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- When monochromatic light is directed on a molecule, the light can be scattered or absorbed. Most of the scattered light will be at the same frequency as the incident light. This is known as Rayleigh scattering or elastic scattering. However, a small fraction of the light (~1 in 107photons) will be inelastically scattered at frequencies different from the incident photons. The energy difference between the incident and scattered light is proportional to the vibrational energy of the scattered molecules. This process of energy exchange between scattering molecules and the incident light is the Raman effect
Pic.1 Raman Effect
Pic.2 Intensity of Raman band
Because most of molecules exist at ground state, as you see from the above graph,
by tracing the intensity of Stokes band, increase and decrease of molecules can be known
Consequently, a location of Stokes band shows energy of vibration mode and an intensity
of band shows quantity of molecules. Finally, we can examine the properties and
structure of the samples, obtaining Raman Spectrum.
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Beam Quality: M2 < 1.2
On Spectroscopic analysis, the laser beam should be focused on the surface of samples using objective lens for obtaining the optimal signal. The quality of laser beam is related to spot size forming on the surface of the sample. Therefore, in case of micro Raman analysis which requires the minimum spot size, M2 should be the below 1.2.
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linewidth : linewidth < 0.01nm
Linewidth is very much related to resolution of spectroscopic analysis. If you want to obtain the analysis data without line broadening, linewidth should be lower than the resolution of spectroscopic analysis. Generally, the resolution of Raman spectroscopy is 1~10cm-1 (0.06~0.6 nm for 785nm), so maximum resolution can be obtained when the linewidth of laser is below 0.01nm.
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wavelength stability < 0.01nm
Wavelength stability is connected with stability of peak location on Raman spectroscopy, which can be very stable in the same condition of resolution
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spectral purity : SMRR and ASE > 90dB
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Raman Spectrum comparison
- Sample : Naphthalene
- Laser filter wasn’t used
- Test in the same environment condition
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Pic.2 Intensity of Raman band
