Polarization effects of confocal Raman microspectrometry of crystal powders using interactive self-modeling analysis
Résumé
The collection by confocal Raman microspectrometry in the motorized point-by-point scanning mode of crystalline samples with heterogeneity larger than the spatial resolution (~1 m) generates large amounts of useable Raman spectral data. The present selected examples of fine, medium, and coarse crystal powders (anhydrite, CaSO4; calcite, CaCO3; gypsum, CaSO4·2H2O; and (NH4)2SO4) treated by the new Simplisma method combining conventional and second derivative data demonstrate the main role of the crystal size on the number and shape of the resolved spectra. With fine crystal powders (size less than 0.5 m), well-known powder Raman spectrum characteristics of each compound were extracted. These spectra were often superimposed on a weak baseline. With medium and coarse crystal powders (size more than 1 m), several significant spectra were extracted for each pure crystalline compound. These spectra were found to be analogous to the polarized spectra previously obtained with oriented large single crystal and were attributed to the several scattering contributions according to the symmetry of the several vibrational modes. The band splittings by the static and dynamic crystal effects can be resolved by the Simplisma approach using the pure variable concept according to the width of the splitting depending on the crystal. These results were explained by theoretical equations deduced from the Raman tensor elements and orientational functions of the crystal-fixed coordinates with respect to the space-fixed coordinates. These findings could provide accurate assignment of the resolved spectra extracted by the Simplisma approach from mixtures with large heterogeneities and can provide meaningful applications to Raman imaging of environmental and material samples.