Showing papers on "Absorption (electromagnetic radiation) published in 1995"

Optical absorption and fluorescence properties of chromophoric dissolved organic matter in natural waters

Abstract: Complete optical absorption and fluorescence spectra were collected for a diverse suite of 0.2-μm-filtered marine, riverine, and estuarine waters, as well as for colored dissolved organic matter (CDOM) isolated from several of these waters by solid-phase C 18 extraction. Absorption and fluorescence parameters for these samples are reported. For surface waters, variations in the fluorescence quantum yields obtained with 355- and 337-nm excitation fell within a narrow window (< 2.5-fold variation about the mean values), demonstrating that fluorescence measurements can be used to determine absorption coefficients of CDOM in the ultraviolet region with reasonably good accuracy. Methods for predicting absorption coefficients and line shapes from the fluorescence data are introduced and tested. The absorption and fluorescence spectra of CDOM extracted from some seawaters differed significantly from those of the original waters, demonstrating that material isolated by hydrophobic adsorption is not necessarily representative of the suite of colored organic matter present in aquatic systems. These results clearly illustrate that great care must be taken when extracted material is used to infer the optical properties of natural waters

Photothermal spectroscopy methods for chemical analysis

Abstract: Absorption, Energy Transfer, and Excited-State Relaxation. Hydrodynamic Relaxation: Heat Transfer and Acoustics. Optical Principles for Photothermal Spectroscopy. Temperature Change and Optical Elements in Homogeneous Samples. Photothermal Spectroscopy in Homogeneous Samples. Analytical Measurement and Data Processing Considerations. Analytical Applications. Photothermal Spectroscopy of Heterogeneous Samples. Index.

Two-photon absorption and optical-limiting properties of novel organic compounds

Abstract: The optical-limiting behavior and two-photon absorption properties of four novel organic compound solutions in tetrahydrofuran have been investigated. An ultrashort laser source with 0.5-ps pulse width and 602-nm wavelength was employed. The transmissivities of the various 1-cm-thick solution samples have been measured as a function of the beam intensity as well as of the solute concentration. The measured results can be fitted on the assumption that two-photon absorption is the only predominant mechanism causing the observed opticallimiting behavior. Based on the intensity-dependent transmissivity measurements, the molecular two-photon absorption coefficients for the four compounds are presented.

Cavity ring-down spectroscopy for quantitative absorption measurements

Abstract: We examine under what conditions cavity ring-down spectroscopy ~CRDS! can be used for quantitative diagnostics of molecular species. We show that CRDS is appropriate for diagnostics of species whose absorption features are wider than the spacing between longitudinal modes of the optical cavity. For these species, the absorption coefficient can be measured by CRDS without a knowledge of the pulse characteristics provided that the cavity ring-down decay is exponential. We find that the exponential ring-down decay is obeyed when the linewidth of the absorption feature is much broader than the linewidth of the light circulating in the cavity. This requirement for exponential decay may be relaxed when the sample absorption constitutes only a small fraction of the cavity loss and, consequently, the sample absorbance is less than unity during the decay time. Under this condition the integrated area of a CRDS spectral line approximates well the integrated absolute absorption coefficient, which allows CRDS to determine absolute number densities ~concentrations!. We determine conditions useful for CRDS diagnostics by analyzing how the absorption loss varies with the sample absorbance for various ratios of the laser pulse linewidth to the absorption linewidth for either a Gaussian or a Lorentzian absorption line shape. © 1995 American Institute of Physics.

Absorption of solar radiation by clouds: observations versus models.

Abstract: There has been a long history of unexplained anomalous absorption of solar radiation by clouds. Collocated satellite and surface measurements of solar radiation at five geographically diverse locations showed significant solar absorption by clouds, resulting in about 25 watts per square meter more global-mean absorption by the cloudy atmosphere than predicted by theoretical models. It has often been suggested that tropospheric aerosols could increase cloud absorption. But these aerosols are temporally and spatially heterogeneous, whereas the observed cloud absorption is remarkably invariant with respect to season and location. Although its physical cause is unknown, enhanced cloud absorption substantially alters our understanding of the atmosphere's energy budget.

