I am currently utilising Raman spectroscopy for the first time, and am struggling to identify some peaks.
I understand that Raman peaks are due to the inelastic scattering of light, whose wavelength corresponds to the vibrational/rotational modes of the sample, and by comparing my shifts to the references reported elsewhere, I am able to identify my material.
One thing I am struggling to understand, however, is the reasons behind shift locations, what causes it? Whats an acceptable shift? and what determines peak breadth?
If anyone could help me with these issues, I would appreciate it greatly.
Best wishes
Jamie
Raman spectroscopy measures the energies of the phonon excitations (vibrational and rotational excitations) in a system. In the lowest order of approximation, these excitations may be viewed as arising from a set of independent harmonic oscillators, with sharply-defined energy spectra. In reality, the phonon excitations are neither fully harmonic, nor independent of each other. Phonons interact with each other, which is another way of saying that phonon excitations are not fully harmonic, and with the electrons in the system. These interaction effects lead to the broadening of the phonon spectral lines. The ambient temperature also plays a role here, since even without any phonon-phonon and phonon-electron coupling, at any non-vanishing temperature what one measures corresponds to an ensemble average (measuring, or collecting data, takes time, which in practice is long in comparison with the inverse of the relevant frequencies, so that by ergodicity one's time-averaged measurements correspond to ensemble-averaged measurements -- over a thermal ensemble), and this leads to broadening of the spectral lines.
For details, you may wish to consult the following books:
[1] G Turrell, and J Corset, Editors, Raman Microscopy: Developments and Applications (Elsevier Academic Press, Amsterdam, 1996).
[2] P Brüesch, Phonons: Theory and Experiments, Vols. I, II, and III (Springer, Berlin, 1982, 1986, 1987).
Raman spectroscopy measures the energies of the phonon excitations (vibrational and rotational excitations) in a system. In the lowest order of approximation, these excitations may be viewed as arising from a set of independent harmonic oscillators, with sharply-defined energy spectra. In reality, the phonon excitations are neither fully harmonic, nor independent of each other. Phonons interact with each other, which is another way of saying that phonon excitations are not fully harmonic, and with the electrons in the system. These interaction effects lead to the broadening of the phonon spectral lines. The ambient temperature also plays a role here, since even without any phonon-phonon and phonon-electron coupling, at any non-vanishing temperature what one measures corresponds to an ensemble average (measuring, or collecting data, takes time, which in practice is long in comparison with the inverse of the relevant frequencies, so that by ergodicity one's time-averaged measurements correspond to ensemble-averaged measurements -- over a thermal ensemble), and this leads to broadening of the spectral lines.
For details, you may wish to consult the following books:
[1] G Turrell, and J Corset, Editors, Raman Microscopy: Developments and Applications (Elsevier Academic Press, Amsterdam, 1996).
[2] P Brüesch, Phonons: Theory and Experiments, Vols. I, II, and III (Springer, Berlin, 1982, 1986, 1987).
Thank you all very much for your feedback.
The peak location in question is around 790cm-1, where as in literature, the peak location for the bond I expect it is is reported between 802 to 830cm-1, is the shift in my data to great to be the same as those reported in the literature.
You are welcome David.
You do not give any specific details for one to form an idea of what you mean by 'to [too] great'. One can have for instance isotope shift, but again to make any sensible suggestion in this direction, one will have to know some details about your sample. What are also the temperatures, that in your measurements and that in the literature you are referring to? Increase in the temperature leads to softening of the phonon modes, ultimately to the melting. Etc.
- Badges
- Science topic