Look for Kramers–Kronig relations, these are integral equations connecting refractive index and absorption (real and imaginary parts of the complex refractive index) together. To my knowledge, they are successfully used in far IR spectroscopy, while UV-Vis ranges have problems with applying them (probably, due to lack of high resolution spectra between FAR and NIR). I do not have ready references, sorry.

Thank you for your response. I have read about kramers-kronig relations. But does it really tells anything about vibrational contribution to refractive index? Can you provide me with a paper or book please? Thank you a lot

Well, I searched a bit and found two very old articles by my friend (one of the authors), who was doing this kind of measurements and calculations. My own research interests were always rather far from this area, so hardly I can give more than that.

DOI: 10.1002/pssb.2221280204

Article Vibrational Spectra of CdTe1-x-ySexSy Solid Solutions. Effec...

My friend Dr. Pyrkov says that he made calculations of refractive index, real and imaginary parts of permittivity from the reflection spectra of bulk samples, basing on formulas 1.4a and 1.4b from the following reference (click on 'PDF...' to get the article).

http://www.mathnet.ru/links/3ea58e258ef4965a36fa5333afbeef6e/mm2400.pdf

It's in Russian, but has an English abstract. All you need is formulas on p.91. Abstract introduces r(w), and N(x) is the free charges density in the semiconductor at point x for non-uniform media. The rest of the article is stating when (1.4b) is correct, and proving that.

He also recommends the following references from the above.

7. Robinson T.S. Optical Constants by Reflection //Proc. Roy Soc. (London). - 1952. - V. 1356.— P. 910-912.

8. Andermann G., Caron A., Dows D.A., Kramers - Kronig dispersion analysis of IR Reflections Bands // JOSA. - 1965. - V. 55, № 10. - P. 1210-1216.

Vladimir I Chukharev thank you a lot my friend. I will surely follow the articles you just sent me and it is a great help.