An alternative approach to absorption measurements of aquatic particles retained on filters

Abstract: We made modifications to the procedure used for absorption measurements of aquatic particles retained on glass-fiber filters that extend the procedure’s application range to “case 2” waters with high suspended sediment content and allow the determination of phytoplankton pigment absorption in situations where the standard solvent extraction method is not effective. The first result was achieved by combining light-transmission and light-reflection measurements (the latter made possible by the use of a commercially available integrating-sphere attachment for the dual-beam spectrophotometer) so as to remove the spurious contribution to the measured absorption caused by sample backscattering. The second result was obtained by bleaching the sample with a NaClO solution-a method that proved satisfactory even with water-soluble pigments (e.g. the important class of the phycobilins) and solvent-resistant Chlorophyceae. This process also allows for depigmentation of the particle suspension, and thus it was used to evaluate the empirical expression for converting the filter-retained sample absorbance to the equivalent particle suspension value. Both features of the modified procedure have been positively tested through measurements carried out on solvent-resistant phytoplankton species and on samples of inorganic suspended sediment. Light-transmission measurements on particles retained in glass-fiber filters (Yentsch 1962) are convenient for determining the light absorption spectrum of particle suspensions consisting mainly of natural phytoplankton and organic and inorganic detritus (Gordon and Morel 1983). The procedure has two basic advantages: particles can be concentrated so that instrumental accuracy requirements can be met regardless of the high dilution occurring in situ, and information is provided on the absorption of the cells in vivo. The main problem in data analysis arises from the large modification of light transmission due to multiple scattering by the filter, which results in an overestimate of particle absorption relative to suspension state (Butler 1962). Several empirical expressions have been derived for converting the absorption of filter-retained particles (for convenience also referred to as “sample” in the following) to the equivalent absorption of particle suspension. A review on this subject is given by Cleveland and Weidemann ( 199 3). Kishino et al. (1984, 1985) proposed a procedure for discriminating absorption by phytoplankton pigments from absorption by detritus based on measurements performed before and after pigment extraction by methanol. Although some doubts remain about the effectiveness of the procedure (Bricaud and Stramski 1990) and its validity range shows some limitations (little or no effect on some algal species), solvent extraction is the method generally used to identify pigment absorption in natural phytoplankton populations. It is current practice to minimize the loss of forward

Spatially varying optical property reconstruction using a finite element diffusion equation approximation

Abstract: A finite element reconstruction algorithm for optical data based on a diffusion equation approximation is presented. A frequency domain approach is adopted and a unified formulation for three combinations of boundary observables and conditions is described. A multidetector, multisource measurement and excitation strategy is simulated, which includes a distributed model of the light source that illustrates the flexibility of the methodology to modeling adaptations. Simultaneous reconstruction of both absorption and scattering coefficients for a tissue-like medium is achieved for all three boundary data types. The algorithm is found to be computationally practical, and can be implemented without major difficulties in a workstation computing environment. Results using simulated data suggest that qualitative images can be produced that readily highlight the location of absorption and scattering heterogeneities within a circular background region of close to 4 cm in diameter over a range of contrast levels. Absorption images appear to more closely identify the true size of the heterogeneity; however, both the absorption and scattering reconstructions have difficulty with sharp transitions at increasing depth. Quantitatively, the reconstructions are not accurate, suggesting that absolute optical imaging involving simultaneous recovery of both absorption and scattering profiles in multicentimeter tissues geometries may prove to be extremely difficult.

Ozone UV spectroscopy. II. Absorption cross-sections and temperature dependence

Abstract: Absolute absorption cross-sections of ozone have been measured at five temperatures (218–295 K) in the UV wavelength region corresponding to the Hartley and Huggins bands (195–345 nm). The aim is to give to the users, a coherent set of data determined under high resolution. A comparison with previous work has been made.

