This is wavelength of the laser used(excitation). The Raman signature and the specific peak position of any material is related to the material’s unique chemical structure and is independent of the excitation wavelength, so the molecular fingerprint will be the same regardless of the excitation laser wavelength. However, different excitation wavelengths provide specific strengths and weaknesses allowing a user to optimize the measurement of different samples by their choice of Raman excitation laser wavelength. So how does one select a laser excitation wavelength for particular applications? There are many different excitation options, but the three most widely used are 532nm, 785nm and 1064nm. The most obvious difference is the excitation efficiency. Raman scattering efficiency is proportional to λ-4, where λ is the laser wavelength. For example, Raman scattering at 532nm is a factor of 4.7 more efficient than at 785nm and 16 times better than at 1064nm, effectively meaningthat scan time is much longer at higher wavelengths as compared to 532nm, assuming that all other conditions remain the same.

This is wavelength of the laser used(excitation). The Raman signature and the specific peak position of any material is related to the material’s unique chemical structure and is independent of the excitation wavelength, so the molecular fingerprint will be the same regardless of the excitation laser wavelength. However, different excitation wavelengths provide specific strengths and weaknesses allowing a user to optimize the measurement of different samples by their choice of Raman excitation laser wavelength. So how does one select a laser excitation wavelength for particular applications? There are many different excitation options, but the three most widely used are 532nm, 785nm and 1064nm. The most obvious difference is the excitation efficiency. Raman scattering efficiency is proportional to λ-4, where λ is the laser wavelength. For example, Raman scattering at 532nm is a factor of 4.7 more efficient than at 785nm and 16 times better than at 1064nm, effectively meaningthat scan time is much longer at higher wavelengths as compared to 532nm, assuming that all other conditions remain the same.

Hi Zeineb Raddaoui , I agree with Talha Nisar . Please take a look at the following application note for raman laser selection from bwtek:

http://bwtek.com/wp-content/uploads/2015/07/raman-laser-selection-application-note.pdf

And also the FAQs from Horiba:

http://www.horiba.com/us/en/scientific/products/raman-spectroscopy/raman-academy/raman-faqs/what-laser-wavelengths-are-used-for-raman-spectroscopy/

Hope this helps!

dear colleague

633 nm corresponds to the energy 1.95 eV

785nm corresponds to the energy 1.58 eV

if you use 633 nm you will observe just peaks whose the energy is under or equal to 1.95 eV

wheras if you use 785nm you will only obseve peaks whose the energy is under or equal to 1.58 eV

that meant that the laser choice depends from what you want to observe and which material you study.

regards

Dr A. Jbeli

Hi Zeineb,

The Raman signature and the specific peak position of any material is related to the material’s unique chemical structure and is independent of the excitation wavelength, so the molecular fingerprint will be the same regardless of the excitation laser wavelength. However, different excitation wavelengths provide specific strengths and weaknesses allowing a user to optimize the measurement of different samples by their choice of Raman excitation laser wavelength.

Further discussions about selection of a laser excitation wavelength for particular applications are given in the attached PDF document entitled "Choosing the Most Suitable Laser Wavelength For Your Raman Application".

From a pedestrian point of view, wavelength doesn't matter. But, Melek Hajji is right. It depends upon a few things and wavelength can matter a lot. Also, the two lasers may have a vastly different power and pulse characteristics. 633 nm suggests HeNe CW laser. 785 nm is maybe a femto-second fiber laser??? Big difference in what you can achieve. What you can get from Raman scattering depends greatly on your sample characteristics, too. Generally, the familiar inverse lambda to the fourth power rule is the first thing to look at and it suggests shorter wavelength. Your spectral peak widths measured by your system can sometimes be decreased as well, increasing resolution. However, fluorescence near 633 nm may drive you toward 785 nm in order to get away from it. Spatial filters in the optical receive end can help (confocal microscopy). Time-resolved Raman spectroscopy with fast pulsed lasers can be used to filter out fluorescence, which occurs in the ~ 500 nano-second time scale and longer. Resonant enhancement can occur near to the laser line (good), which can be seen with high quality spectral filtering, but it is tricky and highly sample and measurement system dependent. Because Raman is a tensor effect, polarization, excitation angle, observation angle, and the design of your specific measurement system can change the spectrum significantly.

I agree with everything above, and as it was already mentioned, the high energy lasers (632, 532 nm) may cause high intensity fluorescence. In case that your Raman machine is not equipped with pulse-mode lasers or other advanced techniques your only way to get rid of fluorescence will be or to decrease the excitation intensity or to move to the higher laser wavelengths. But still everything depends on Your samples and very often you need to optimize the measurement parameters individually for each sample.

Alright.

Thank you!