Observed Absorption Coefficients Research Articles

Size-dependent correction factors for absorption measurements using filter-based photometers: PSAP and COSMOS

Filter-based absorption photometers have been widely used to measure mass concentrations of black carbon (BC) by measurement of the absorption coefficient of BC. In these techniques, correction for the effect of multiple scattering by the filter medium is necessary, even if only BC particles are extracted by evaporating co-existing volatile compounds using a heated inlet. The correction depends on particle size, because it varies with the aerosol penetration depth into the filter. The size dependence has not, however, been taken into account in previous studies. For the first time, we quantify the particle size dependence of the sensitivities of two filter-based photometers, PSAP and COSMOS, using mono-disperse nigrosin particles, which were generated by the combination of a differential mobility analyzer and an aerosol particle mass analyzer. At diameters smaller than 200 nm, the absorption coefficients measured by PSAP and COSMOS were much larger than those calculated by Mie theory. The size-dependent correction factors for PSAP and COSMOS are determined by comparing the observed absorption coefficients at a flow rate of 0.7 standard liter per minute with those calculated by Mie theory. The correction factors to the mass absorption cross-section are also estimated for typical size distributions of ambient black carbon particles. The new factors reduce the mass absorption cross-sections measured by PSAP and COSMOS by 28–36% for typical ambient black carbon particles observed with an inlet heated to 400 °C.

Bio‐optical properties of the Labrador Sea

Three cruises were conducted during fall and spring in the Labrador Sea to investigate the effects of bio‐optical properties on satellite retrievals of phytoplankton chlorophyll in this important high‐latitude ecosystem. Taxon‐specific and regional differences were found. Diatoms had ∼1.5 lower chlorophyll‐specific absorption but significantly higher reflectance ratios than prymnesiophytes. Particulate absorption at 443 nm for total, phytoplankton, and “detrital” fractions was related to chlorophyll, but values were lower than reported for lower latitudes. Decreased particulate absorption is attributed primarily to pigment packaging, while low backscattering to scattering ratios result from a lower relative abundance of bacteria and picophytoplankton with more large phytoplankton. Soluble absorption was not related to chlorophyll. A four‐component model with low, variable backscatter fractions and the observed absorption coefficients for phytoplankton, “detritus,” and soluble materials reproduces the measured reflectance spectra. Global chlorophyll algorithms tend to underestimate biomass at high latitudes, whereas regionally tuned algorithms provide more reliable retrievals. Taxon‐specific algorithms show promise, but given limited ranges, small sample sizes, and overlapping reflectance ratios they remain premature.

Simultaneous electrodeposition of Cu–In–Se–Te thin films

Cu–In–Se–Te thin films have been produced by electrodeposition on molybdenum electrodes supported on glass substrates. The composition of the films is a function of the growth conditions. The best conditions for a near-stoichiometric composition are a deposition potential of −0.5 V vs. SCE and a time greater than 4000 s. After electrodeposition the samples are amorphous. However, crystallization of the films can be achieved by various thermal treatments, for example, annealing in a Te atmosphere at 550 °C for 30 minutes followed by annealing under vacuum at 400 °C for 1 h. This produces crystalline quaternary phases, as confirmed by X-ray diffraction. In order to study the optical properties of the films, some samples were grown on indium tin oxide (ITO) electrodes and annealed at 550 °C for 30 min. The observed absorption coefficients showed two regions corresponding to two energy gaps having values of Eg1 = 0.81 eV and Efg1 = 1.28 eV. The same films exhibited photoelectrochemical responses with flatband potentials of −0.15 V. All samples exhibited p-type conduction.

