I will just give a hint for the answer: Raman is a scattering process in which the income light strikes the atoms. So, you will have some interesting features when you detected the outcoming light. The energy difference between the outcoming and incoming light is related to its absorption by the atoms which you can describe it not only in terms of their vibration (the most common situation), but also by some electronic and/or electronic+vibration excitation, depending on the amount of the absorbed energy. The easiest part of an interpretation of the Raman spectra is related to the atomic vibrations, once the theory is well developed and accurate for that.

They are excited for very short time (to so called virtual level) and relaxed to some of vibration sublevels of ground state (for Stokes component), if you have real level to excite to, you'll get resonant Raman scattering.

I believe your question relates to the difference between molecular excitation and atomic excitation?  Mainly Raman is a molecular excitation technique.  The Raman photons represent the difference in energy levels between vibration modes of a molecule.  No molecular bond -- no Raman!  This is why there is no Raman signal for He, Ar, and so forth.  One exception is hydrogen.  For hydrogen you see atomic effects, not because of atomic excitation but because there exists different spin isomers of hydrogen.  So you get more Raman bands than you would expect from a simple diatomic molecule.  Bottom line -- for commonly used Raman lasers, there is no atomic excitation.

Thanks for all who's on  comment  on the thread .

@ Patrick Wiegand, yes exactly, I'm confused between of the molecular excitation and atomic excitation and effect of Raman shifts, when atoms excited the results is molecules vibrated, i don't get your idea about Argon and Helium ?

Atoms do not get excited in molecular vibrations, the bonds between the atoms are what is affected.  When at atom is excited the electrons surrounding the atoms move into higher energy orbitals.  For example, atomic spectroscopy using plasmas or flames cause excited atomic states, and spectroscopies based on this use atomic lines.  Raman shift normally results from the bond energies changing.  Think of a tightly stretched rope beteen two points.  The endpoints are the atoms and the rope is the bond.  The lowest "rope energy" is where the center goes up and down.  However, if you put more energy into the rope you can get it to vibrate so that the center is still and the points 1/4 and 3/4 of the distance along the rope are going up and down.  With molecules, the atoms do move, but not because they are excited.  They move because the bond between them is in an excited state.  Hope this helps.

I believe you can perform Raman on atoms but these are performed using microwaves.

http://quantum-technologies.iap.uni-bonn.de/en/diplom-theses.html?task=download&file=89&token=61410be31efd891035e7832a473728af

Though this is stimulated raman spectroscopy so perhaps not exactly related to your question, though worth adding here.

Affar Shahid Karimullah , thank you so much for your concern and answer,

this question was for 5 years ago since April 16, 2014 .

thank you so much

Mustafa