i have recorded raman spectra of strontium oxide thin film. the spectra were recorded in the range 400 to 4000 cm-1. i have reference to peaks upto 1000cm-1. but i couldn't find literature for higher wavenumber. and the raman spectra is usually given in terms of wavenumber. also what is the significance the raman is recorded in wavenumber and not in wavelength?
The Raman effect is an inelastic scattering of photons, hence the absolute wavelength of the Raman peaks will depend on the used laser wavelength. Therefore, Raman peaks (stokes or anti-stokes) for a green laser will be found at different absolute positions than those for a red laser. However, due to energy conservation (and for most cases) the wavenumber shift (or frequency shift for that matter) is independent of the used laser wavelength.
The usage of wavenumbers in cm^-1 is for historical and practical reasons. The Raman effect is considered to be complimentary to IR spectroscopy, where the use of wavenumbers in cm^-1 is quite common. This therefore allows an easy and fast comparison. Further, typically wavenumbers for Raman shifts or IR spectra between typically 0-4000 cm^-1 are observed, making this a very practical unit for graphs, plots and data comparsion due to the easy to get numbers. In principle, you could give the shift in frequencies as well, which although often leads to large numbers.
Finally to your material: For many solid state systems wavenumbers of less than 1000 cm^-1 are usually observed, as phonons usually show relatively "soft" restoring forces. This is in contrast to molecular vibrations. Here, very strong binding forces, e.g. think about C-O or C-double and tripple bonds, are common, which have extremely strong restoring forces and hence, very high wavenumbers in the 2000-3000 cm^-1 range. As Raman is a quite common technique in chemistry as well, many spectrometers are made with exactly those wavenumbers in mind.
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