**31st Annual Meeting of the DPS, October 1999**

*Session 50. Labratory Results Posters*

Poster Group II, Thursday-Friday, October 14, 1999, , Kursaal Center
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## [50.05] Experimental Determination of Scattering Matrices of Olivine

*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
(Mg_{2y}Fe_{2-2y}SiO_{4} 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 (F_{ij}), 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
-F_{21}/F_{11}, 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

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