13.3 Raman Spectroscopy Raman spectroscopy is based on inelastic scattering of light by matter. Monochromatic light scattered by matter contains radiations with frequencies different from the exciting light. This effect, predicted by Smekal (1923), was demonstrated by Raman (1928) and named after him [27]. A Raman spectrum is the plot of light intensity expressed as arbitrary units, or counts, versus the frequency of scattered light (i.e., Raman vibrational modes) expressed in frequency units (wave-numbers ~ν ¼ν/c ¼1/λ in cm 1, where c is the velocity of light). The Rayleigh scattered frequency (i.e., light-source wave-number) lies at 0 cm 1, whereas Raman frequencies are expressed as relative wave numbers, or Raman shifts (Fig. 13.4). In Raman technique, spectra frequencies correspond to the energy levels of different molecular vibrations and are independent from the wavelength of the laser beam. A spectrum comprises one or more bands which correspond to the vibration energies of the molecules of the analyzed sample; these in turn are related to the nature of the bonding. The most important molecular vibrations include stretching and bending modes, stretching frequencies being generally higher than bending frequencies. In order to obtain precise references for the test in compression on calcite specimens, some measurements with Raman spectroscopy before and after their fracture are carried out. The analyses are performed at the Interdepartmental Center “G. Scansetti” of Universita` di Torino where a Micro-Raman device is installed. The equipment is composed by an optical petrographic microscope (Olympus BX41), a motorized table sample holder, a spectrometer (Horiba Jobin Yvon HR800), a CCD detector, two polarized lasers (HeNe red—λ ¼633 nm, 20 mW; Nd solid state green—λ ¼532 nm, 250 mW) together with a set of interference filters, a console managing the laser and a PC workstation equipped with Labspec 5 software. In order to ensure the repeatability in the measurements pre and post fracture, on the three orthogonal faces of the specimens parallel lines of different colors (red, blue and black) are drawn (Fig. 13.5a). In total on a single intact sample thirty Raman spectra are acquired (Fig. 13.5b). No differences in terms of frequency shift between the Raman spectra on the three analyzed Anti-Stokes Anti-Stokes hν1 hν1 h(ν1+νm) h(ν1−νm) hν1 hν1 Rayleigh Scattering Stokes Raman Scattering n=2 n=1 n=0 Raman Scattering Rayleigh Raman shift Virtual energy level cm-1 0 Stokes Fig. 13.4 Raman spectra frequencies and energy levels corresponding to different molecular vibrations 96 G. Lacidogna et al.
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