Raman spectroscopy is based on the inelastic light scattering of a laser source in the near IR–UV range. Backscattered photons come out with a lower (ν₀ − ν_vibr) or higher (ν₀ + ν_vibr) energy with respect to the incoming laser photons (ν₀). The energy difference (ν_vibr) corresponds to the vibrational mode. A Raman active vibrational mode should involve a change in the electric polarizability. This powerful technique is widely adopted in the characterization of carbon-derived materials, both crystalline [e.g., HOPG (highly oriented pyrolytic graphite), nano-graphite] and amorphous (e.g., turbostratic carbon, carbon fibers etc.).

The Raman spectrum of perfect graphite (e.g., HOPG) with large crystalline domains is dominated by a sharp peak centred at 1580 cm⁻¹ (historically indicated as the G peak, first order, see Figure 4.1(a), top), which is attributed to a E_2g mode (Figure 4.1(c), part A). In addition, a complex envelope of bands appears in the 2800–2600 cm⁻¹ region (second order), mostly due to combination modes. In particular, two main peaks are observed in the spectrum of perfect graphite, originally indicated as G^′₂ (high frequency side, sharp) and G^′₁ (low frequency side, broad).^1–3