  Rubidium adsorption on the surface of the topological insulator Bi$_2$Se$_3$
is found to induce a strong downward band bending, leading to the appearance of
a quantum-confined two dimensional electron gas states (2DEGs) in the
conduction band. The 2DEGs shows a strong Rashba-type spin-orbit splitting, and
it has previously been pointed out that this has relevance to nano-scale
spintronics devices. The adsorption of Rb atoms, on the other hand, renders the
surface very reactive and exposure to oxygen leads to a rapid degrading of the
2DEGs. We show that intercalating the Rb atoms, presumably into the van der
Waals gaps in the quintuple layer structure of Bi$_2$Se$_3$, drastically
reduces the surface reactivity while not affecting the promising electronic
structure. The intercalation process is observed above room temperature and
accelerated with increasing initial Rb coverage, an effect that is ascribed to
the Coulomb interaction between the charged Rb ions. Coulomb repulsion is also
thought to be responsible for a uniform distribution of Rb on the surface.
