Effect of Oxygen on the Work Function of Barium

Abstract

While a high sensitivity of the work function to gaseous contaminants is generally characteristic of metals which adsorb the common gases strongly, barium combines outstanding work function stability with an exceptional affinity for these gases. The paradox is investigated by administering measured quantities of oxygen to vapor-deposited barium surfaces at known rates, with concurrent monitoring of the work function. It is found (1) that when clean barium is exposed suddenly to a massive dose of oxygen, its work function is lowered by 0.32 ev; (2) that the work function of the heavily oxygenated surface remains constant for 5 days and then drifts very slowly toward the clean-metal value, and (3) that when oxygen is administered to a barium surface slowly (2\ifmmode\times\else\texttimes\fi{}${10}^{13}$ molecules per ${\mathrm{cm}}^{2}$ per hour) no change of work function occurs during or after the absorption of a quantity of oxygen equivalent to 100 complete monolayers of the oxide.The results are interpreted as stemming from two processes which compete for control of the surface; the expected surface oxidation and a restorative process involving engulfment of oxygen ions. The mechanism of engulfment is discussed.