![Figure 1: Balmer-alpha line absorption spectra. (A) The seven allowed transitions between …[Credits : From T.W. Hansch, A.L. Schawlow, and G.W. Series, "The Spectrum of Atomic Hydrogen," copyright by …]](http://media-2.web.britannica.com/eb-media/01/5701-004-46E02C09.gif)
Figure 1: Balmer-alpha line absorption spectra. (A) The seven allowed transitions between the n = 2 and n = 3 energy levels of hydrogen. (B) The Doppler-broadened profile of the absorption spectra. Only two components can be distinguished. (C) An early example of Doppler-free spectra. Peaks resulting from four of the seven transitions can be resolved; the fifth peak marked as a crossover resonance is not significant. The frequency scale on this data is relative to an arbitrary starting point, but subsequent measurements have determined the frequency ν of these transitions to an uncertainty δν/ν of less than one part in one billion.
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