  Li metal has been deposited on the surface of a Ru(0001) single crystal
containing patches of monolayer-thick epitaxial graphene islands. The use of
low-energy electron microscopy and diffraction allowed us to {\em in situ}
monitor the process by measuring the local work function as well as to study
the system in real and reciprocal space, comparing the changes taking place on
the graphene with those on the bare Ru(0001) surface. It is found that Li
deposition decreases the work function of the graphene islands but to a much
smaller degree than of the Ru(0001) surface, as corresponds to its
intercalation below the graphene overlayer. Finally, the diffusion process of
Li out of the graphene islands has been monitored by photoelectron microscopy
using a visible-light laser.