Line mixing effects in solar occultation spectra of the lower stratosphere: measurements and comparisons with calculations for the 1932-cm^−1 CO_2Q branch

Line mixing effects have been observed in a CO(2)Q branch recorded in 0.01-cm(-1) resolution IR solar occultation spectra of the lower stratosphere. The spectral data were obtained by the Atmospheric Trace Molecule Spectroscopy Fourier transform spectrometer during the Spacelab 3 shuttle mission in the spring of 1985. Analysis of the 1932.47-cm(-1) (11102)?(00001) band Q branch of (12)C(16)O(2) shows absorption coefficients below the band origin approximately 0.62 times those calculated using a standard Voigt line shape function. Calculations of line mixing using the Lorentz halfwidths of the lines and a simple energy gap scaling law to parametrize rotational energy transfer reproduce the observed absorption coefficients to ~10%. The present results provide the first quantitative information on air-broadened line mixing effects in a Q branch at low temperatures (~210 K) and show that these effects are significant even at the low pressures of the lower stratosphere (~100 mbar).

Self- and N 2-broadened spectra of water vapor between 7.5 and 14.5 μm

Self- and N 2-broadened absorption spectra of water vapor have been measured in the infrared between 7.5 and 14.5 μm at 294, 311, 333 and 339 K. The so-called continuum aspect of the observed absorption coefficients is interpreted in terms of existing theoretical models.

Line mixing in a N2-broadened CO2 Q branch observed with a tunable diode laser

Line-mixing effects have been observed in the infrared Q branch of the (1110,0310)I–0000 band of CO2 at 2076 cm−1. A tunable diode laser spectrometer was used to record spectra of CO2 broadened by N2 and O2 at total pressures ranging from 100 to 720 Torr. The observed absorption coefficients are up to 65% lower than those calculated using an isolated Lorentzian line approximation. A simple energy gap scaling law is used to determine the off-diagonal relaxation matrix elements from the known pressure-broadening coefficients. The spectra calculated using these matrix elements reproduces the observed absorption coefficients to within several percent.

Rotational collisional narrowing in an infrared CO2 Q branch studied with a tunable-diode laser

Rotational collisional narrowing, or line mixing, has been observed in the infrared absorption spectrum of the Q branch of the (1110,0310)II – 0000 band of CO2 at 1932.47 cm−1 using a tunable diode laser. Spectra were recorded at pressures ranging from 100 to 745 Torr both inside the Q branch and below the band origin. The observed absorption coefficients below the band origin were up to 80% less than those calculated using an isolated Lorentz line approximation. A simple fitting law for the state-to-state rotationally inelastic rate constants is determined using the known line broadening coefficients. A calculation of line mixing using these rates agrees with the observed absorption coefficients to within several percent.

The conduction bands in 6H and 15R silicon carbide II. Absorption measurements

It is shown, on the basis of classical theory, that the free carrier absorption in a uniaxial semiconductor having an anisotropic effective mass and relaxation time is characterized by two independent absorption coefficients. For a general multivalley model these absorption coefficients are shown to be quite complicated functions of the effective mass and relaxation time tensors; these functions are considerably simplified if the energy band extrema are located on the symmetry axis. In this case the anisotropy of the effective mass and also that of the relaxation time may be obtained from the free carrier absorption coefficients if these are measured at high frequencies (w 2 >r 2 ≫ 1) and also in the low frequency region (w 2 r 2 ≪ 1) in the same specimen. At low frequencies the free carrier absorption coefficients no longer have a near quadratic dependence upon wavelength but tend to limiting values. Results of measurements in the high frequency region are presented for a number of samples of both the 6H and 15R forms of silicon carbide. At wavelengths for which the absorption is due to free carriers the absorption coefficient for radiation with its electric vector (X) perpendicular to the symmetry axis (c axis) is greater than that for X || c by a factor of ˜ 4 in the 15R polytype and by a factor of ˜ 15 in the 6H form. The wavelength dependences of the absorption coefficients in the high-frequency region are discussed in terms of the probable scattering mechanisms. In one specimen of each polytype the m easurem ents were extended to ˜ 20 μm and the absorption coefficients were observed to approach limiting values as predicted by the theoretical expressions for low frequencies. Quite good agreement was found between the magnitudes of the calculated and observed absorption coefficients, and the onset of low-frequency behaviour was found to occur in approximately the wavelength region predicted by classical theory. From these results the effective m ass in the direction of sym m etry axis was found to be 1.5 ± 0*2 m 0 in the 6H polytype and ˜ 0-53 m 0 in the 15R polytype; the relaxation time appropriate to the axial direction was found to be greater than that for the transverse plane by a factor of 2.5 in the 6H form and 1.6 in the 15R form. Additional absorption bands for X ||c were found at 19.9 μm in 6H SiC and a t 17.5 μm in the 15R polytype and were of magnitude ca . 100 cm -1 and ca . 150 cm -1 respectively; their origin is as yet unknown. All the results reported relate to room temperature.

