Diffusion and Wall Loss of Magnesium in Decaying Plasma

The plasma-surface interactions are of considerable interest, induced by the successful application of plasmas in layer deposition, surface treatment and other technologies. The reflection coefficients of plasma atoms and molecules incident to a surface (or the complementary sticking coefficients) are generalised quantities, which characterise these interactions. They are closely related to the motion and nature of particles in the gas phase and the surface characteristics, such as roughness, existence of layers of adsorbed particles, inert, or reactive with those in the bulk, temperature, etc. The relevant data, available in literature up to now are scarce. In a previous work [1] we have developed a method for simultaneous measurement of the diffusion coefficient of particles in gaseous media and their reflection coefficient at the wall of the container. Here we present an experimental investigation, based on this method, as well as some numerical simulations of the particles' diffusion, aimed at obtaining the diffusion coefficient of ground-state Mg atoms in argon and their reflection coefficient at a magnesium surface. The experiment is carried out recording the decay curves of the Mg atoms' density in the afterglow of a pulsed hollow cathode gas discharge in argon. The cylindrical hollow cathode is made of magnesium, which is sputtered in the active phase of the discharge to give a Mg-Ar mixture plasma. The decay curves are recorded by an automated multi-channel photon counting system. The usage of a hollow cathode is a way to obtain the diffusion and reflection coefficients of Mg atoms in (rare) gases in the temperature region below the melting point of magnesium, down to room temperatures and even below. Experimental data for both of these parameters in this large region are lacking in the literature.