Photoelectric and Thermionic Emission from Cobalt

Abstract

The total photoelectric emission from a strip of cobalt foil excited by the undispersed radiation from a quartz mercury arc was studied during an extended outgassing process and after stable conditions had been reached. The sensitivity rose quickly in the initial period, then decreased to a stable value. A change in the long-wave limit consistent with the change in the sensitivity was observed. Curves are plotted showing the variation of the full arc sensitivity with temperature. At a temperature near 850°C abrupt changes in these curves were observed. X-ray analyses seemed to indicate that this resulted from a structural change. The long-wave limit for cobalt which had been cooled suddenly from above 850°C and which was known by x-ray analysis to have a face-centered cubic structure was between 2967A and 3022A (4.25±0.08 volts). A sample which was cooled slowly from above 850°C and which was known from x-ray analysis to have a hexagonal close pack structure had a long-wavelimit between 2757A and 2967A (4.12±0.04 volts). A plot of the thermionic current as a function of temperature indicates that there is an abrupt change in the Richardson ($A$) or ($b$) and perhaps both at a temperature near 850°C.