Lyman-alpha
spectral line of hydrogen in the Lyman series
Lyman-alpha, typically denoted by Ly-α or Lyα, is a spectral line of hydrogen (or, more generally, of any one-electron atom) in the Lyman series. A photon is emitted when the atomic electron transitions from an n = 2 orbital to the ground state (n = 1), 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 about 2.46607×1015 Hz, places Lyman-alpha in the ultraviolet (UV) part of the electromagnetic spectrum, with a photon energy of 10.2 eV. More specifically, Ly-α lies in vacuum UV (VUV), characterized by a strong absorption in the air.
Fine structure
Because of the spin–orbit interaction, the Lyman-alpha line splits into a fine-structure doublet with the wavelengths of 1215.668 and 1215.674 angstroms. These components are called Ly-α3/2 and Ly-α1/2, respectively.
The eigenstates of the perturbed Hamiltonian are labeled by the total angular momentum j of the electron, not just the orbital angular momentum l. In the n = 2, l = 1 orbital, there are two possible states, with j = 1/2 and j = 3/2, resulting in a spectral doublet. The j = 3/2 state has a higher energy and so is energetically farther from the n = 1 state to which it is transitioning. Thus, the j = 3/2 state is associated with the more energetic (having a shorter wavelength) spectral line in the doublet.
Observation
Since the hydrogen Lyman-alpha radiation is strongly absorbed by the air, its observation in laboratory requires use of vacuumed spectroscopic systems.
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