The Effect of Alkali Ions on the Photoelectric Emissivity of Tungsten

Known quantities of ${\mathrm{Na}}^{+}$, ${\mathrm{K}}^{+}$, ${\mathrm{Rb}}^{+}$ and ${\mathrm{Cs}}^{+}$ ions were deposited on tungsten and the changes in the photoelectric current measured. For wave-lengths below 2800A the current increases proportional to the fraction of the surface covered as long as $f$ is small. The emissivity for longer wave-lenths is low at first, and then rises sharply beyond some critical value of $f$, which increases with the wave-length. The results indicate that the work function is not uniform over these composite surfaces; this makes it impossible to fix a definite threshold for the various values of $f$. The emissivity increases with temperature to about 500°C, where fatigue becomes appreciable. As the filament fatigues for photoelectric emission the positive ion emissivity increases, reaching a maximum when the threshold returns to that for clean tungsten. The results show that the alkali dissolves so rapidly in the tungsten that the dependence of the work function on $f$ cannot be determined at temperatures above 500°C.