The Effect of AdsorbedK+Ions on the Photoelectric threshold of Iron

The photoelectric emissivity of a pure iron filament, which has just previously been used as a cathode for ${\mathrm{K}}^{+}$ ions, is increased directly proportional to the positive ion current, for all surfaces where the computed fraction of the surface covered is $\theta \leqq 0.03$. For larger values of $\theta$ the emissivity for $\lambda 3022$ and $\lambda 3132$ increases rapidly with $\theta$ while $\lambda 2804$ remains proportional to $\theta$ until the fraction of the iron covered with potassium exceeds 0.08. The solution of potassium in the iron beginning above $\theta \leqq 0.03$, as well as the vaporization of potassium by the ion bombardment prevent the formation of a surface for which $\theta$ exceeds 0.10. The data have been interpreted as indicating that for $\theta \leqq 0.03$ every ${\mathrm{K}}^{+}$ ion striking the iron remains on the surface as an ion, while for larger values of $\theta$ both potassium atoms and ions are present. The increase in the photoelectric current is due both to the lowering of the work function by the presence of the positive ions and to the emissivity from the adsorbed atoms.