Raman spectroscopy is a method that enables real-time reaction monitoring and characterization of compounds in a non-contact manner. This method is based on the so called Raman effect which describes the inelastically scattered light on elementary excitation of a solid material like lattice vibrations (phonons) or plasma oscillations (plasmons). The name is after the Indian physisist Raman who reported the effect in 1928.
In a Raman experiment the solid (see Figure 10.1), e.g. semiconductor, surface is illuminated by a monochromatic laser light and the resulting scattered light is analyzed. As a consequence of the scattering process, among the incident frequencies (Rayleigh scattering) also frequency lines are observed which are shifted relative to the incident light (Raman scattering). From the energy shift, intensity and polarization of the scattered light it is possible to get a lot of information about the elementary excitations and correspondingly about the properties of the solid itself like crystallinity, orientation, composition, mechanical stress, temperature, doping etc.
Figure 10.1: A schematic representation of the Raman effect.
From the classical point of view, the oscillating electric field of the incident light generates in the solid an oscillating electrical polarization, which at its turn is also a source for electromagnetic radiation, i.e. the scattered light. The relation between the incident electrical field and polarization is given by P=εE, where ε is the susceptibility. The susceptibility is modulated with the frequency of elementary oscillations is such a way that in the polarization and as a consequence in the scattered light, the difference (stokes) as well as the sum (anti-stokes) of frequencies of elementary excitations are present. Thus, the process is similar to a frequency modulation.
From the quantum mechanics point of view this can be explained as follows. The incident photon is absorbed and an electron-hole pair is generated. The generated electron or hole will interact with the lattice generating (stokes) or destroying (anti-stokes) a phonon. Subsequently, the electron-hole pair will recombine by generating a photon. Due to energy conservation the difference between the energies of the incident and scattered photons will be equal to the generated/destroyed phonon. This process is schematically described in Figure 10.2.
Figure 10.2: A schematic representation of the Raman effect.