Thermionic electron emission from 2000−4000 Å thick films of K+(cryptand[2.2.2])e- and Rb+(cryptand[2.2.2])e- was studied as a function of temperature and time. The total charge collected was generally several hundred nanocoulombs with peak currents in the range of 5−100 pA. Although electron emission was not a direct result of thermal decomposition of the films, the peak current correlated with the decomposition rate. At a given temperature, the current increased gradually to a peak value and then decreased to near zero. Interrupting the process by cooling before complete decay, followed by a return to the same temperature, restored the previous current−time behavior. Once the current had decayed to near zero, the only way to restore emission was to raise the temperature above the previous value. The process of growth and decay was then repeated. The origin of emission is unknown, but the behavior suggests the presence of surface defect sites at grain boundaries that are initially empty. Sample decomposition fills these sites, which lie only a few tenths of an electronvolt below the vacuum level. Continued decomposition destroys the sites, but new defect sites can be produced by raising the temperature, probably by forming additional grain boundaries.
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