[Previous] | [Session 50] | [Next]
O. Munoz (Dep. of Astron., Free University, Amsterdam and Inst. Astrof. Andalucia, CSIC, Spain.), H. Volten, K. Vermeulen, J.F. De Haan, W. Vassen, J.W. Hovenier (Dep. of Astron., Free University, Amsterdam.)
Mid- and far-infrared spectra have provided striking evidence for the presence of crystalline olivine in comets, circumstellar envelopes and planetary nebulae. In many cases the light scattering properties of these particles are required for the interpretation of observations of these objects. Since many data on the scattering behaviour of irregular mineral particles in general, and of crystalline olivine in particular, are lacking in the literature, we have measured the whole scattering matrix as a function of the scattering angle of a sample of Olivine (Mg2yFe2-2ySiO4 y=0.9) at two wavelengths (633nm and 442nm).
In our experimental setup we use a laser as a light source. This light is scattered by randomly oriented particles located in a jet stream. We employ polarization modulation in combination with lock-in detection to obtain all elements of the four-by-four Mueller matrix (Fij), which describes the scattering process. The elements depend on the wavelength of the laser light, the scattering angle as well as size, shape and complex refractive index of the particles. The sample has been prepared so that the measurements have been repeated for three different size distributions: d=6, 32 and 100 \mu m respectively, where d is the median diameter for volume equivalent spheres. One of the interesting results pertains to the element ratio -F21/F11, which equals the degree of linear polarization for unpolarized incident light. This shows negative values at angles close to the backward direction for the three size distributions and at both wavelengths. Such negative polarization has been observed for many objects in the Solar System.
We intend to compare the measured matrix elements with calculated elements using the T-matrix method.
The author(s) of this abstract have provided an email address for comments about the abstract: olga@nat.vu.nl