Structure in the X-RayKAbsorption Edge of Metallic Potassium

For the alkali metals sodium and potassium the free electron approximation should give the energy states in the conduction band closely. Calculations based on this approximation predict an arctangent shape for the potassium $K$ absorption edge. By use of a two-crystal vacuum x-ray spectrometer, the absorption edge of metallic potassium has been explored with high resolving power. Details of the experimental procedure are given. The shape of the absorption edge is in satisfactory agreement with the theoretical arctangent curve, and the width of the edge agrees with the predicted value. Some evidence is found of a secondary structure not given by the simple theory and the free electron approximation. The wave-length of the midpoint of the edge has been redetermined.