  We report the Raman spectroscopy of 12C/13C graphene isotope superlattices
synthesized by chemical vapour deposition. At large periods the Raman spectra
corresponds to the sum of the bulk 12C and 13C contributions. However, at small
periods we observe the formation of mixed 12C/13C modes for Raman processes
that involve two phonons, which results in the tripling of the 2D and 2D' Raman
peaks. This tripling can be well understood in the framework of real space
Raman spectroscopy, where the two emitted phonons stem from different regions
of the superlattice. The intensity of the mixed peak increases as the
superlattice half period approaches the mean free path of the photo-excited
electron-hole pairs. By varying the superlattice period between 6 and 225 nm we
have a direct measure of the photo-excited electron mean free path, which was
found to be 18 nm for suspended graphene and 7 nm for graphene on SiO2
substrates.
