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The structure of the soft x-ray absorption of thin films of sodium metal, NaF, NaCl, NaBr, and NaI has been photographed in the region between 250A and 430A with a plane grating vacuum spectrograph having a resolving power of 0. 1 electron volt. The structure is much narrower and more complicated than found in the K-absorption spectra for the same compounds. The sodium halides have narrow intense absorption "lines" beginning near 380A and extending toward shorter wave-lengths. These absorption "lines" of these crystals have widths at half-maximum intensity as small as 0. 3 ev and almost the same wave-length as the emission of NaII in the vacuum spark. Sodium metal has a sharp edge at 405A and relatively faint structure at shorter wave-lengths. Continuous absorption in the halides starts near 45 ev corresponding to the ionization potential of the free ion at 47 ev and probably is due to transitions to the conduction states of the crystal lattice. These similarities between the energy levels of the sodium ion in these crystalline solids and in the free ions indicate a highly ionic character of the halides and surprisingly little broadening and displacement by neighboring ions in the crystals.
H. M. O’Bryan (1940) studied this question.