Angular-resolved desorption of potassium ions from basal graphite surfaces. Ionization of Rydberg species from adsorbed and molecular beam supplied atoms

Abstract

The thermal desorption of K⁺ ions from a sample of well ordered pyrolytic graphite is studied at temperatures from 1000 to 1500 K, measuring, primarily, angular distributions due to a thermal K atom beam as well as those due to K atoms previously absorbed in the graphite on exposure to the beam. The two sources of desorbing K⁺ give completely different angular distributions. The K atoms emanating from the bulk mainly penetrate out to the prism surfaces and leave the sample, as ions, from the edges. This process involves a localized state of K, proposed to be a covalent Rydberg state with antibonding character. The atoms from the K atom beam instead enter a mobile state on the basal surface. Rapid diffusion into the bulk is observed above 1100 K, and the desorbing fraction is observed in a lobe centred around the normal of the basal surface. A large fraction of the flux from the basal surface seems to be in Rydberg states. This gives rise to charge-exchange processes outside the sample, observed as less than complete acceleration of the ions and a narrowing of the lobe with increased external field strength. Several features give evidence for the formation and ionization of Rydberg atoms: the non-equilibrium ionization of the K atoms from the bulk at the sample edges, and the very large energy spread, > 5 eV, of the ions formed from the beam atoms being the most obvious. The new results complement previous extensive studies of the surface kinetics of alkali-metal atoms on graphite, and also agree with previous studies of formation of Rydberg atoms of K on non-metallic surfaces.