References contained in this database are from Bibliography on Atomic Energy Levels and Spectra, NBS Special Publication 363 and Supplements, as well as current references since the last published bibliography collected by the NIST Atomic Spectroscopy Data Center (http://www.nist.gov/physlab/div842/grp01/asdc_info.cfm). These references pertain to atomic structure and spectra that arise from interactions or excitations involving electrons in the outer shells of free atoms and atomic ions, or from inner shell excitations corresponding to frequencies up to the soft x-ray range. Please note that this database does not contain references to atomic transition probabilities, line intensities, or broadening. These references can be found in two other bibliographic databases maintained by the same Data Center: NIST Atomic Transition Probability Bibliographic Database (http://physics.nist.gov/fvalbib) and NIST Atomic Spectral Line Broadening Bibliographic Database (http://physics.nist.gov/linebrbib). References to publications containing critically compiled data can be found in a separate database of NIST compilations of atomic spectroscopy data (http://physics.nist.gov/PhysRefData/datarefs/datarefs_search_form.html).