Optical absorption studies of surface plasmons and surface phonons in small particles

Abstract

Starting from the Mie equations one can derive many resonant conditions for a sphere, including surface mode resonances. Surface phonon and surface plasmon modes are derived using Lorentz and Drude models of the dielectric function in the small particle theories. Infrared absorption studies of small particles dispersed in KBr matrices show, the expected sphere mode for amorphous SiO2 particles, but measurements on irregular shapes must be modeled with a distribution of shapes such as ellipsoids. Cubic MgO particles finally show the calculated surface phonons for cubes if enough effort is put into disrupting the particle aggregation. Surface plasmon absorption in Mg smoke particles shows a rare blue color. Because no sphere calculations produce this effect it is probably due to the nonspherical shape, perhaps with clustering. Size and shape distributions and clustering make it very difficult to observe quantum size effects in abosrption by ensembles of particles. Observations of apparently well isolated particles in interstellar space are interpreted as infrared emission from surface phonon modes in SiC showing shape effects, and ultraviolet extinction interpreted as surface plasmons in graphite. The origin of many narrow interstellar absorption bands in the visible is still a mystery more than 60 years after the first discovery.