Electric field control of the rapid diffusion of K atoms into a graphite surface

We show that the steady-state diffusion rate of potassium atoms into a graphite surface in the temperature range 1100–1500 K is increased strongly if the electric field outside the surface is directed so that no positive ions can leave the surface. This means that the thermal process of removing K atoms from a basal graphite surface is changed by the direction of the external electric field. If ions are allowed to leave the surface, most K atoms leave in the form of ions or easily ionizable Rydberg species, but if the electric field is reversed the K atoms disappear from the surface by diffusion into the material, not by desorption. The mechanism is not yet understood in its details, but the thermal barrier for this process is the same as that required to form a Rydberg state, as shown in recent kinetic studies. We speculate that the rate of diffusion into a metal surface may be increased by a graphite layer, which could greatly increase the hydrogen diffusion in metals used for hydrogen storage.