1. M. A. El’yashevich, Spectra of Rare-Earth Elements (Gostekhizdat, Moscow, 1953) [in Russian].
2. A. Przybylski, Nature 189(4766), 739 (1961).
3. A. A. Radtsig and B. M. Smirnov, Parameters of Atoms and Atomic Ions: A Handbook (Energoatomizdat, Moscow, 1986).
4. NIST Atomic Spectra Database: Atomic Energy Levels (2010).
5. M. Miyabe, I. Wakaida, and T. Arisawa, J. Phys. 29(18), 4073 (1996).