Description:
Absorption spectra, mainly in the visible and near ultraviolet regions, were taken of the following anhydrous salts at room temperature and at that of liquid nitrogen: uranium tetrachloride, neptunium tetrachloride, plutonium trichloride, americium trichloride (and americium tribromide). The spectra of these salts have the features characteristic of rare earth spectra, sharp lines clustered in groups. It follows that the heavy elements have their least stable electrons within the inner 5f shell when their ions are in activated as well as in their basic electronic states. The spectra of the salts of americium are among the sharpest known from crystals—even at room temperature. In this respect they are matched, as far as we know, only by the salts of trivalent europium ion. The spectra confirm the resemblance drawn between these ions on other grounds and make it very likely that trivalent americium has six electrons in the 5f shell with 7F0 for its basic electronic state. The experimental method requires at most several micrograms of the salt for obtaining spectra of considerable dispersion.