Two general solutions are applied to avoid strong heating effects in Raman scattering of highly absorbing materials. They are both based on the same principle of moving the focus spot relative to the sample surface, which decreases the illumination time per sample volume. The first way is a fast movement of the sample, as is realized in the most popular technique by sample rotation, but it is obvious that this method is difficult to apply in a wide temperature and pressure range. The second way offers a solution for applications in temperature and pressure cells by scanning the beam across a fixed sample. Here we found two realizations in the literature. One method is to move a lens in an optical system in backscattering geometry. It moves the focus across the sample by moving a lens and using an off-axis beam. This arrangement cannot use the whole aperture of the monochromator, so that it is characterized by decreasing intensity and resolution. The other method is scanning the focus across the surface of a sample by using a rotating refractor plate. Here the disadvantages are, first, the astigmatism of the focus and, second, the change of the image on the entrance slit while the plate is tilted. In order to investigate highly viscous liquids and glass transition phenomena in a wide temperature range, we developed a new technique, which is moving the laser beam by a rotating mirror.
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