**5. Outlook**

formalism developed by Albrecht et al. is commonly employed in the interpretation of the

**Figure 3.** Schematic representation of two electronic states (ground and excited) and their respective vibrational levels.

and ν<sup>s</sup>

the stokes and anti-stokes components, respectively) are the laser line and the scattered frequencies. The equation that describes the resonance Raman scattering is formed in the numerator part by transition dipole moment integrals between the electronic ground state (g, for the vibrational m or n states) and an excited electronic state (e, for any vibrational v states). The sum is done over all possible (e,v) states. The denominator part is the difference or sum of the scattered and incident light, added by the dumping factor (iΓev) that contains information about the lifetime of the transition states.

(the scattered frequency is composed of: νev,gm and νev,gn,

Generally, the tensor of polarizability is described as shown in **Figure 3**. The equation is formed in the numerator part by transition dipole moment integrals between the electronic ground state (g, for the vibrational m or n states) and an excited electronic state (e, for any vibrational v states). The sum is done over all possible (e,v) states. In the denominator part is the difference or sum of the scattered and incident light, added by the dumping factor (i Γev) that contents information about the lifetime of the transition states. This enormous intensification makes, in principle, the Raman spectrum easy to be acquired. But, in a state of resonance, a lot of radiation is absorbed, leading to a local heating and frequently can be observed a decomposition of the sample. Despite this problem, the RR spectroscopy has been largely used in the study of the different chromophore units present in many compounds varying from conducting polymers, nanocomposites, carbon allo-

resonance Raman [6–8, 16].

6 Raman Spectroscopy

The arrows indicated the possible transitions. In figure ν<sup>o</sup>

tropes to DNA [17–21].

Hence, this introduction tries to summarize the principles of main mechanisms of intensification of the Raman signal. These two effects (Resonance Raman and SERS) can be used separately or combined in the structural studies. In addition, the attachment of powerful microscopes with atomic/molecular resolution can also amplify the Raman signal, and this is the case of Tip-enhanced Raman spectroscopy (TERS). As a consequence, this combination opens the opportunity to study the Raman signal at unparalleled spatial resolution. In the present book, some examples of the state-of-the-art applications of Raman spectroscopy in characterization of materials and biomaterials, mainly through resonance Raman (RR) and surface-enhanced Raman spectroscopy (SERS) are deeply discussed.
