of species over a range of temperatures and pressures, it shows promise for industrial application.

Effects of π centers and symmetry on two-photon absorption cross sections of organic chromophores

Abstract: We have theoretically examined a series of organic molecules that exhibit large two-photon absorption cross sections in the visible region and that have been synthesized in different laboratories. One- and two-photon absorption cross sections of the four lowest excited states of each molecule have been calculated at the same theoretical level using ab initio response theory. It is found that the molecular length and the one-photon absorption intensity are quite strongly correlated factors, but that a corresponding correlation for the two-photon absorption is much weaker or is missing. In contrast, a most crucial role for large two-photon absorption is played by the π center. For molecules with a given π center a symmetrical structure with strong donor groups can result in a maximum two-photon absorption cross section. Our theoretical findings are consistent with some recent experimental observations. The chromophore based on dithienothiophene as π center attached with symmetrical N,N-diphenylamine donors is found to have the largest two-photon cross section in the visible region among all known one-dimensional two-photon organic materials that have been reported in the literature.

Complex resonance absorption structure in the X-ray spectrum of IRAS 13349+2438

Abstract: The luminous infrared-loud quasar IRAS 13349+2438 was observed with the XMM - Newton Observatory as part of the Performance Verification program. The spectrum obtained by the Reflection Grating Spectrometer (RGS) exhibits broad (FWHM - 1400 km/s) absorption lines from highly ionized elements including hydrogen- and helium-like carbon, nitrogen, oxygen, and neon, and several iron L - shell ions (Fe XVII - XX). Also shown in the spectrum is the first astrophysical detection of a broad absorption feature around lambda = 16 - 17 A identified as an unresolved transition array (UTA) of 2p - 3d inner-shell absorption by iron M-shell ions in a much cooler medium; a feature that might be misidentified as an O VII edge when observed with moderate resolution spectrometers. No absorption edges are clearly detected in the spectrum. We demonstrate that the RGS spectrum of IRAS 13349+2438 exhibits absorption lines from two distinct regions, one of which is tentatively associated with the medium that produces the optical/UV reddening.

High-temperature (1000–7000 K) collision-induced absorption of H2 pairs computed from the first principles, with application to cool and dense stellar atmospheres

Abstract: The collision-induced absorption (CIA) spectra of H2–H2 and H2–He are known to play an important role for modelling of low-metallicity cool and dense stellar atmospheres. In this paper we present collision-induced absorption spectra of H2–H2 complexes in the rototranslational (Δv=0), the fundamental (Δv=1), the first (Δv=2) and the second (Δv=3) overtone bands in the temperature range from 1000 to 7000 K, and in the frequency region from 0 to 20 000 cm−1. The translational spectral density functions are computed quantum mechanically, based on: (1) the newly developed ab initio collision-induced H2–H2 dipole functions of Zheng (Computational study of collision induced dipole moments and absorption spectra of H2–H2. Ph.D. thesis, Michigan Technological University, 1997), which account for the short-range H2–H2 intermolecular distances (as small as 2.5 a.u.) and for larger H2 internuclear distances (as large as 2.15 a.u.); (2) semiempirical isotropic H2–H2 potential (Ross et al, J Chem Phys 1983;79(3):1487) suitable for high temperatures. We include the collision-induced absorption coefficient of the vibrational transitions as v1,v2,v′1,v′2≤3 which we computed rigorously. We also give our estimate for the collision-induced absorption coefficients of single vibrational transitions such as v i v′ i >3 in the first and second overtone bands. The dependence of CIA spectra on rotational states of H2 molecules is accounted for in our computations. We have previously (Borysow et al, Astronom Astrophys. 1997;324:185–95) studied the effect of CIA for stars of a wide range of fundamental stellar parameters (effective temperature, gravity, and chemical composition), and determined for which combinations of these parameters it is necessary to include CIA in the model and spectrum computation. These calculations showed that CIA from H2–H2 plays an important, and often even a dominating role for stellar atmospheres of a wide range of stars. The approximate character of the estimates of the H2–H2 absorption coefficient we used in our previous work combined with the large effect CIA had on the stellar atmospheres, were the main inspirations to initiate the more accurate computations of the absorption coefficient we present here. The absorption coefficient we compute in the present analysis is in qualitative agreement with our preliminary estimates (Borysow et al, Astronom Astrophys 1997;324:185–95), but in some spectral regions of high importance for the stellar structure, our computed absorption coefficient is up to a factor of 3 larger than our preliminary estimates. We therefore fully confirm our previous suspicion that H2–H2 CIA will have a pronounced effect on the atmosphere for a wide range of stars. In this paper we therefore quantify the effect the new data have on a typical cool dense stellar atmosphere, and compare our new results with our previous estimates.