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In Search of Strong Infrared Extinction in Aerosols
ARMY ARMAMENT RESEARCH AND DEVELOPMENT COMMAND ABERDEEN PROVING GROUND MD CHEMICAL SYSTEMS LAB
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When particle size becomes small compared to wavelength, extinction not only depends on geometric cross sections but also becomes a very strong function of complex refractive index. Regions of resonant extinction were located in the complex refractive index plane for a variety of spheroidal and confocally coated spheroidal particles using the Rayleigh ellipsoidal approximation. The strongest most extensive is found for high aspect ratio metal prolate and oblate spheroids approximating thin needles and disks. Even stronger extinction was found to occur when metal needles and disks have a dielectric core. On the other hand a dielectric coating reduces extinction. The unbounded peaks of extinction resonances, located where the real part of the complex refractive index becomes zero, were proven to be inaccessible to any material obeying the Kramers-Kroenig relation which requires absorption at restrahl. Finally the six resonances of a cubic particle were explored in the complex refractive index plane. Because a cube is an example of where the Rayleigh ellipsoidal approximation fails most severely, the extinction was predicted using a result taken from the dielectric continuum and the lattice dynamical theories.
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