What is the difference between phonon-infrared scattering and molecular-infrared scattering? Are the both process same?,if not how do they differ in their mechanism,selection rules.
 A: First, you are talking about optical phonons, that are, out of phase movements of the atoms in the lattice, one atom to the left and one (its neighbour) to the right. This occurs if the lattice basis consists of two or more atoms. They are called optical, because, in ionic crystals, they are excited by infrared radiation.
On the other hand, molecular infrared scattering is a type of inelastic scattering. it is a common misconception, where people think materials are heated up because of only absorption of EM waves. The truth is, heating up happens mostly because of inelastic scattering of infrared wavelength photons. The molecules of the material gain vibrational energies from the photons.
To answer your question, the two scattering processes are not the same. 
Phonon infrared scattering is in your case with crystal lattice structured materials, that have phonons (out of phase movements in the lattice), and these scatter off infrared wavelength photons.
Molecular infrared scattering (Raman scattering) happens, when a material is heated up because the photons are inelastically scattered off the molecules, and the molecules gain vibrational (heat up) energy. In this case the molecules in the material do not have out of phase movements.
