Laboratory measurements of light scattering by single levitated ice crystals

Abstract

The authors have measured the differential light-scattering cross sections and phase functions of single vapor-grown hexagonal ice particles levitated in an electrodynamic balance. The ice particles, grown at temperatures -5° > T > -10°C, were typically ~50 μm in diameter and tended to orient with the c axis either nearly vertical (parallel to the scattering plane normal) or horizontal (in the scattering plane). Helium-neon laser light scattered by a levitated crystal was collected in the angular ranges 20°-65°and 115°-160°with a 1024-element linear photodiode array with an angular resolution of about 0.05°. The particle size and orientation were measured a few seconds before and after the scattering measurements with top- and side-view video telemicroscopes. Three basic features are found in the scattering from vertically aligned crystals: (i) a strong 'halo' peak between about 21°and 35°, (ii) a secondary peak with ripple structure between about 30°and 70°, and (iii) a weaker peak in the backscatter between 115°and 160°. The ripple structure is interpreted in terms of two-beam interference and is shown to provide a sensitive measure of crystal dimensions. The experimentally measured peak positions are compared with a simple model, and the authors discuss the effects of surface roughness, crystal imperfections, and tilted orientations on the measured scattering cross sections.