Particle Extinction Research Articles

Laboratory Measurements on the Infrared Features of Interstellar Silicate Grains

Thin layers of an amorphous silicate coating were obtained by ion sputtering of crystalline olivine on KBr substrates. The dielectric functions of the coating were determined from reflection and transmission measurements in the wavelength range between 5 and 25 μ. The small particle extinction of the amorphous silicate shows a strong feature at 9.7 μ and a weaker one at 17.5 μ. In wavelength position both absorption bands agree with astronomically observed features.

Optoacoustic measurements of diesel particulate emissions

Infrared absorption by acetylene and diesel smoke particles was measured using a differential optoacoustic cell driven by a 3.4-W CO2 laser. Real-time particle mass concentration measurements are demonstrated. In addition we found the average particle extinction cross sections per unit mass of acetylene and diesel smoke were 0.94±0.1 m2/g and 0.83±0.2 m2/g, respectively, at the 10.6-μm wavelength.

Extinction by a spherical particle in an absorbing medium

The extinction cross section for an arbitrary spherical particle that is embedded in an absorbing medium is derived by considering that which is measured in an actual extinction experiment. The resultant “optical theorem” is similar in form to the optical theorem for particles in a nonabsorbing medium. Calculations of extinction by germanium particles that are embedded in a polyethylene medium at room temperature have been performed for far-infrared frequencies (50–200 cm −1). If the imaginary part of the medium refractive index is ignored, then particulate extinction is underestimated by factors of between 2 and 4 over the frequency range of interest for 0.02-μm-radius spheres. In addition, there is a significant shift of a bulk germanium absorption maximum, which is not predicted when the medium is assumed to be nonabsorbing. For larger particles, the relative error decreases but still may be significant.

Infrared extinction of heavy ion irradiated and amorphous olivine, with applications to interstellar dust

A smooth surface layer of highly disordered olivine, (Mg, Fe)2SiO4, has been produced by exposure of polished, natural olivine to a dose of 5×1016 cm−2 of 1.5 MeV neon ions from a Van de Graaff accelerator. The dielectric functions of the disordered silicate in the wavelength range from 8 to 30 μm have been determined from analysis of specular reflectance data, and extinction for Rayleigh particles of such disordered olivine has been calculated. Extinction measurements for amorphous olivine smoke collected on a substrate are also presented. The small particle extinctions of both kinds of structurally disordered olivine are shown to agree well with the main features of the absorption and emission spectra from interstellar grains in the 10 and 20 μm region.

Particle sizing by forward lobe scattered intensity-ratio technique: errors introduced by applying diffraction theory in the Mie regime

Hodkinson proposed that Fraunhofer diffraction theory could be used as an approximation to Mie theory in particle size measurement by a forward lobe scattering intensity ratio technique. This paper describes a technique for quantifying theoretical errors involved in this approximation. Theoretical errors are computed for monodisperse latex spheres in air and in water at angle pairs of 5 degrees /2.5 degrees , and 10 degrees /5 degrees , and 20 degrees /10 degrees . Errors oscillate with particle size in a manner related to oscillations in particle extinction coefficient and are generally within +/-40%. These are somewhat greater than suggested previously. Oscillations are stronger and errors generally larger in air than in water.

Atmospheric Particulate Properties Inferred from Lidar and Solar Radiometer Observations Compared with SimultaneousIn SituAircraft Measurements: A Case Study

Abstract Particulate size and height distributions, complex refractive index and mass loading have been measured and inferred from direct aircraft and indirect lidar-solar radiometer observations made during a unique joint experiment conducted the week of 18 November 1974 in Tucson, Ariz. The aircraft and lidar-solar radiometer measurements were first analyzed independently and the results were then intercompared. Vertical profiles of particulate extinction obtained from the lidar (monostatic) and aircraft measurements were found to be in excellent agreement on both a relative and absolute basis. Lidar (bistatic and monostatic) inferences of particulate mass loading agreed favorably with the aircraft mass monitor measurements. The aircraft and lidar (bistatic) size distribution determinations were found to be similar in shape and agreed in absolute value within an order of magnitude. The mean particle refractive index inferred from the lidar (bistatic) measurements (n = 1.40 − i0.000) agreed with the inde...

