Thermal desorption of Ba from tungsten

Scandate cathodes have demonstrated extremely high electron emission densit ies, and developments are underway to improve the emission uniformity and t he replenishment of the active surface layer under ion bombardment. A thoro ugh understanding of the emission mechanism is needed to guide the approach of the development. Much of the surface studies characterize the equilibra ted films, and little understanding has been known about the dynamics of th e interactions. Some insight into the desorption dynamics can be gained thr ough temperature-programmed desorption studies in which a direct-line-of-si ght quadrupole mass analyzer is employed to monitor a specific desorbing at omic or molecular species as a function of the substrate temperature. In th is paper, we discuss the experimental method in applying the technique to s tudy Ba/BaO on a tungsten substrate. Multiple adsorption states were observ ed and each appeared as a distinct desorption feature, from which one is ab le to determine separately for each state the binding energy, the desorptio n kinetics and the population. In ascending order of binding energy, we hav e observed the multilayer Ba adsorption state, Ba-on-W adsorption state, an d oxygen-enhanced adsorption state. (C) 1999 Elsevier Science B.V. All righ ts reserved.