The choice of laser wavelength used in Raman spectroscopy can have a significant impact on the resulting Raman spectra. The most important factors that are affected by the laser wavelength are the intensity of the Raman scattering signal, the fluorescence background, the resolution and the specificity of the measurement.
Intensity of the Raman scattering signal: The intensity of the Raman scattering signal is directly proportional to the fourth power of the laser wavelength. This means that using a laser with a shorter wavelength will increase the intensity of the Raman scattering signal, making it easier to detect weak signals from samples.
Fluorescence background: Fluorescence is a type of scattered light that can interfere with the Raman measurement and can be caused by the excitation of the sample with a laser. The fluorescence background is generally higher at shorter laser wavelengths, making it harder to detect weak Raman signals. Using a laser with a longer wavelength will decrease the fluorescence background, making it easier to detect weak Raman signals.
Resolution: The resolution of the Raman spectrum is inversely proportional to the laser wavelength. This means that using a laser with a shorter wavelength will increase the resolution of the measurement and make it possible to resolve close peaks in the Raman spectrum.
Specificity: Different chemical compounds have unique Raman scattering cross sections and different groups of chemical compounds will have different scattering cross sections at different laser wavelengths. This means that using a specific laser wavelength will increase the specificity of the measurement for some compounds and decrease it for others.
In conclusion, the choice of laser wavelength used in Raman spectroscopy is a trade-off between sensitivity, resolution, fluorescence background and specificity. The choice of laser wavelength should be made based on the specific problem being studied, the type of sample, and the available equipment. Additionally, it is important to take into account that Raman spectroscopy can also be performed with multi-wavelength excitation, where more than one laser wavelength is used, this approach offers a more comprehensive data and also helps to reduce the fluorescence background.