RESUMEN
Most of the Mie-scattering calculations have been done for a particle embedded in a nonabsorbing host medium. Generalization to an absorbing host medium can be achieved (a) by modifying the calculation of the spherical Bessel functions to account for a complex argument and (b) by accounting properly for the net rate of incident, scattered, and absorbed energy. We present an extended formalism of Mie scattering for the case of an absorbing host medium. Numerical calculations show that for a large spherical particle embedded in an absorbing host medium the extinction efficiency approaches 1 compared with 2 for a nonabsorbing host medium. We conjecture that this difference is due to the suppression of diffraction when the radius of the sphere is large.
RESUMEN
We derive and examine the general expression for the scattering asymmetry parameter g. For aggregate particles, the asymmetry parameter is made up of two terms. One term accounts for interference effects of the electromagnetic fields radiating from the individual subsystems. The other term contains the effects of the interaction of the electromagnetic fields between these subsystems. Enhanced backscatter is one phenomenon resulting from these interactions. Numerical results demonstrate that interference effects play a dominant role when the separation distance between two-sphere aggregates is smaller than half the incident wavelength. As the separation distance becomes large, both interference and interaction effects drop off and the asymmetry parameter approaches that of the individual particle constituents.
RESUMEN
We have carried out the transmission spectroscopy and obtained the imaginary part of the refractive index k of sulfate and nitrate aqueous solutions in the spectral range between 0.7 and 2.6 microm for several concentrations at temperatures of T = 24 degrees C and T = -24 degrees C. A linear interpolation with volume fraction is found to reproduce the measured k spectra of the ammonium solutions.
RESUMEN
New accurate values of the imaginary part of the refractive index k of polycrystalline ice at T = -22 °C are reported. The k spectrum in the 1.43-2.89-µm region was found to be in excellent agreement with the most recent study, and the data in the 3.35-7.81-µm range eliminate the large existing uncertainty in the 3.5-4.3-µm region.
RESUMEN
Effective medium theories are valuable tools that can provide effective refractive indices of composite media whose constituents are much smaller than the wavelength of the illuminating radiation. Extended theories have been developed to remove this limitation on the constituent size. Although these extended theories are not derived without additional limitations, useful regions of applicability do exist. We examine an extended effective-medium approximation and show that its predicted absorption efficiencies obtained agree well with exact theoretical results obtained for a naturally occurring system of interest, water droplets containing carbon inclusions.
RESUMEN
New accurate values of the imaginary part, k, of the refractive index of water at T = 22 °C, supercooled water at T = -8 °C and polycrystalline ice at T = -25 °C are reported. The k spectrum for water in the spectral region 0.65-2.5 µm is found to be in excellent agreement with those of previous studies. The k values for polycrystalline ice in the 1.44-2.50-µm region eliminate the large uncertainties existing among previously published conflicting sets of data. The imaginary part of refractive index of supercooled water shows a systematic shift of absorption peaks toward the longer wavelengths compared with that of water at warmer temperatures.
RESUMEN
The effect of absorption on microdroplet resonance emission line intensities was studied in 15-microm-diameter Rhodamine 6G/ethanol solution droplets. Absorption was controlled by varying the concentration of the additive nigrosin. Spectrally integrated intensities of resonant features are found to be proportional to a droplet cavity mode efficiency Q(a)/(Q(a)+Q(o)) expressed in terms of cavity output coupling and absorption factors Q(o) and Q(a), respectively. These Q's are determined from linewidths calculated from Lorenz-Mie theory by using combinations of the real and complex indices of refraction. An experimental upper limit of Q for first-order modes was determined to be 10(8) from the data.
RESUMEN
Laser-induced breakdown threshold intensities for helium, argon, xenon and clean air were measured as a function of pressure (p < 900 Torr) at wavelength lambda = 0.532 microm using the Nd:YAG laser with 6.5-ns pulse duration. Pressure dependence of the breakdown of a 50-microm diam water droplet in these gases was also investigated. For pure gases, different free electron generation processes and electron loss processes dominate in different pressure regions. The water droplets decrease the breakdown thresholds up to 3 orders of magnitude depending on the pressure of the particular gas surrounding the droplet. For the droplet in He, Ar, and clean air for p < 800 Torr, the breakdown at the threshold intensity occurs inside the droplet and is independent of pressure. For the droplet in Xe, the breakdown occurs inside the droplet for p < 140 Torr; however, for p < 140 Torr, the breakdown occurs outside the droplet and is dependent on pressure. Transition from the breakdown inside to outside the droplet takes place in the pressure region where the breakdown thresholds of the bulk liquid and the pure gas are approximately equal.
RESUMEN
We point out errors in the derivation of the periodicity of the interference structure of the Mie extinction curve, recently published in ABSORPTION AND SCATTERING OFLIGHTBY SMALL PARTICLES, by C. F. Bohren and D. R. Huffman (Wiley, New York, 1983).
RESUMEN
Aerosol-induced loser breakdown thresholds have been measured for liquid droplets at wavelengths lambda= 1.064, 0.532, 0.355, 0.266 microm using a Nd:YAG laser with 5-10-ns pulse duration. Breakdown thresholds are 2-3 orders of magnitude below those for clean air and range from 4 x 10(7) to 3 x 10(9) W cm(-2) for nominal 50-microm diam droplets, depending on laser wavelength and droplet composition. Thresholds decrease with decreasing wavelength; they also decrease for droplets having a higher real refractive index. For water droplets the breakdown threshold intensity varies approximately as lambda(0.5). The wavelength dependence of breakdown thresholds can be qualitatively explained by considering (1) the effect of enhancement of internal fields and energy density within and near droplets and (2) the increasing importance of multiphoton absorption processes at shorter wavelengths. Laser transmission losses through the breakdown plasma and observations of the suppression of stimulated Raman scattering by the addition of small quantitites of absorbing material to water and carbon tetrachloride droplets are also reported.
RESUMEN
The diameter and refractive index of micrometer sized spherical dielectric particles are simultaneously deduced using the wavelength dependence of backscattering data from optically levitated particles. The accuracy of the results is set by experimental errors in the determination of the wavelength of backscatter resonance peaks and the ratio of slopes of specified peaks. At present the refractive index and diameter can be deduced with relative errors of 5 x 10(-5). This represents the most accurate determination of absolute size and refractive index yet made by light scattering. A reduction of these errors by an order of magnitude is possible. We assume a priori knowledge of diameter and refractive index with accuracy of 10(-1) and 5 x 10(-3), respectively.
RESUMEN
The general belief that the sphere of equal volume provides a better approximation for the extinction cross section of a nonspherical particle than a sphere of equal surface area at small values of the size parameters is not correct. At some values of x, the equal volume sphere is a better approximation; at others, the equal surface area sphere is better. Details depend on the shape, size, and refractive index. For strongly absorbing particles at x > pi, the extinction cross section of an equal volume sphere sigma(EV) provides the lower bound, and sigma(EV)S(N)/S(EV) (where S(N) is the surface area of considered nonspherical particle, and S(EV) is the surface area of equal volume sphere) provides the upper bound on the extinction cross section of an arbitrarily shaped nonspherical particle.
RESUMEN
Absorption or extinction measurements at wavelengths of 0.5145 and 10.6 microm lead to determination of the mass concentration of carbonaceous smokes with an accuracy of ~20%. The results do not depend on the details of the particle size distribution or on particulate to void ratio.