Elastic Constants of Single-Crystal Benzene

The elastic coefficients of benzene are determined from sound velocities measured with the use of an ultrasonic pulse technique over a temperature range of 170° to 250°K. At 250°K the results are (in units of 1010 dyn/cm2): c11 = 6.14; c22 = 6.56; c33 = 5.83; c44 = 1.97; c55 = 3.78; c66 = 1.53; c12 = 3.52; c13 = 4.01; c23 = 3.90. These elastic coefficients generally increase as the temperature is lowered. The observed absorption coefficients are relatively large, thus confirming data reported by L. N. Liebermann. However, it was found that the absorption is not isotropic and that the absorption for shear waves is comparable with that for longitudinal waves. Some of the experimental techniques used are described.

Elastic Constants of Benzene

The elastic coefficients of benzene have been determined from sound velocities measured with the use of an ultrasonic-pulse technique over a temperature range of 170° to 250°K. At 250°K, the results are (in units of 1010 dyn/cm2): C11=6.14; C22=6.56; C33=5.83; C44=1.97; C55=3.78; C66=1.53; C12=3.52; C13=4.01; C23=3.90. These elastic coefficients generally increase as the temperature is lowered. The observed absorption coefficients are relatively large, thus confirming data reported by L. N. Liebermann. However, it was found that the absorption is not isotropic and that the absorption for shear waves is comparable with that for longitudinal waves. Some of the experimental techniques used are described. [Supported by the U. S. Office of Naval Research.]

Absorption of Rayleigh Waves in Low-Loss Media

Many materials are characterized by internal dissipation parameter 1/Q«1. An expression is derived for such media relating the Rayleigh wave absorption coefficient to compressional and shear wave absorption coefficients with the elastic velocities as parameters. Ultrasonic experiments are described in which the three absorption coefficients are measured in thin Plexiglas sheets. The theoretically derived expression satisfactorily relates the observed absorption coefficients.

The Absorption of Sound in Air

A detailed review will be presented, summarizing available data on absorption of sound in homogeneous air and its main constituents as a function of frequency, pressure, temperature, and humidity. Disagreement exists between results of the various investigators, especially when different techniques are used. It is customary to compare experimentally observed absorption coefficients αexp with theoretical values αtheor, where the latter include the effects of viscosity, heat conduction, heat radiation, diffusion, and vibrational thermal relaxation. Observed values of r = αexp/αtheor vary from slightly less than unity up to about 4 for measurements in the laboratory and up to somewhat higher values for measurements out-of-doors. Even in carefully dried air, nitrogen, and oxygen, reported values of r vary over a wide range, depending on the measurement technique employed. It is interesting that in recent measurements, using direct source-to-receiver systems where the sound field is approximately a spherical wave, r values of about 1.2 are obtained. This approaches the result expected from classical kinetic theory for rotational relaxation.

The Absorption of High Energy Quanta. I.

The absorption of quanta between 11 and 20 Mev has been measured with the x-rays from the 22-Mev betatron as a source. The high energy tip of the x-ray spectrum has been isolated for these measurements by using a threshold detector method. The detectors used were copper, iron, and carbon, with which absorption coefficient values were obtained at 11.04, 13.73, and 19.10 Mev, respectively. Aluminum, iron, copper, and lead were used as absorbers. The observed absorption coefficients are in good agreement with existing theory except in the case of lead. The absorption in lead is smaller than predicted. This is probably due to the failure of the Born approximation in the calculation of the pair-production cross section.