A typical spectrum of Raman intensity as a function of wavenumber is shown in
Fig.9.16. The difference between the frequency of the incident laser light and that
of the Raman-shifted light is equal to the frequency of the vibrational bond that was
excited. Molecules of different substances have Raman spectral peaks at specific
frequencies, which are unique to the chemical bonds of that particular substance.
A Raman spectrum thus serves as a type offingerprint for specific biomolecules.
Traditionally the Raman frequency shifts are recorded in wavenumbers (in units of
cm−^1 ) with the spectrum of vibrations ranging from about 600–3000 cm−^1.
Based on Boltzmann distribution statistics, at room temperature the vibrational
ground state typically is significantly more populated than the higher vibrational
excited states. As a result, the intensity of the Stokes Raman spectrum tends to be
much higher than the anti-Stokes Raman spectrum. An example of this is shown in
Fig.9.17for CCl 4. Using Boltzmann statistics, the ratio of the anti-Stokes intensity
Fig. 9.16 A typical spectrum
of Raman intensity as a
function of wavenumber
Fig. 9.17 Anti-Stokes and
Stokes frequency spectra of
Raman scattering in CCl 4
based on argon laser
excitation (Reproduced with
permission from R. Menzel,
Photonics, Springer, 2nd ed.,
2007)
278 9 Spectroscopic Methodologies