Laboratory measurements of light scattering by single levitated ice crystals

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 degrees > T > -10 degrees C, were typically approximate to 50 mu m in diame ter and tended to orient with the c axis either nearly vertical (parallel t o the scattering plane normal) or horizontal (in the scattering plane). Hel ium-neon laser light scattered by a levitated crystal was collected in the angular ranges 20 degrees-65 degrees and 115 degrees-160 degrees with a 102 4-element linear photodiode array with an angular resolution of about 0.05 degrees. The particle size and orientation were measured a few seconds befo re and after the scattering measurements with top- and side-view video tele microscopes. Three basic features are found in the scattering from vertical ly aligned crystals: (i) a strong "halo" peak between about 21 degrees and 35 degrees, (ii) a secondary peak with ripple structure between about 30 de grees and 70 degrees, and (iii) a weaker peak in the backscatter between 11 5 degrees and 160 degrees. The ripple structure is interpreted in terms of two-beam interference and is shown to provide a sensitive measure of crysta l dimensions. The experimentally measured peak positions are compared with a simple model, and the authors discuss the effects of surface roughness, c rystal imperfections, and tilted orientations on the measured scattering cr oss sections.