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2. E.U. Condon, G.H. Shortley, The Theory of Atomic Spectra (Cambridge University Press, New York, 1935)
3. J.C. Slater, Quantum Theory of Atomic Structure (McGraw-Hill Book Company, New York, 1960)
4. E.U. Condon, H. Odabasi, Atomic Structure (Cambridge University Press, London, 1980)
5. R.C. Powell, Physics of Solid-State Laser Materials (Springer, New York Inc, New York, 1998)