Optical properties of particulates

Optical properties of small particles of olivine (less than 0.1 μ) have been studied in the ultraviolet as an example of an insulating solid. Very little structure survives in the ultraviolet extinction curves for such small particles. By contrast ‘surface modes’, observed for graphite small particles in the ultraviolet and for olivine particles in the infrared, produce dominant and persistent structure in extinction. The general trend of optical properties of graphite is surprisingly similar to the behavior required to explain all features of the interstellar extinction and albedo curves from near visible to 1000 A. Measured extinction of small olivine particles in the infrared agrees with calculations based on newly measured optical constants, but dominant sharp structure in the 10μ region still presents a bit of a problem in explaining ‘silicate’ features in astronomical data.

Optical extinction of Ag particles in KCl crystals

The optical extinction bands due to silver Colloids in KCl crystals have been calculated theoretically taking into account the size distribution of the particle in Mie theory. The optical constants for the bulk metal have been modified to take into account the small size of the particles. The experimental absorption curve agree well with those calculated theoretically.

Method for the determination of the index of refraction of particles suspended in the ocean*

It is shown that the complex index of refraction of a given particle-size distribution may be calculated if the particle extinction coefficient and the particle absorption coefficient are known. If the particles are assumed to be nonabsorbing, a real index of refraction may be calculated from the ratio of light scattering at 45° from the forward for two wavelengths. Application of the method to two stations off Ecuador indicates that the particle index of refraction can be determined with sufficient accuracy to become an important parameter in the study of the oceans.

Detection and Sizing of Small Particles in an Open Cavity Gas Laser

The open cavity of a He-Ne gas laser oscillating at 632.8 microm was used to detect and size aerosol particles. This was done for two transverse modes of the laser, the TEM(oo) and TEM(o1) *. Plots of particle extinction loss vs laser power are shown to agree with the theory for an inhomogeneously broadened line under the condition of small saturated gain per pass. The TEM(ol)*, doughnut, mode is shown to be preferable for use in an operational instrument. Extrapolation of the measurement to particles smaller than the wavelength of the laser radiation is discussed.

Evidence of Particulate Extinction in the Near Infrared Spectrum of the Sun

Abstract The optical thickness a(λ) of atmospheric particulate matter has been investigated in a near infrared wavelength interval. The instrument (a monochromator with a Newtonian telescope) was located at 1100 m MSL on the Appennine chain. We found an increase of a(λ) by comparison with the Bouger-Lambert law with increasing solar elevation. The distribution of a(λ) as a function of wavelength, computed at different times, shows that particulate matter either forms locally in the morning or arrives from over the Po Valley. According to the theory of Deirmendjian condensation of water also occurs on the atmospheric particles.

Evaluation of a light transmission technique for testing a two-stage electrostatic dust precipitator

A light transmission technique has been employed for evaluating the performance of a two-stage electrostatic dust precipitator, using fly-ash as the test dust. Collection efficiency of the second (ie collector) stage of the precipitator was obtained in terms of the surface area of the dust as a function of the applied voltage. Experiments using light of various wavelengths throughout the visible region indicate clearly the limitations of the technique when the mean particle diameter becomes of the same order of magnitude as the wavelength of the light, and some semi-quantitative assessments are made with respect to the behaviour of the particle extinction coefficient in that régime. Finally, some results obtained with white light are compared with some efficiency values obtained in terms of dust weight by isokinetic sampling.

Solid Hydrogen Coated Graphite Particles in the Interstellar Medium--I

Solid para hydrogen coated graphite particles expulsion into interstellar medium from star formation regions, considering mantles stability and particles extinction efficiency, albedo and phase function

Graphite as Interstellar Matter

GRAPHITE consists of hexagonal crystals. Thus the optical constants depend on the direction of the incident light. For light with an electric vector parallel to the basal planes of the crystals graphite behaves like a metal. In the perpendicular direction, for light propagation through a crystal with an electric vector along optic axis, it behaves like a dielectric particle. Many calculations on graphite as interstellar matter have been made. In these cases the electric vector is in the basal plane. Lenham and Treherne have measured the optical constants for graphite with the electric vector along the optic axis (Figs. 1 and 2)1. These figures show a sharp minimum in the optical conductivity 2 nk/λ at a wavelength λ = 0.64µ and a maximum at the same wavelength in the real part of the dielectric constant n2–k2. In the computer programme of Q extinction for spherical particles, “n” and “k” have to be used. They are calculated from 2 nk/λ and n2–k2 at different wavelengths. The optical constants “n” and “k” have sharp maximum respectively minimum at λ = 0.64&µ which produces the typical appearance of the Q extinction curves (Fig. 3). The diagram shows a broad and deep depression around λ = 0.64µ between the wave-numbers l.5µ−1 and 2.0 µ−1 for different particle radii. No other material with a similar appearance has been reported which could be interstellar grains. Pure ice is also a dielectric material, but both “n” and “k” are independent of wavelength and give no depression in the Q curves.

