Lyman-alpha, typically denoted byLy-α orLyα, is aspectral line ofhydrogen (or, more generally, of anyone-electron atom) in theLyman series. A photon is emitted when the atomicelectron transitions from ann = 2orbital to theground state (n = 1), wheren is theprincipal quantum number. In hydrogen, itswavelength of 1215.67angstroms (121.567 nm or1.21567×10−7 m), corresponding to afrequency of about2.46738×1015 Hz, places Lyman-alpha in theultraviolet (UV) part of the electromagnetic spectrum. More specifically, Ly-α lies invacuum UV (VUV), characterized by a strong absorption in theair.
Because of thespin–orbit interaction, the Lyman-alpha line splits into afine-structure doublet with the wavelengths of 1215.668 and 1215.674 angstroms.[2] These components are called Ly-α3/2 and Ly-α1/2, respectively.
The eigenstates of theperturbedHamiltonian are labeled by thetotalangular momentumj of the electron, not just theorbital angular momentuml. In then = 2,l = 1 orbital, there are two possible states, withj = 1/2 andj = 3/2, resulting in a spectral doublet. Thej = 3/2 state has a higher energy and so is energetically farther from then = 1 state to which it is transitioning. Thus, thej = 3/2 state is associated with the more energetic (having a shorter wavelength) spectral line in the doublet.[3]
Since the hydrogen Lyman-alpha radiation is strongly absorbed by the air, its observation in laboratory requires use of vacuumed spectroscopic systems. For the same reason, Lyman-alpha astronomy is ordinarily carried out by satellite-borne instruments, except for observing extremely distant sources whoseredshifts allow the line to penetrate theEarth atmosphere.
The line was also observed inantihydrogen.[4] Within the experimental uncertainties, the measured frequency is equal to that of hydrogen, in agreement with predictions ofquantum electrodynamics.