Physisorption kinetics of electrons at plasma boundaries

Plasma-boundaries floating in an ionized gas are usually negatively charged. They accumulate electrons more efficiently than ions which leads to the formation of a quasi-stationary electron film at the boundaries. We propose a physisorption-inspired quantum-kinetic description of the build-up of surface charges at inert plasma boundaries, where other surface modifications, in particular, implantation of particles, reconstruction or destruction of the surface due to high energy particles, etc. do not occur. The electron sticking coefficient se and the electron desorption time τe, which play an important role in determining the quasi-stationary surface charge, and about which little is empirically and theoretically known, can then be calculated from a relatively well-defined microscopic model for the electron-wall interaction. Within this approach, it turns out that the polarization-induced short-range part of the electron-wall interaction determines the binding energy of the electron, whereas the inelastic processes at the wall determine se and τe. In particular the inelastic channels depend strongly on the material. As an example we specifically discuss a metallic boundary. Assuming the plasma electron to loose (gain) energy at the surface by creating (annihilating) electron-hole pairs in the metal, we find se ≈ 10 −4 and τe ≈ 10 s. The product seτe ≈ 10 s has the order of magnitude expected from our earlier results for the charge of dust particles in a plasma but individually se is unexpectedly small and τe is unexpectedly large. However, since the binding energy of the electron turns out to be much larger than kTs, where Ts is the wall temperature, this, at first sight perhaps unexpected result, can be understood. Nevertheless, further investigations, both theoretical and experimental, are clearly needed. If se is indeed as small as our exploratory calculation suggests, it would have severe consequences for the way the formation of surface charges should be described. To identify what we believe are key issues of the electronic microphysics at inert plasma boundaries and to inspire other groups to join us on our journey is the purpose of this colloquial presentation. PACS. 52.27.Lw Dusty or complex plasmas – 52.40.Hf Plasma-material interaction, boundary layer effects – 68.43.-h Chemi-/Physisorption: adsorbates on surfaces – 73.20.-r Electron states at surfaces and interfaces