Editing Lyman-alpha

{{Short description|Spectral line of hydrogen in the Lyman series}}
[[File:All-sky New Horizons Alice Lyα maps.jpg|thumb|upright|The All-sky [[New Horizons]] Alice Lyα maps in ecliptic coordinates centered on the anti-Sun direction, and the lower map is in Galactic coordinates. The c.&nbsp;90,000 stars in the M. A. Velez et al. (2024) catalog are overlaid as black dots, where the size of the dot is proportional to the logarithm of the expected Alice count rate from each star. The map in Galactic coordinates indicates the outlines of four of the important LISM clouds ("LIC", in red; "Aql" in green; "Blue", in blue; and "G", in tan).<ref name=lyman>{{Cite journal |last1=Gladstone |first1=G. Randall |last2=Shull |first2=J. Michael |last3=Pryor |first3=Wayne R. |last4=Slavin |first4=Jonathan |last5=Kammer |first5=Joshua A. |last6=Becker |first6=Tracy M. |last7=Lauer |first7=Tod R. |last8=Postman |first8=Marc |last9=Spencer |first9=John R. |last10=Parker |first10=Joel Wm. |last11=Retherford |first11=Kurt D. |last12=Velez |first12=Michael A. |last13=Versteeg |first13=Maarten H. |last14=Davis |first14=Michael W. |last15=Froning |first15=Cynthia S. |date=April 2025 |title=The Lyα Sky as Observed by New Horizons at 57 au |journal=The Astronomical Journal |language=en |volume=169 |issue=5 |pages=275 |doi=10.3847/1538-3881/adc000 |doi-access=free |issn=1538-3881}}</ref>]]
'''Lyman-alpha''', typically denoted by '''Ly-α''' or '''Lyα''', is a [[spectral line]] of [[hydrogen]] (or, more generally, of any [[Hydrogen-like atom|one-electron atom]]) in the [[Lyman series]]. It is emitted when the atomic [[electron]] transitions from an ''n''&nbsp;=&nbsp;2 [[atomic orbital|orbital]] to the [[ground state]] (''n''&nbsp;=&nbsp;1), where ''n'' is the [[principal quantum number]]. In hydrogen, its [[wavelength]] of 1215.67 [[angstrom]]s ({{val|121.567|u=nm}} or {{val|1.21567e-7|u=m}}), corresponding to a [[frequency]] of about {{val|2.47e15|u=Hz}}, places Lyman-alpha in the [[ultraviolet]] (UV) part of the electromagnetic spectrum. More specifically, Ly-α lies in [[vacuum ultraviolet|vacuum UV]] (VUV), characterized by a strong absorption in the [[air]].

==Fine structure==
[[image:Hydrogen fine structure2.svg|thumb|The Lyman-alpha doublet.]]
Because of the [[spin–orbit interaction]], the Lyman-alpha line splits into a [[fine structure|fine-structure]] doublet with the wavelengths of 1215.668 and 1215.674 angstroms.<ref>{{Citation| publisher = National Institute of Standards and Technology| last1 = Kramida| first1 = Alexander| last2 = Ralchenko| first2 = Yuri| title = NIST Atomic Spectra Database, NIST Standard Reference Database 78| accessdate = 2021-06-27| date = 1999| url = http://www.nist.gov/pml/data/asd.cfm}}</ref> These components are called Ly-α<sub>3/2</sub> and Ly-α<sub>1/2</sub>, respectively.

The eigenstates of the [[Perturbation theory (quantum mechanics)|perturbed]] [[Hamiltonian (quantum mechanics)|Hamiltonian]] are labeled by the ''total'' [[Angular momentum#Angular momentum in quantum mechanics|angular momentum]] ''j'' of the electron, not just the [[Azimuthal quantum number|orbital angular momentum]] ''l''. In the ''n''&nbsp;=&nbsp;2, ''l''&nbsp;=&nbsp;1 orbital, there are two possible states, with ''j''&nbsp;=&nbsp;{{sfrac|1|2}} and ''j''&nbsp;=&nbsp;{{sfrac|3|2}}, resulting in a spectral doublet. The ''j''&nbsp;=&nbsp;{{sfrac|3|2}} state has a higher energy and so is energetically farther from the ''n''&nbsp;=&nbsp;1 state to which it is transitioning. Thus, the ''j''&nbsp;=&nbsp;{{sfrac|3|2}} state is associated with the more energetic (having a shorter wavelength) spectral line in the doublet.<ref>{{Cite book |first=Bruce T. |last=Draine |year=2010 |title=Physics of the Interstellar and Intergalactic Medium |url=https://books.google.com/books?id=FycJvKHyiwsC&pg=PA83 |location=Princeton, N.J. |publisher=[[Princeton University Press]] |isbn=978-1-4008-3908-7 |oclc=706016938 |page=83}}</ref>

==Observation==
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 whose [[redshift]]s allow the line to penetrate the [[Earth]] atmosphere.

The line was also observed in [[antihydrogen]].<ref>{{cite journal |title=Observation of the 1S–2P Lyman-α transition in antihydrogen |first1=M. |last1=Ahmadi |display-authors=etal |journal=Nature |volume=560 |issue=7720 |pages=211–215 |date=22 August 2018 |doi=10.1038/s41586-018-0435-1 |pmid=30135588 |pmc=6786973 |doi-access=free }}</ref> Within the experimental uncertainties, the measured frequency is equal to that of hydrogen, in agreement with predictions of [[quantum electrodynamics]].

==See also==
{{cmn|colwidth=30em|* [[Lyman limit]]
* [[Lyman-alpha forest]]
* [[Lyman-alpha emitter]]
* [[Lyman-alpha blob]]
* [[Lyman-break galaxy]]
* [[Siegbahn notation#K-alpha|K-alpha line]]
}}

== References ==
{{Reflist}}

{{DEFAULTSORT:Lyman-Alpha Line}}
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[[Category:Emission spectroscopy]]
[[Category:Atomic physics]]
[[Category:Hydrogen physics]]
[[Category:Astronomical spectroscopy]]