  We present the results of a systematic study of Knudsen s flow of Rb atoms in
cylindrical capillary cells coated with a polydimethylsiloxane (PDMS) compound.
The purpose of the investigation is to determine the characterization of the
coating in terms of the sticking probability and sticking time of Rb on the two
types of coating of high and median viscosities. We report the measurement of
the sticking probability of the Rb atom to the coating that is equal to 4.3 10
-5 and corresponds to the number of bounces of 2.3 10 4 at room temperature.
These parameters are the same for the two kinds of PDMS used. We find that at
room temperature the sticking times for high viscosity and median viscosity
PDMS are 22 +- 3 mks and 49 +- 6 mks, respectively. These sticking times are
about million times larger than the sticking time derived from the surface Rb
atom adsorption energy and temperature of the coating. A tentative explanation
of this surprising result, based on the bulk diffusion of the atoms that
collide with the surface and penetrate inside of the coating, is proposed. The
results can be important to many resonance cell experiments, such as the
efficient magneto-optical trapping of rare elements or radioactive isotopes and
in experiments on the light-induced drift effect.
