Raman Spectroscopy is a non-destructive chemical analysis technique which provides detailed information about chemical structure, phase and polymorphy, crystallinity and molecular interactions. It is based upon the interaction of light with the chemical bonds within a material.
In Raman spectroscopy, laser choice is important. The laser wavelength chosen will impact Raman intensity, spatial resolution, background fluorescence, acquisition time, and the potential cost of a Raman system.
Lasers used in Raman spectroscopy range from the UV into the near-infrared. Different wavelength regions offer advantages and disadvantages with the final laser selection. Some samples can be analysed at any wavelength with no issues, for example toluene, but for many samples laser wavelength choice is crucial for high quality Raman spectra .
UV (254nm laser, ) and visible lasers (532nm laser, 638nm laser ) require shorter accumulation times and can be used at lower laser power than near-infrared lasers(785nm laser).
Figure 1 shows the difference in Raman intensity between a 638 nm laser and a 785 nm laser using a silicon sample under the same conditions.
The most commonly used laser wavelength in Raman spectroscopy is 785 nm which offers low fluorescence whilst retaining relatively high Raman intensity. However, for samples which suffer from large fluorescence backgrounds, such as dyes, a 1064 nm laser may be needed. This laser would generally only be used if the fluorescence is extremely high due to the reduction in Raman intensity and risk of sample damage from a more powerful laser.
Figure 2 shows the same material measured with two different excitation wavelengths (532 nm and 785 nm). The spectra reveal the fluorescence suppression that can be achieved when using the most appropriate laser. The 785 nm laser reveals peaks that are masked by the high fluorescence background seen when using the 532 nm laser.
Deep UV lasers below 300 nm also offer fluorescence suppression. This is because the Raman spectrum sits closer to the laser line and fluorescence tends to lie at higher wavelength due to Kasha’s Rule, preventing them from overlapping.
Raman spectroscopy requires a concentrated light source such as a laser to generate Raman signal from a sample, which is often referred to as a 'Raman laser' or 'Raman excitation laser'. Its characteristics impact the quality of the Raman spectrum which can be acquired.
Some diode lasers can be used for Raman Spectroscopy.
532nm DPSS Green Laser System for Raman spectroscopy Green Solid State Diode Pump Laser.
The most commonly used laser wavelength in Raman spectroscopy is 785 nm which offers low fluorescence whilst retaining relatively high Raman intensity. However, for samples which suffer from large fluorescence backgrounds, such as dyes, a 1064 nm laser may be needed.