Laser-induced alteration of Raman spectra for micron-sized solid particles

The Raman Laser Spectrometer (RLS) instrument on board of the future ESAs ExoMars mission will analyze micron-sized powder samples in a low pressure atmosphere. Such micron-sized polycrystalline solid particles might be heated by the laser during the Raman measurements. Here, we report on the temperature-induced alteration of Raman spectra from micron-sized polycrystalline solid particles by comparing Raman spectra on silicon and the rock forming minerals olivine and pyroxene taken at different laser intensities and different ambient temperatures. Our analyses indicate that laser-induced heating results in both broadening and shifting of characteristic Raman lines in the Stokes and anti-Stokes spectral regions. For elementary crystalline silicon a significant local temperature increase and relevant changes in Raman spectra have been observed in particles with median sizes below 250 µm. In comparison, significantly weaker laser-induced Raman spectral changes were observed in more complex rock-forming silicate minerals; even for lower grain sizes. Laser power densities realized in the RLS ExoMars instrument should cause only low local heating effects and, thus, negligible frequency shifts of the major Raman lines in common silicate minerals such as olivine and pyroxene.