Optical limiting effect in a two‐photon absorption dye doped solid matrix

Abstract: We recently reported a new lasing dye, trans‐4‐[p‐(N‐ethyl‐N‐hydroxylethylamino)styryl]‐N‐methylpyridinium tetraphenylborate (ASPT), which has also been shown to possess a strong two‐photon absorption (TPA) and subsequent frequency upconversion fluorescence behavior when excited with near infrared laser radiation. Based on the TPA mechanism, a highly efficient optical limiting performance has been demonstrated in a 2 cm long ASPT‐doped epoxy rod pumped with 1.06 μm Q‐switched laser pulses at 50–250 MW/cm2 intensity levels. The measured nonlinear absorption coefficient reached 6 cm/GW for the tested sample of dopant concentration d0=4×10−3 M/L. The molecular TPA cross section of ASPT in the epoxy matrix is estimated as σ2=2.5×10−18 cm4/GW or σ2′=4.7×10−46 cm4/photon/s, respectively. Two‐photon pumped cavity lasing is also observed in an ASPT‐doped polymer rod.

Monte Carlo Modeling of Light Transport in Tissues

Abstract: Monte Carlo simulations of photon propagation offer a flexible yet rigorous approach toward photon transport in turbid tissues. This method simulates the “random walk” of photons in a medium that contains absorption and scattering. The method is based on a set of rules that govern the movement of a photon in tissue. The two key decisions are (1) the mean free path for a scattering or absorption event, and (2) the scattering angle. Figure 4.1 illustrates a scattering event. At boundaries, a photon is reflected or moves across the boundary. The rules of photon propagation are expressed as probability distributions for the incremental steps of photon movement between sites of photon—tissue interaction, for the angles of deflection in a photon’s trajectory when a scattering event occurs, and for the probability of transmittance or reflectance at boundaries. Monte Carlo light propagation is rigorous yet very descriptive. However, this method is basically statistical in nature and requires a computer to calculate the propagation of a large number of photons. To illustrate how photons propagate inside tissues, a few photon paths are shown in Fig. 4.2.

Synthesis and Characterization of InP, GaP, and GaInP2 Quantum Dots

Abstract: Quantum dots of InP, GaP and GaInP{sub 2} with diameters ranging from 20-65 {Angstrom} were synthesized as well-crystallized nanoparticles with bulk zinc blende structure. The high sample quality of the InP and GaP QDs results in excitonic features in the absorption spectra. The GaP and GaInP{sub 2} QD colloids exhibited very intense (quantum yields of 15-25%) visible photoluminescence at room temperature. The photoluminescence for InP preparations showed two emission bands: one band in the visible at the band edge of the QD (50 nm Stokes shift), and a second band above 800 nm. The near-IR PL is attributed to deep traps, presumably phosphorus vacancies on the QD surface. This band disappears after activation of particles by controlled addition of etchant. In that case very intense band-edge emission (quantum yield 30% at room temperature and 60% at 10 K) was obtained. The QDs were characterized by TEM, SAXS, AFM, powder x-ray diffraction, steady state optical absorption and photoluminescence spectroscopy, ps to ns transient photoluminescence spectroscopy, and fs to ps pump-probe absorption (i.e., hole-burning) spectroscopy. Results will also be reported on efforts to produce ordered arrays of InP QDs.

Optical gap of CuO.

Abstract: The optical absorption edge of CuO single crystals has been investigated by reflectance and transmittance measurements at different temperatures. An exponential decay of the absorption has been detected, while temperature dependence of the edge and of the steepness between 10 K and room temperature indicate a large coupling of electronic states with lattice vibrations. At the lowest temperatures, excitonic structures have been revealed.

Infrared absorption by the B nitrogen aggregate in diamond

Abstract: Two approaches have been adopted in attempting to determine the strength of the one-phonon infrared absorption by the B nitrogen aggregate in diamond. On the one hand, we have heat treated purely type IaA specimens with known infrared absorption strengths (and thus also known nitrogen concentrations) to bring about partial aggregation of the A centres to B centres, monitoring changes in the strengths of the A, and resultant B, one-phonon components by decomposition of the ensuing infrared spectra. Secondly, we have made direct chemical assays of nitrogen in diamonds that show dominant B absorption features. This approach is complicated because diamonds in which the B aggregate is the only point defect also contain extended defects that may or may not involve nitrogen. These defects may or may not, in turn, contribute to absorption in the one-phonon region. Nevertheless, the nitrogen concentrations have been measured for two distinct groups of diamonds that both uncritically might at first glance ...

