ORIENTATION IN RAMAN SPECTROSCOPY

Polarized Raman spectroscopy on oriented samples records atomic vibrations into a spectrum that can be related to the crystal structure. The goal of our study is to investigate whether we can take an unknown, unoriented sample, identify it and determine its orientation with one spectrum analysis. This is of particular importance in the Martian environment where rock forming mechanisms and textures can be inferred from Raman data, without the need to return any mineral samples.

In the general procedures of this experiment, an oriented crystal is mounted on a pin, polished, and placed onto the rotational spectrometer stage which allows precise rotation around the axis of the laser. We then take spectrum shots from at 10º intervals between each shot. The procedure is then repeated along all appropriate crystallographic axes.

We examined the Raman spectra of beryl, which has hexagonal symmetry. When collecting data down the c-axis, regardless of polarization, the same Raman spectrum was obtained, consistent with hexagonal crystal symmetry. Next, spectra were collected with the polarized laser pointed down the a, a*, and b axes. The results were again identical with each other, consistent again with hexagonal symmetry. However, when the laser is oriented along the a axis and the polarization is changed from c to the a-b plane, complex variations in peak intensities are observed. These variations will be analyzed to understand their systematics and provide a model for determination of orientation.

We also report oriented Raman spectrum of garnets, in particular almandine. Almandine is generally cubic, so there should be little change in Raman intensities as a function of orientation. This is observed for a sample of almandine from Ontario, Canada; but not for a sample of almandine from Gore Mountain, New York, USA. In this latter sample we find significant Raman peaks in the OH region.