Mary Masterman - Oklahoma Biological Survey - University of ...

Raman Instrumentation
4
Raman systems are generally composed of several main parts, as follows.
A. The excitation source (laser).
Lasers make modern Raman feasible because they give a coherent beam of monochromatic light and can have very high power.
They must be of sufficient intensity to produce the desired and detectable amount of Raman scatter, and they should be free of
extraneous bands. They should thus exhibit good wavelength stability and low background emission.
The frequency of a Raman shift is independent of the laser wavelength used for excitation. This is an important concept in
Raman. In Raman, unlike electronic spectroscopy, wavelengths are not normally measured in Angstroms or nanometers. Since the
frequency shift, not the actual frequency, is what is important, Raman spectra are generally plots of intensity versus wavenumber:
, where c is the speed of light in cm/s and v is the frequency in s^-1. (4) Since it is independent
of the laser wavelength, the wavenumber will be the same for a 532 nm or a 785 nm laser. The wavelength difference for a
particular band, however, is not necessarily the same. For instance, a wavenumber of 1000 cm^-1 for a 532 nm laser would result in
a Stokes band ~30 nm away from the laser line. But the same wavenumber for a1064 nm laser would result in a Stokes band ~130
nm away from the laser line.
Intensity of the Raman signal is inversely proportional to the 4 th power of the laser wavelength (3):
. It would seem,
then, that laser radiation of a high frequency would be ideal. However, since samples tend to fluoresce at higher-frequency
wavelengths, and since fluorescence is a much more efficient process than Raman, this is not necessarily true. Thus, infrared lasers
have become increasingly popular in Raman, despite their low frequencies. On the other hand, it has also been demonstrated that at
wavelengths under 260 nm, there is essentially no fluorescence interference (3); so ultraviolet lasers have the benefits of stimulating
strong Raman scattering and not causing samples to fluoresce.
B. System to irradiate sample and collect scattered light (three types of filters).
Light from the laser is generally passed through an excitation filter in Raman systems. The excitation filter purifies the laser
beam by permitting light only of a very small selected wavelength range to pass through, eliminating background emission (1).
Excitation filters may be Short-Wave-Pass (only extraneous light of longer Stokes wavelengths than the laser is blocked) or Laser-
Line (extraneous light of wavelengths shorter and longer than the laser is blocked) (5).
After passing through an excitation filter, the laser beam is usually focused on the sample in order to increase the intensity of the
electric field on the sample surface (the magnetic component of the electromagnetic radiation is generally not relevant to Raman)
(4). Raman scattered light is then collected and passed through a barrier filter. The barrier filter prevents Rayleigh-scattered light of
the same wavelength as the laser form passing on to the slit of the spectrograph (the Rayleigh scattering is so much more intense
than the Raman that it would saturate the CCD chip).