Particle Sizing by Means of the Forward Scattering Lobe

The angular distribution of scattered light in the main lobe of the Fraunhofer diffraction pattern of a particle changes rapidly with its size, but is largely independent of its refractive index. Even when the particle is so small (diameter less than a few wavelengths) that its forward lobe must be calculated from the Mie theory, the relative angular distribution within the central part is found to be given approximately by the Fraunhofer formula with obliquity factor, even for particles smaller than one wavelength, except near a minimum in the particle extinction efficiency factor calculated from the Mie theory. A measurement of the ratio between the scattered intensities at a pair of convenient angles within the lobe can thus give a useful estimate of the size of spherical and nonspherical particles without knowing their refractive index. The sizing error caused by the narrowing of the lobe near an extinction minimum is acceptable for many practical purposes, and can in any case be detected by comparing the size estimates derived from measurements at two different wavelengths. Families of curves for sizing by this simple procedure have been prepared. Sideways scattering instruments have been preferred hitherto because of their easier construction. A realization of their possibilities for sizing unknown particles should, however, stimulate the development of suitable forward scattering instruments.

Some observations on light extinction by spherical particles

The definition of the particle extinction coefficient or efficiency factor, its behaviour according to the Mie theory and previous determinations of its value for spheres larger than the wavelength of light are briefly reviewed. Further measurements, of the extinction of monochromatic and white light by both monodisperse and polydisperse polystyrene spheres, are reported.

Interstellar polarization by absorbing particles.

view Abstract Citations References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Interstellar polarization by absorbing particles. Smith, Elske V. P. ; van de Hulst, H. C. Abstract absorbing cylinders is applied to test the hypothesis that such particles are responsible for the observed polarization. Among the requirements imposed by the observations on any theory of the interstellar polarization are the facts that the amount of the polarization is independent of the wave length and that there is a certain correlation with the reddening. This correlation is such that the reddening is always present when polarization is observed, though the converse is not true. The theory for cylinders is analogous to the Mie theory of scattering by spheres. The efficiency factor for extinction varies with the direction of the electric vector of the incident light. The extinction is greatest when the electric vector is parallel to the cylinder axis, and least when it `is perpendicular. The ratio of the greatest to the least efficiency factor can be considered to define the amount of polarization. In computing the efficiency factors of cylinders we sum over the coefficients of extinction which appear in the solution of the scalar wave equation. These coefficients are in turn calculated from certain phase angles. The phase angles had already been calculated previously' for some real refractive indices. Our problem was to calculate the efficiency factors for slightly absorbing cylinders. A representative refractive index for such particles is m = 1.50 - 0.10 ~, for which the imaginary part is small. The computations were greatly simplified by using the results for real refractive indices and calculating the changes of the phase angles with small changes in the index. A graphical method of solution was used. Curves are given for theoretical extinction of particles with the refractive index I .500. lOt for the electric vector parallel and perpendicular to the cylinder axis, respectively. These curves follow an approximate I/N law such as that observed for the reddening. These particles could therefore produce the observed reddening. Curves were also calculated by Mr. Grootendorst and the second author for m = 1.414-1.414 t, a value representative for metallic particles. Comparison of the ratios of the extinction produced by the two alternative directions of the incident electric vector with the maximum observed values, indicate that the margin is even smaller for partially absorbing particles than for pure dielectric cylinders. It had been hoped that the opposite would be true, since the particles are presumably not perfectly aligned and may not be as long as we have assumed, and therefore would not show the maximum polarization as predicted by theory. I.van de Hulst, Ap. J. 112 1, 1949. Harvard College Observatory, Cambridge, Mass., and Leiden Observatory, Leiden, Netherlands. Publication: The Astronomical Journal Pub Date: 1951 DOI: 10.1086/106622 Bibcode: 1951AJ.....56R.141S full text sources ADS |