Basics of Raman Spectroscopy
When a sample is illuminated with a laser and the scattered light is dispersed with a spectrograph, the output of the spectrograph will show a strong line at the excitation wavelength, and weaker lines appearing on both lower and higher frequencies of the strong line. Light scattered at the incident wavelength is called Rayleigh scattering. The shifted features are called Raman Stokes and Raman anti-Stokes lines, respectively, and have their origin in the interchange of energy between the light and the molecules causing the scattering. Because actual electronic or vibrational states are not accessed in this process, the frequency shifts observed in Raman spectroscopy are independent of excitation wavelength.
In accounting for the origin of the observed shifts, both classical and quantum explanations are useful, but both descriptions rely on the property of polarizability. Polarizability is the relative tendency of a charge distribution to be distorted from its normal shape by an external electric field. It increases as the volume occupied by the bond electrons increases.
In the classical explanation of Raman spectroscopy, the electric vector of the incident illumination induces an oscillating electrical dipole, by virtue of the bond’s native polarizability, which subsequently emits radiation. If the atoms in the molecule are not moving with respect to one another, the induced dipole, and therefore the scattered light, has the same frequency as the excitation laser. If the molecule is in motion, vibrating or rotating, the amplitude of the dipole will depend on the positions of the atoms within the sample molecule. In this case, the rotational and vibrational frequencies of the molecule will influence the scattered light such that the outgoing radiation consists of light of frequencies equal to the sum and difference between the incident beam and anti-Stokes and Stokes features.
The quantum explanation is also related to a change in polarizability.
In both the classical and quantum descriptions of Raman scattering, actual scattering of light by molecules is governed by the relative polarizability of the electron distribution associated with the molecule. The Raman intensity is proportional to the square of the induced dipole moment (i.e. the square of the polarizability derivative).
If the vibration does not appreciably change the polarizability, the induced dipolemoment will be small, and the Raman emission feature weak. Vibrations involving bonds that are already polar In this description, the incoming photons promote the molecule into a higher lying energy state (usually a virtual state). The transition moment between the lower and higher states is the polarizability tensor. Energy available to the emitted, or scattered, photon is the energy of the excitation beam (ν0) plus or minus the vibrational state of the molecule prior to excitation (νvib). The scattered photons therefore have frequency of ν0+/- νvib. These processes are shown in Figure 1. The efficiency of Raman scattering is usually quite low; only about 1 out of every 106-108 incident photons is frequency shifted. Because near room temperature, most molecules are near their vibrational ground states, the signal intensity in the Stokes features is normally much larger than in the anti-Stokes. Indeed, for most applications, the anti-Stokes features are not even collected. (i.e. the electrons are shared unevenly between the atoms of the bond) before the arrival of the photon, such as C-O, N-O and O-H, are weak scatterers. Bonds that are relatively neutral, however, such as C-C, C-H and C=C, undergo large changes in polarizability during a vibration. These bonds have very active Raman features. This explains the strong Raman features of molecules with large pi bonds and ring structures, as well. Illustrative of this principle, a spectrum of benzene is presented in Figure 2. The feature at 992 cm-1 is the ring breathing vibrational mode.
Molecular fluorescence is sometimes competitive with Raman scattering. If the material under test has a component with a real electronic energy level accessible by the Raman excitation beam, a portion of the excitation beam will be absorbed. The electronic energy level will, like the ground state, have a plethora of vibrational/rotational energy levels. A gradual descent through the vibrationally excited levels of the upper electronic state will occur (intra-system crossing), and then a photon will be emitted as the molecule relaxes into the ground state. Because of the large number of vibrational states in both the excited and ground states, the outgoing photons will be emitted in a wide array of frequencies, and therefore a broad band. The tail of this broad band can overlap the Raman shifts of interest, making strategies to deal with sample fluorescence of extreme importance in Raman spectroscopy.