  Though graphene has been intensively studied by Raman spectroscopy, in this
letter, we report a study of second-order overtone and combinational Raman
modes in an unexplored range of 1690-2150 cm-1 in nonsuspended commensurate
(AB-stacked), incommensurate (folded) and suspended graphene layers. Based on
the double resonance theory, four dominant modes in this range have been
assigned as 2oTO (M band), iTA+LO, iTO+LA and LO+LA. Differing to AB-stacked
bilayer graphene or few layer graphene, the M band disappears in single layer
graphene. Systematic analysis reveals that interlayer interaction is essential
for the presence (or absence) of M band whereas the substrate has no effect on
this. Dispersive behaviors of these "new" Raman modes in graphene have been
probed by the excitation energy dependent Raman spectroscopy. It is found that
the appearance of the M band strictly relies on the AB stacking, which could be
a fingerprint of AB-stacked bilayer graphene. This work expands the unique and
powerful abilities of Raman spectroscopy on study of graphene and provides
another effective way to probe phonon dispersion, electron-phonon coupling, and
to exploit electronic band structure of graphene layers.
