  Bi$_2$Se$_3$ is a topological insulator whose unique properties result from
topological surface states (TSS) in the band gap. The adsorption of Cs onto a
Bi$_2$Se$_3$ surface is investigated by low energy ion scattering and work
function measurements. Much of the deposited Cs quickly diffuses to the step
edges forming one-dimensional chains of positively charged adatoms, along with
some deposition on the terraces. The work function decreases until a coverage
of 0.1 ML is reached, beyond which it increases slightly. The minimum in the
work function is due to depolarization of the dipoles induced when the
concentration of adatoms in the chains reaches a critical value. A slow
diffusion of adsorbed Cs from the terraces to the step edges is also marked by
changes in the neutralization of scattered Na$^+$ and work function over time.
The spatial distribution of the conductive charges in the TSS, which are
primarily positioned between the first and second atomic layers, is confirmed
by comparison of the neutralization of Na$^+$ scattered from Bi and Se.
