  Low-energy reflectivity of electrons from single- and multi-layer graphene is
examined both theoretically and experimentally. A series of minima in the
reflectivity over the energy range of 0 - 8 eV are found, with the number of
minima depending on the number of graphene layers. Using first-principles
computations, it is demonstrated that a free standing n-layer graphene slab
produces n-1 reflectivity minima. This same result is also found experimentally
for graphene supported on SiO2. For graphene bonded onto other substrates it is
argued that a similar series of reflectivity minima is expected, although in
certain cases an additional minimum occurs, at an energy that depends on the
graphene-substrate separation and the effective potential in that space.
