Introduction to Spectroscopy - Vibrational Spectroscopy for Tissue Analysis - page 5

the scattered photon has less or more than the energy of the incident photon
(the energy is shifted up or down). This is inelastic or Raman scattering and
was first described in 1928 by Raman, who received the Nobel prize two
years later for his work in this field [11,12,13]. Photons that undergo elastic
Rayleigh scattering with wavelengths close to the laser line are filtered out
while the rest of the collected light is dispersed onto a detector. A very small
portion (1 in 10 10 ) [14] or (1 in 10 8 ) [15] of the light, however, is inelastically
scattered at a different wavelength to the incident light.
In theory, the Raman effect occurs when light hits a molecule and inter-
acts with the electron cloud and the bonds of that molecule. Subsequently,
a photon excites the molecule from the ground state to a different energy
level. When the molecule relaxes, it returns to a different vibrational state
and emits a photon. Due to the difference between the energy of the original
state and the energy of the new state, the emitted photon has a shifted and
different frequency from the excitation wavelength. If the final vibrational
state of the molecule is more energetic than the initial state, then the emit-
ted photon is shifted to a lower frequency in order for the total energy of
the system to remain balanced. This shift in frequency is referred to as a
Stokes shift [6,7,16,17]. In other words, the scattered photons can be red-shifted
by providing energy to the bond vibration (Stokes Raman scattering) [15]
If the final vibrational state is less energetic than the initial state, then the
emitted photon will be shifted to a higher frequency, which is designated
as an anti-Stokes shift [6,7,16,17]. Anti-Stokes Raman scattering is referred
to as blue-shifting [15]. Raman scattering is an example of inelastic scatter-
ing because of the energy transfer between the photons and the molecules
during their interaction. The difference in energy between the incident and
scattered photons corresponds to the energy of the molecular vibration [15].
If the proton has a higher frequency and therefore lower energy than the
incident light, this is known as Stokes-Raman and is due to the change in the
vibrational mode of the molecule. Figure 1.4 depicts different Raman scat-
tering processes. The Raman signal can be enhanced by up to 15 orders of
magnitude in this way [18].
Raman spectra are plots of scattered intensity as a function of the energy
difference between the incident and scattered photons, and are obtained
by pointing a monochromatic laser beam at a sample. When light strikes a
molecule, most of the light is scattered at the same frequency as the incident
light (elastic scattering). As mentioned, only a small fraction is scattered at
a different wavelength (inelastic or Raman scattering) due to light energy
changing the vibrational state of the molecule. The loss (or gain) in the pho-
ton energies corresponds to the difference in the final and initial vibrational
energy levels of the molecules participating in the interaction. The resultant
spectra are characterised by shifts in wave numbers (inverse of wavelength
in cm −1 ) from the incident frequency. The frequency difference between
incident- and Raman-scattered light is termed the Raman shift , which is
unique for individual molecules and is measured by the machine's detector