SPECTROSCOPY
Figure 2: Polar plots
of the biotite νTO4 stretching and δT6
modes in two areas of a black mica cross section. Left: Measurement positions. Right: Polar orientation plots with data fits (lines).
O6 ring
physical properties, exhibits a molecular fibre structure. In an experiment conducted at the Empa, polarisation-resolved Raman spectroscopy was employed to investigate the effects of a pulling force on the PE molecules in a plastic foil. The foil was stretched from an initial length of 15 mm to 40 mm, and a full polarisation series was recorded by obtaining a Raman spectrum at every degree (total 360 spectra per condition) using a 532 nm excitation laser and a 50x/0.55 LWD objective. The obtained Raman spectra
(Figure 1) exhibited characteristic peaks related to the vibrational modes of PE, including the asymmetrical C-C stretching (1063 cm-1
C-C stretching (1130 cm-1 twisting (1297 cm-1 1441, and 1464 cm-1 (2,849 and 2,883 cm-1
), CH2
), symmetrical ), CH2
bending (1417,
), and C-H stretching ). Notably,
after stretching, the Raman bands associated with CH2
bending and
C-H stretching showed polarisation- dependent intensity changes. The signals for the C-C stretching mode also exhibited polarisation-dependent effects, but in an inversed pattern. These findings suggest a pulling force-induced alignment of PE fibres, with the C-C and C-H bonds pointing to perpendicular directions.
BIOTITE MINERAL ORGANISATION IN BLACK MICA The crystallographic structure of minerals in a rock gives valuable insights into the geological formation processes. In this example, polarisation-resolved Raman spectroscopy revealed regions of different biotite alignments in a black mica biotite cross section that indicate separate phases in rock formation. Polarisation series were recorded from each 5 biotite grains of two areas of the cross section, obtaining one Raman spectrum every 5° through a full rotation, using a 532 nm excitation laser and a 50x/0.8 NA objective. The polar plots in Figure 2 display
the Raman intensities of the crystal’s νTO4
stretching (green) and δT6 O6
ring modes (blue) at the marked positions relative to the polarisation angle. For each individual position, a bipolar pattern is visible, caused by the anisotropic crystal structure in biotite grains. In region (i) (upper panel), the single patterns show no preferred angle in the polar plots, indicating a random orientation of the grains. In contrast, the polar plots from the measurement positions in area (ii) (lower panel) follow the same direction, with the νTO4
stretching mode perpendicular to the δT6
O6
ring modes. This reveals
the high alignment of biotite crystals in this region, a structure that was induced by high pressure during rock formation. The random organisation in region (i) originates likely from a later event of magmatic infiltration and undirected crystallisation.
SUMMARY Polarisation-resolved Raman analyses unveil information about molecular orientation, crystallographic alignments, and anisotropy in materials. A Raman microscope that enables controlling and selecting the polarisation angle of light is thus a valuable tool for structural analyses in diverse scientific fields.
AUTHORS: Thomas Meyer – Senior Application Scientist – Oxford Instruments
Ute Schmidt – Applications Manager – Oxford Instruments
Stefan Kreißl – Application Scientist – Oxford Instruments
Judith Beer – Technical Marketing Specialist – Oxford Instruments
For more information visit:
https://raman.oxinst.com
www.scientistlive.com 47
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