Raman spectroscopy is very useful for material structure analysis. However, some questions still bother me:
1. As shown in the attached figure, there are 2 peaks in the 0-200 cm-1. However, few authors have investigated these peaks. What are their origins?
2. The spectrum is highly asymmetric. Why?
3. How should you fit this spectrum by multi-peak fitting? Some people used asymmetric lineshape. However, the vibrational spectra seems to be symmetrical. Is this fitting correct?
Hello there,
unfortunately I can not answer question 1, but let me help on questions 2 and 3: The vibrational spectrum seems assymetric, but it consists of symetrically broadend vibrational modes (wagging, bending, streching, ring, etc...) . In this publication: https://www.researchgate.net/publication/259638541_Compositional_dependent_response_of_silica-based_glasses_to_femtosecond_laser_pulse_irradiation?ev=prf_pub one can find a list of the origin of these vibrational modes and a list of corresponding references (Tab2), as we fitted the peaks to the spectrum (Fig2) with "Fityk", an open source software.
Hope that helps,
Moritz
Article Compositional dependent response of silica-based glasses to ...
I have a guess as to the answer to #1, but first I need to know the specifics of the spectrometer. Do you have any information about the edge or notch filter used to reject your laser line from the sampled spectrum? Unless you have a monochrometer specifically designed for low wavenumber sampling, there is a good chance that these features are due to laser interference with the notch filter, rather than truly spectral characteristics, which tend to be much broader in the low wavenumbers.
By "interference" in this 0-200 cm-1 region, what I mean is that you are right on the edge of your notch filter, spectrally speaking. This means that the intensity of your scattered signal beam is rapidly decreasing from 100% to 0% as it approaches 0 cm-1 shift. Since (I assume) the spectrum you've shown is the average of several scans taken by your system, the software has averaged the values obtained for each wavenumber. In the 0-200cm-1 region, right at the edge of your filter, this averaging process is not reliable, due to this "interference" from the filter, which can cause the system to average very different values over the course of a multi-accumulation scan. This would not impact the rest of the spectrum, which is obtained in the 100% transmission region of your filter. Just a guess, but something I have seen on a number of different instruments and sample types.
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