1. T.G. Harrison, QJRAS 25, 65 (1984)
2. D.H. Menzel, Selected Papers on Physical Processes in Ionized Plasmas (1962)
3. D.H. Menzel, E.A. Milne, Selected Papers on the Transfer of Radiation (1966)
4. L. Spitzer, Physical Processes in the Interstellar Medium (Wiley-Interscience, New York, 1978), p. 333
5. A.G.G.M. Tielens, The Physics and Chemistry of the Interstellar Medium (Cambridge University Press, Cambridge, 2005)