Lyman-alpha line
In physics, the Lyman-alpha line, sometimes written as Ly-α line, is a spectral line of hydrogen, or more generally of one-electron ions, in the Lyman series, emitted when the electron falls from the n = 2 orbital to the n = 1 orbital, where n is the principal quantum number. In hydrogen, its wavelength of 1215.67 angstroms (121.567 nm or 1.21567×10−7 m), corresponding to a frequency of 2.47×1015 hertz, places the Lyman-alpha line in the vacuum ultraviolet part of the electromagnetic spectrum. Lyman-alpha astronomy must therefore ordinarily be carried out by satellite-borne instruments, except for extremely distant sources whose redshifts allow the hydrogen line to penetrate the atmosphere.
Because of fine structure perturbations, the Lyman-alpha line splits into a doublet with wavelengths 1215.668 and 1215.674 angstroms. Specifically, because of the electron's spin-orbit interaction, the stationary eigenstates of the perturbed Hamiltonian must be labeled by the total angular momentum j of the electron (spin plus orbital), not just the orbital angular momentum l. In the n = 2 orbital, there are two possible states, j = 1/2 and j = 3/2, resulting in a spectral doublet. The j = 3/2 state is of higher energy (less negative) and so is energetically farther from the n = 1 orbital to which it is transitioning. Thus, the j = 3/2 state is associated with the more energetic (shorter wavelength) spectral line in the doublet.
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