Raman

Raman scattering spectra are recorded for the inelastic scattering of a light on the materials. When light is scattered from a molecule, most photons are elastically scattered. The

Figure 12. HRTEM image showing the shell/core structure of Fe(B) nanocapsules with an amorphous boron oxide shell of about 4.0 nm thick and a crystalline Fe core with d = 2.0268 A. After [122], Z. D. Zhang et al., Phys. Rev. B 64, 024404 (2001). © 2001, American Physical Society. Image courtesy of J. G. Zheng, Northwestern University, USA.

Figure 12. HRTEM image showing the shell/core structure of Fe(B) nanocapsules with an amorphous boron oxide shell of about 4.0 nm thick and a crystalline Fe core with d = 2.0268 A. After [122], Z. D. Zhang et al., Phys. Rev. B 64, 024404 (2001). © 2001, American Physical Society. Image courtesy of J. G. Zheng, Northwestern University, USA.

Figure 13. HRTEM image showing the shell/core structure of Gd(C) nanocapsules with a graphite shell and crystalline Gd core. The shell is characterized by curved lattice fringes of interplanar spacing 0.34 nm corresponding to the (0002) lattice plane of graphite carbon.

scattered photons have the same energy (frequency) and, therefore, wavelength, as the incident photons. However, a small fraction of light (approximately 1 in 107 photons) is scattered at optical frequencies different from, and usually lower than, the frequency of the incident photons. The process leading to this inelastic scatter is termed the Raman effect. Raman scattering can occur with a change in vibra-tional, rotational, or electronic energy of a molecule. The difference in energy between the incident photon and the Raman scattered photon is equal to the energy of a vibration or a rotation of the scattering molecule. A plot of intensity of scattered light versus the energy difference is a Raman spectrum.

The Raman effect arises when a photon is incident on a molecule and interacts with the electric dipole of the molecule. In classical terms, the interaction can be viewed as a perturbation of the molecule's electric field. In quantum mechanics, the scattering is described as an excitation to a virtual state lower in energy than a real electronic transition with nearly coincident deexcitation and a change in vibra-tional (or rotational) energy. The scattering event occurs in 10-14 seconds or less. Numerically, the energy difference between the initial and final vibrational (or rotational) levels, V, or Raman shift in wave numbers (cm-1), is calculated through

0 0

Post a comment