Influence of blood vessels on the measurement of hemoglobin oxygenation as determined by time-resolved reflectance spectroscopy.

Abstract: We report the development of a heterogeneous resin-tube model to study the influence of blood vessels on the apparent absorption of the system, mu a(sys), using a time-resolved technique. The experimental results show that mu a(sys) depends on the absorption inside the tubes, mu a(tube), tube diameters, and tube-to-sample volume ratios. A mathematical expression relating mu a(sys) and mu a(tube) is derived based on the experimental results and is verified by time-resolved Monte Carlo simulations for heterogeneous models. This analytical formula predicts that the apparent absorption coefficient measured on a biological organ is a volume-weighted sum of the absorption coefficients of different absorbing components. We present some apparent absorption coefficients measured in vivo in animals and humans and discuss improved algorithms that calculate the hemoglobin saturation by including background-tissue absorption and blood vessel distribution.

Validation of general circulation model radiative fluxes using surface observations

Abstract: The surface radiative fluxes of the ECHAM3 General Circulation Model (GCM) with T2 1, T42, and T 106 resolutions have been validated using observations from the Global Energy Balance Archive (GEBA, World Climate Program-Water Project A7). GEBA contains the most comprehensive dataset now available for worldwide instrumentally measured surface energy fluxes. The GCM incoming shortwave radiation at the surface has been compared with more than 700 long-term monitoring stations. The ECHAM3 models show a clear tendency to overestimate the global annual-mean incoming shortwave radiation at the surface due to an underestimation of atmospheric absorption. The model-calculated global-mean surface shortwave absorption around 165 W m−2 is estimated to be too high by 10–15 W m−2. A similar or higher overestimate is present in several other CYCMS. Deficiencies in the clear-sky absorption of the ECHAM3 radiation scheme are proposed as a contributor to the flux discrepancies. A stand-alone validation of the radi...

Initial assessment of a simple system for frequency domain diffuse optical tomography

Abstract: Diffuse optical tomography is an imaging technique whereby spatial maps of absorption and scattering coefficients are derived from the characteristics of multiply scattered light transmitted through the object. The system described here used four intensity-modulated light sources and measurements of the intensity and phase (relative to each source) at 16 or 20 detectors on the surface of a 10 cm diameter cylinder. An iterative Newton-Raphson algorithm was used to estimate the absorption and scattering coefficients at each pixel in a 17 x 17 array minimizing the difference between measured and calculated values of the intensity and phase at the measurement sites. Forward calculations of the intensity and phase were based on a multigrid finite-difference solution of the frequency domain diffusion equation. Numerical simulations were used to examine the resolution, contrast, and accuracy of the reconstructions as well as the effects of measurement noise, systematic uncertainties in source-detector location, and accuracy of the initial estimates for the optical properties. Experimental tests also confirmed that the system could identify and locate both scattering and absorbing inhomogeneities in a tissue-simulating phantom.

Fluorescence of coumarins and xanthenes after two-photon absorption with a pulsed titanium–sapphire laser

Abstract: Fluorescence emission after two-photon absorption of coumarins and xanthenes in an alcoholic solution was measured in the tuning range of a femtosecond-pulsed titanium–sapphire laser (750–840 nm). Xanthenes, which have a low one-photon absorption in the near UV, show a higher fluorescence signal after two-photon absorption than the UV-excitable coumarins. When fluxes of 1028 photons/(cm2 s) are used, the two-photon absorption cross sections for xanthenes are 1 order of magnitude higher than the two-photon absorption cross sections of the coumarins. Absolute cross sections have been estimated for three coumarins and three xanthenes. For the xanthenes a significant wavelength-dependent departure from the expected fluorescence intensity square law was observed. The coumarins follow the square-law dependence. The consequences of the findings are discussed for analytic and diagnostic methods. An especially important result is that the resolution in two-photon microscopy of xanthenes is worse than expected.

