Raman Theory - PerkinElmer RamanMicro 300 Series Getting Started Manual

Raman Theory

When photons from a monochromatic light source impinge on a sample, a small fraction are
scattered in all directions. Some 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).
Sample
Lens Lens
Laser Photons in Laser Photons in
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).
Excited Energy
ν= 0
Rayleigh
Figure 4 Rayleigh, Stokes and Anti-Stokes scattering
Raman History, Theory and Instrumentation . 23
No change in energy
no useful information
Photons gain energy
but, signal is very small,
(anti-Stokes)
Shift from excitation line -
Shift from excitation line - in
in wavenumber/cm wavenumber / cm
Virtual Energy State
ν= 0 ν= 0
Stokes Anti-Stokes
Loss of energy
spectra (Stokes)
O
O
Wavenumber/ cm Wavenumber/ cm - 1 - 1
-1
- 1