Physisorption kinetics of electrons at plasma boundaries

Abstract

Abstract. Plasma-boundaries floating in an ionized gas are usually negatively charged. They accumulate electrons more efficiently than ions leading to the formation of a quasi-stationary electron film at the boundaries. We propose to interpret the build-up of surface charges at inert plasma boundaries, where other surface modifications, for instance, implantation of particles and reconstruction or destruction of the surface due to impact of high energy particles can be neglected, as a physisorption process in front of the wall. 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 microscopic models for the electron-wall interaction. Irrespective of the sophistication of the models, the static part of the electron-wall interaction determines the binding energy of the electron, whereas inelastic processes at the wall determine se and τe. As an illustration, we calculate se and τe for a metal, using the simplest model in which the static part of the electron-metal interaction is approximated by the classical image potential. Assuming electrons from the plasma to loose (gain) energy at the surface by creating (annihilating) electron-hole pairs in the metal, which is treated as a jellium half-space with an infinitely high workfunction, we obtain se ≈ 10 −4 and τe ≈ 10 −2 s. The product seτe ≈ 10 −6 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 somewhat large. The former is a consequence of the small matrix elements occurring in the simple model while the latter is due to the large binding energy of the electron. More sophisticated theoretical investigations, but also experimental support, are clearly needed because if se is indeed as small as our exploratory calculation suggests, it would have severe consequences for the understanding of the formation of surface charges at plasma boundaries. 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

Towards a Microscopic Theory of Particle Charging

“…Both parameters depend on the inelastic, that is, the dynamic interaction between the electron and the grain. As discussed in [30], a rigorous calculation of (sτ ) e has to be based on a microscopic model for the electron-grain interaction taking elementary excitations of the grain into account. Quantum-kinetic equations have then to be solved to obtain s e and τ e separately.…”

“…In principle the coupling to other elementary excitations of the grain (plasmons, electron-hole pairs, ...) can compensate for the lack of coupling to lattice vibrations but how efficient this coupling really is is not at all obvious. In a recent exploratory calculation we found, for instance, that the electron sticking coefficient at metallic boundaries arising from the coupling to internal electron-hole pairs is also extremely small, at least when the Coulomb interaction between an internal and an external electron is realistically screened [30].…”

“…To calculate the product (sτ ) e requires therefore a microscopic model for the electrongrain interaction. First steps in this direction were taken in [30].…”

Mass changes of microparticles in a plasma observed by a phase-resolved resonance method

“…Nevertheless, for high floating potentials ͑ f Ϸ −1 keV͒, Pavlů et al 19 found that the particle charge is limited by field emission of electrons, which strongly depends on the surface properties ͑material͒. Furthermore, Bronold et al 20 proposed a charging model that uses a quantum mechanical ansatz to describe the physisorption process of electrons at the particle surface. Here, the particle charge depends on the surface states of the material.…”

Phonon-mediated desorption of image-bound electrons from dielectric surfaces

“…It is only until recently that we pointed out 39,40 that the charging of surfaces in contact with an ionized gas, as it occurs, for instance, in the interstellar medium, [41][42][43] in the upper atmosphere, 44 in dusty laboratory plasmas, 45,46 and in dielectrically bounded low-temperature plasmas, [47][48][49][50][51][52] could be perhaps microscopically understood as an electronic physisorption process.…”