Experimental Phonon Band Structure of Graphene using C$^{12} and   C$^{13}$ Isotopes
  Using very uniform large scale chemical vapor deposition grown graphene transferred onto silicon, we were able to identify 15 distinct Raman lines associated with graphene monolayers. This was possible thanks to a combination of different carbon isotopes and different Raman laser energies and extensive averaging without increasing the laser power. This allowed us to obtain a detailed experimental phonon dispersion relation for many points in the Brillouin zone. We further identified a D+D' peak corresponding to a double phonon process involving both an inter- and intra-valley phonon. 
Simon Bernard, Eric Whiteway, Victor Yu, D. G. Austing, Michael Hilke
cond-mat.mes-hall