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2. EXPERIMENTAL METHODS 2.6 Raman Spectroscopy Raman scattering is a vibrational spectroscopy sensitive to molecular vibrations. The incident radiation absorbed by a molecule induces a polarization according to equation 2.4 P = αE. (2.4) where P is the strength of induced polarization, α is the polarizability of the molecule and E is the impinging oscillating electric field (75). The polarized molecule, a dipole, radiates a light scattered in various directions in an elastic or inelastic way. Once the ultraviolet (UV), visible or near infrared (NIR) radiation falls on a material, the vibrational levels of molecules are excited to virtual lev- els. While the molecule transitions back to molecular energy levels, a radiation quantum is emitted. Such scattering may occur in three different paths depending on the energy of emitted radiation - elastic Rayleigh scattering, and non-elastic Stokes and anti-Stokes scattering type. The frequency of radiation scattered by the molecule νR, the Raman shift, is calculated from equation 2.5 hνR = hν0 − hνL. (2.5) where ν0 is the frequency of incident radiation, νL - frequency of selected energy level, νR - frequency of Raman shift. In anti-Stokes scattering, the transition is from higher to a lower vibrational level, while in Stokes scattering transition occurs from lower to higher energy level. The collected variation of scattered intensities as a function of frequency represents the values of Raman shift that occur in the studied material. In normal conditions, it is more probable to collect Stokes than anti-Stokes Rayleigh scattering (98). Comparison between transitions in various types of vibrational spectroscopy are depicted in 2.13. The exciting radiation in vibrational spectroscopy is usually laser radiation. The laser radiation should be of intensity high enough for the Raman effect to be visible because the Raman scattering is typically about 10−10 the intensity of mid-IR absorption (75). Structural analysis in temperature-dependent mode (2 K/1min, about 1 K stabilization) has been performed from 293 K to 473 K (during the heating and cooling) using WITec confocal Raman microscope CRM alpha 300 R equipped with an air-cooled solid-state laser (λ = 532 nm) and a CCD camera. The excita- tion laser radiation was coupled into a microscope through a single-mode optical 56PDF Image | Investigation of metal-insulator transition in magnetron sputtered samarium nickelate thin films
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