  Plasma walls accumulate electrons more efficiently than ions leading to wall
potentials which are negative with respect to the plasma potential.
Theoretically, walls are usually treated as perfect absorber for electrons and
ions implying perfect sticking of the particles to the wall and infinitely long
desorption times for particles stuck to the wall. For electrons we question the
perfect absorber model and calculate, specifically for a planar dielectric
wall, the electron sticking coefficient $s_e$ and the electron desorption time
$\tau_e$. For the uncharged wall we find $s_e\ll 1$ and $\tau_e\approx
10^{-4}s$. Thus, in the early stage of the build-up of the wall potential, when
the wall is essentially uncharged, the wall is not a perfect absorber for
electrons. For the charged wall we find $\tau_e^{-1}\approx 0$. Thus, $\tau_e$
approaches the perfect absorber value. But $s_e$ is still only of the order of
$10^{-1}$. Calculating $s_e$ as a function of the wall potential and combining
this expression with the quasi-stationary balance equations for the electron
and ion surface densities we find the selfconsistent wall potential, including
surface effects, to be 30% of the perfect absorber value.
