Barium Ultrafine Particles in Barium Oxide Crystal

The excess barium, which is indispensable to a well activated oxide cathode, can exist in two forms in BaO crystal: oxygen vacancies and barium ultrafine particles (UFPs). When the density of the oxygen vacancies exceeds the critical value, the formation and the growth of the barium UFPs take place. Some experimental results related to BaO crystal with excess barium are interpreted, and the role of barium ultrafine particles is accentuated. The optical absorption of BaO around photon energy 2 0eV is attributed to barium UFPs embedded in BaO, and the low work function of the oxide cathode is considered to arise from the presence of oxygen vacancies. The physical origin of the photoemission from the oxide cathode bears a resemblance to that of the photoemission from Ag O Cs cathode in visible and infrared region.