The excitation potentials of light metals. I.—Lithium

Abstract During recent years a large volume of work has been published on the excitation of soft X-rays from various metals. But the simplest elements, namely lithium and beryllium have received comparatively little attention. Holtsmark has investigated these elements in the form of compounds, a state not altogether satisfactory for the purpose because of its non-conducting nature. McLennan and Clark, Levi and Rollefson, it is true, have studied metallic lithium, but in neither case under very good experimental conditions, and, as the present work will show, with but partially correct results. The results obtained with the heavier elements, e. g., Fe, Ni, Co, Cu, W, are extremely complex. Even with a relatively light element, C, there has been so far no clear correlation with the spectroscopic data obtained with the vapour in the extreme ultra-violet. It seemed, therefore, important to investigate carefully these simple cases, in which all the excitation potentials of the free atom could be calculated with a good degree of approximation. So work was begun on lithium. A preliminary notice of an interesting result was published in 'Nature.’ Briefly Stated, it is the fact that the minimum excitation potential for the K-radiation of Li is 9 volts below the K-ionisation potential of the Li atom. This implies that it is possible to excite a K-resonance radiation in the metal, just as would be the case with free atoms. A numerical correlation between the results on the excitation of K-radiation in the metal and the calculated properties of the free atom has been successfully achieved. Some data from a preliminary investigation of the radiation from lithium metal have also been obtained. At the end of the paper, a theoretical discussion is given on the interpretation of soft X-ray discontinuities under more complicated conditions than those prevailing in lithium; and an attempt is made towards a theory of the approximately linear relationship which is observed between the intensity of the radiation emitted from a metal and the voltage of the exciting electron-beam.