The Surface Ionization of Potassium on Tungsten

The surface ionization of potassium on tungsten has been investigated by a method which employed a ray of potassium atoms. The use of an atomic ray eliminated photoelectric currents and permitted observations of the degree of ionization to be made up to temperatures as high as 2800°K. Between temperatures of 1350° and 2170°K the graph of the logarithm of the ion-atom ratio versus $\frac{1}{T}$ is linear, and when corrected to zero field yields a value of work function for tungsten of 4.51 ±0.01 v. The intercept on the ${log}_{10} \frac{i_{+}}{i_a}$ axis has a value very near to zero instead of the value ${log}_{10} \frac{1}{2}$, expected from theory. This difference between the experimental and the theoretical intercept is interpreted as evidence for a temperature coefficient of the work function. If $\phi ={\phi }_0+\alpha T$ expresses the temperature dependence of $\phi$, $\alpha$ is calculated to have the value 5.6×${10}^{-5\mathrm{}}$ volt ${\mathrm{degree}}^{-1\mathrm{}}$. It is suggested that the departure from linearity at high temperatures may be accounted for by assuming a temperature dependence in that range of the form $\phi ={\phi }_0+\alpha T+\beta T^2$.