PerkinElmer Raman IdentiCheck, Getting Started Manual

Page: 29 / 78

Raman History, Theory and Instrumentation . 29
Raman Theory
When photons from a monochromatic light source impinge on a sample, a small fraction are
scattered in all directions. Most of the scattered photons are of the same energy as those of
the incident beam, and hence the same frequency, giving rise to Rayleigh scattering
(Figure 3).
Figure 3 The Raman effect: Irradiation of sample and subsequent scatter
Some photons will be inelastically scattered and will be detected at either longer wavelengths
(lower frequencies, ν ’, Stokes scattering) or shorter wavelengths (higher frequencies, ν ’’,
Anti-Stokes scattering). With Stokes scattering, some of the incident photons give up a
portion of energy to the molecules and are detected at a lower frequency (longer
wavelength) than the Rayleigh radiation (Figure 4).
Figure 4 Rayleigh, Stokes and Anti-Stokes Scattering
Virtual Energy State
Rayleigh Anti-Stokes Stokes
Excited Energy
ν
= 0
ν
= 0
ν
= 0
Wavenumber/ cm - 1
O
No change in energy
no useful information
Shift from excitation line - in
wavenumber / cm - 1
Int
e
ns
it
y
Wavenumber/ cm - 1
O
Shift from excitation line
-
in wavenumber/cm
−
1
Loss of energy
spectra (Stokes)
Photons gain energy
but, signal is very small,
(anti-Stokes)
Laser Photons in
Sample
Lens
Laser Photons in
Lens