Infrared excited-state absorption and stimulated-emission cross sections of Er 3+ -doped crystals

Abstract: Detailed spectra of the excited-state absorption and the stimulated-emission cross-sections of Er3+ -doped Y3Al5O12, YAlO3, LiYF4, and KYF4 crystals are analyzed in the infrared wavelength regions from 700 to 2100 nm. The spectra were measured with a pump and probe technique employing a double modulation scheme. For Er (2%): Y3Al5O12 also the stimulated emission at 3 µm and the reabsorption due to excited-state absorption from the lower laser level are investigated.

Electromagnetically induced transparency in a three-level Λ-type system in rubidium atoms

Abstract: An electromagnetically induced transparency is observed at one arm of a three-level \ensuremath{\Lambda}-type system in a rubidium D1 line (794.8 nm) with an 85% reduction in absorption, when a pumping field is present at the other arm. This reduction in absorption for the weak probe field is due to the atomic coherence produced by the strong pumping field. This experiment is done in a Rb vapor cell at room temperature with cw diode lasers for both pumping and probe beams in a Doppler-free configuration. A simple theoretical treatment including Doppler broadening is in good qualitative agreement with the experimental measurement.

Absorption spectroscopy in tissue-simulating materials: a theoretical and experimental study of photon paths

Abstract: A diffusion model of noninvasive absorption spectroscopy was used to determine how the change in signal resulting from a point absorber depends on the position of that absorber relative to the source and detector. This is equivalent to calculating the relative probability that a photon will visit a certain location in tissue before its detection. Experimental mapping of the point-target response in tissue-simulating materials confirmed the accuracy of the model. For steady-state spectroscopy a simple relation was derived between the mean depth visited by detected photons, the source-detector separation, and the optical penetration depth. It was also demonstrated theoretically that combining a pulsed source with time-gated detection provides additional control over the spatial distribution of the photon-visit probability.

Far-infrared terahertz time-domain spectroscopy of flames.

Abstract: We present what is to our knowledge the first comprehensive far-infrared absorption measurement of flames. These measurements, covering the region of 7–88 wave numbers (0.2–2.65 THz) are only now made possible by the technique of terahertz time-domain spectroscopy. We observe a large number of absorption lines— including those of water, CH, and NH3—in a stationary, premixed, propane–air flame. The absorption strength permits the determination of species concentration along the beam path. The flame temperature is determined by comparison of the relative strengths of the water vapor lines.

Investigations of optical limiting mechanisms in carbon particle suspensions and fullerene solutions

Abstract: Liquid suspensions of carbon act as optical limiters in the presence of high-intensity Q-switched laser pulses. The optical limiting is a result of optical breakdown, which is initiated by absorption in the small carbon particles. We measured the total energy scattered during the breakdown process and the angular distribution of the scattered light as a function of input energy. We also compared the relative scattered energy created during limiting in the carbon suspensions with that generated in solutions of fullerenes in toluene. The ratio of scattered to absorbed energy is different for the suspensions from that for the fullerene solutions, indicating that different mechanisms dominate the limiting processes in the two media.

Light absorption by phytoplankton, photosynthetic pigments and detritus in the California Current System

