The Wonders of Raman Spectroscopy

Abstract

The simplest, most common light scattering process is that of Rayleigh scattering. In this case an incident photon, of frequency V0, is absorbed by an atom or molecule—pumping it up to an excited electronic state. This will be an unstable configuration, and the atom or molecule will decay back to the original state—releasing a photon of identical frequency as that of the incident photon. Since the incident and emitted photons have the same energy (frequency), Rayleigh scattering is an elastic process. In contrast, Raman scattering is an inelastic process, arising from transitions between adjacent vibrational levels of a molecule. When a sample is illuminated by a laser beam, both Rayleigh scattering and the much weaker Raman scattering occur. The intensity of the Raman scattered light is ~10^5 to 10^6 of the incident beam. This Raman scattering consists of frequencies V0 ± Vvib, where Vvib is the frequency of some vibration of the molecule. Thus, what is observed in Raman spectroscopy are two separate peaks: the Stokes (V0 − Vvib) and anti-Stokes (V0 + Vvib) peaks. Raman spectroscopy measures the vibrational frequency Vvib as a shift away from the incident frequency V0. This methodology has been extensively used on organic and inorganic systems. We will explore how Raman spectroscopy, unlike spectroscopic systems such as FTIR (Fourier Transform Infrared Spectroscopy), enables observations of both vibrational features as well as defects, therefore allowing identification of nanoscale structures and bonding in selenium, tellurium, and nanotubes.