Line Radiation of Neutral Hydrogen
Most atomic transitions give rise to spectral lines at wavelengths in the infrared or shorter. With the exception of radio recombination lines (Chap. 13), atomic radio lines are rare. The energy levels are described by the scheme 2S+1LJ . In this description, S is the total spin quantum number, and 2S + 1 is the multiplicity of the line, that is the number of possible spin states. L is the total orbital angular momentum of the system in question, and J is the total angular momentum. For the lighter elements, the energy levels are best described using LS coupling. This is constructed by vectorially summing the orbital momenta to obtain the total L, then combining the spins of the individual electrons to obtain S, and then vectorially combining Land S to obtain J. If the nucleus has a total spin, I, this can be vectorially combined with J to form F. For an isolated system, all of these quantum numbers have a constant magnitude and also a constant projection in one direction. Usually the direction is arbitrarily chosen to be along the z axis, and the projected quantum numbers are referred to as MF , MJ , ML and Ms
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