Experimental infrared spectroscopic measurement of light extinction for agglomerate dust grains

Optical properties of aggregate dust grains hold a key to understand many astrophysical problems. Scattering and absorption features are strongly influenced by morphological aspects of grains (particle size, shape, and agglomeration). We have experimentally investigated the morphological effects on the 9.0μm extinction band in the infrared region for amorphous silica (SiO2) particles which are of 0.5, 1.0, and 1.5μm in diameter. Deviations of the peak position of this band have been detected depending on environment (air or KBr), particle size, and manufacturer of the samples. We found that the peak position difference of silica particles with a diameter of 0.5 and 1.5μm was 0.04μm in wavelength whereas the effect of different environments were approximately 0.1μm. In addition, the measured band profiles were compared with calculations for five cluster shapes applying three different theoretical approaches (Mie, T-matrix, and DDA). The calculated extinction efficiencies beyond the peaks rose as the particle size and the particle numbers in the clusters increased. The same rise compared to the Mie calculation was observed in the experimental spectra, which was, therefore, interpreted as a result of the clustering of the particles.