1. D.C. Frost, C.A. McDowell, J. Chem. Phys. 29, 503 (1958)
2. I.S. Buchel’nikova, Sov. Phys. JETP 35, 783 (1959) [Zh. Eksperim. i Teor. Fiz. 35, 1119 (1958)]
3. L.G. Christophorou, Atomic and Molecular Radiation Physics (Wiley, New York, 1971)
4. For earlier studies of these systems see, for example, references [5,6]
5. K. Rohr, J. Phys. B 10, 1849 (1978)