Links
NIST: Atomic Spectra Database - Energy Levels Form
Peter van Hoof's Atomic Line List
Catalog of mostly long-wavelength/low-frequency transitions,
tailored for ALMA
Up-to-date list of detected IS molecules,
part of the Cologne Database for Molecular Spectroscopy (CDMS)
NASA Goddard list of Interstellar molecules and ices
-also hosts a database of infrared spectra and optical constants
for dust constituents