Abstract: Pigment-specific absorption by total particulates, detritus and phytoplankton was measured throughout the euphotic zone at > 275 stations on three cruises off California in late 1991 and early 1992. A new spectral fluorescence method for assessing photosynthetically active absorption in natural samples was developed and applied. Spatial variability in specific absorption coefficients at the mesoscale was found to be as high as previously observed between mid- and high latitudes, while differences between cruises were very low. In surface waters, the highest values of specific absorption were found in warm, low-pigment surface waters offshore and in the Southern California Bight. Vertical sections reveal that low values occur near the surface only where the pycnocline and nitracline slope toward the sea surface. The highest values of phytoplankton specific absorption occurred at shallow optical depths for stations with deep nitraclines, whereas the lowest values always occurred close to or below the depth of the nitracline. Specific absorption generally increased with increasing temperature, but there were large differences in the relationships between cruises. In the context of previous laboratory observations, these results imply that nutrient availability plays a greater role than direct temperature effects in controlling natural variance in phytoplankton specific absorption. Specific absorption of photosynthetically active phytoplankton pigments was found to be less variable than that of total phytoplankton and showed no systematic trends with temperature, optical depth, or distance from the nitracline. This result leads to a new version of a bio-optical model for primary production which is based only on the photosynthetically active component rather than total phytoplankton absorption.

The variable effect of clouds on atmospheric absorption of solar radiation

Abstract: A four-year global record of solar flux observed from both space and the Eartha¤™s surface allows an examination of the effect of clouds on the atmospheric absorption of solar radiation. The results indicate that, contrary to some recent suggestions, the effect of clouds is highly variable and present general circulation models should be able to incorporate cloud absorption into climate simulations.

Regional models for phytoplankton absorption as a function of chlorophyll a concentration

Abstract: Empirical relationships for predicting phytoplanktonic absorption at 676 and 436 nm from water column chlorophyll a concentration are presented for distinct geographic regions defined by latitude The forms of the predictive equations are controlled by underlying biological mechanisms and lend insight into these mechanisms These region-specific models allow prediction of phytoplanktonic absorption from more easily measured parameters such as chlorophyll a concentration or in situ fluorescence and increase accuracy in modeling optical properties or primary production rates from specific absorption coefficients Phytoplanktonic absorption can be predicted from chlorophyll a concentrations estimated from satellite-based ocean color measurements Temperate and tropical regions exhibited statistically indistinguishable relationships at low chlorophyll so these regions were combined and treated as one Nonlinear relationships between phytoplanktonic absorption at both 436 and 676 nm and chlorophyll a concentration for the combined temperate/tropical region suggested that pigment packaging effects were important and variable Higher slopes between absorption and chlorophyll a at low chlorophyll supported the concept of low pigment packaging effects (thus higher specific absorption) in oligotrophic, low chlorophyll a waters Subpolar waters displayed a distinct pattern and were defined as a separate region Near-linear and linear relationships between phytoplanktonic absorption at 436 and 676 nm and chlorophyll a concentration indicated that influences of pigment packaging on phytoplankton specific absorption coefficients were relatively constant and uncoupled from water column chlorophyll a concentration in the subpolar region Optical depth correlated inversely with specific absorption at 436 nm in the subpolar region, illustrating the role of photoadaptation in determining specific absorption and predictive relationships Differences between predictive quadratic equations for particulate and phytoplanktonic absorption indicated that detritus made only a small contribution to the nonlinear nature of the relationships Absorption by detritus at 436 nm ranged from 25 to 90% of the total particulate absorption The proportion of absorption by detritus did not exhibit any strong patterns as a function of chlorophyll a concentration, arguing against the previously invoked explanation that proportional increases in absorption by detritus as environments become more oligotrophic cause the observed nonlinearity between absorption and chlorophyll a concentration

Does silicon exist in association with organic compounds in rice plant

Abstract: Silicon-organic compounds in the cell walls of rice seedlings grown in solutions with different levels of CaCl2 and with or without silicon were investigated by IR and UV absorption spectroscopy. Levels of the lignin-carbohydrate complexes (LCC) in cell walls decreased by silicon deficiency. A bathochromic effect, namely, shifting of the absorption maximum to a longer wavelength, a hyper-chromic effect, evident as an increase in the maximum absorbance in the absorption spectrum, and an absorbance band at 940 cm-1 in the IR spectrum, suggesting the combination between an aromatic ring and Si-O, were observed in the LCC from the cell walls supplied with silicon, while only a hyperchromic effect was observed when calcium was supplied. Thus, in the cell walls of rice plants, evidence was provided for the possible presence of silicon combined with some organic compounds.