Atomic Energy Levels and Other Spectroscopic Data

The key advances in our understanding of atomic structure were made early in the 20th Century. Niels Bohr developed his revolutionary theory of the hydrogen atom in 1913, still largely on classical terms. Then, in the 1920s, the generalization to other atoms and ions came with the development of quantum mechanics, or wave mechanics, by Schroedinger, Heisenberg and others, after de Broglie postulated that all elementary particles also have wave properties. Important later generalizations and refinements were the relativistic extensions by Dirac, as well as quantum electrodynamics by Feynman, Schwinger, and others. Even before these developments took place, many highly precise spectroscopic data had been obtained experimentally, and these data could now be fully understood in terms of atomic structure and quantum numbers. Thus, laboratory and astrophysical spectra could for the first time be interpreted as specific classified transitions between energy levels of atoms or positive ions. The three volumes of tables in the Atomic Energy Levels series cited here [1-3] represented a synthesis of the data derived from studies of atomic spectra in the first half of the century—studies which played a major role in the revolution in physics just described. The effort can be traced to an initiative of the National Research Council, which in 1924 created a Committee on Line Spectra of the Elements, with one of its main goals being to encourage work on the analysis of atomic spectra and to collect atomic structure data in a systematic manner. A first, still rather incomplete, tabulation was undertaken by Bacher and Goudsmit in 1932 with a book entitled Atomic Energy States as Derived from the Analysis of Optical Spectra [4]. In the following years, spectroscopic data were accumulated on a vast scale, especially for somewhat heavier atoms and ions, and in the middle 1940s, with the strong support of the Line Spectra Committee, NBS Director E. U. Condon agreed to undertake a new data compilation project at NBS, after Bacher and Goudsmit declined to update their book. Charlotte E. Moore, already an expert in the compilation of atomic spectra and author of the Princeton Observatory Multiplet Tables of Astrophysical Interest of 1933 and 1945 [5], accepted a position at NBS in 1945 to prepare a handbook of atomic energy levels. This project achieved a first milestone in 1949 with the publication of Volume I of Atomic Energy Levels, containing the spectra of hydrogen, (atomic number Z=1) through vanadium (Z=23), 309 pages strong [1]. In 1952, Volume II with the elements chromium (Z=24) through niobium (Z=42) followed [2], and in 1958 a third volume containing the spectra of molybdenum (Z=42) through actinium (Z=89) completed this series [3], also known as NBS Circular 467. In all these tables, the principal data presented are the atomic energy levels (or “energy eigenstates”) compiled from experimentally determined (published or unpublished) material. This was done for those stages of ionization for which reliable data existed. All data were critically compiled, which means that in cases where several experimental results were available, only the best value was selected after critical evaluation. In addition to the level values, the spectroscopic configuration and term assignment, the total angular momentum quantum number, and the ionization energy of the atom or ion were presented, and all this material was arranged in a highly organized, easily readable format.