Barium and oxygen: incorporation characteristics at very low pressures

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Anderson and Hunt have demonstrated that oxygen may be incorporated in barium films, leaving the contact potential difference with respect to some reference surface unchanged. A kinetic theory of the balance between adsorption and incorporation of gas is suggested. Thus it is deduced that there is a critical pressure below which there will be an increase in the amount of oxygen incorporated during the formation of a very thin protective layer of oxide. Such critical pressures have been found in experiments, for two different temperatures. Hence the heat of binding and the activation energy of mobility of oxygen on the surface have been estimated. Also, the sticking factor for oxygen on barium increases with pressure below 10-7 torr. These results have a practical importance: at the very low pressures obtained in sealed-off devices, with the aid of barium getters, the capacity for incorporating oxygen is greater than expected from experiments done at higher pressures (>10-7 torr).