View source for Physics:Lyman-alpha line

{{Short description|Spectral line of hydrogen in the Lyman series}}
The '''Lyman-alpha line''', typically denoted by '''Ly-α''', is a [[Physics:Spectral line|spectral line]] of [[Chemistry:Hydrogen|hydrogen]] (or, more generally, of any [[Physics:Hydrogen-like atom|one-electron atom]]) in the [[Physics:Lyman series|Lyman series]]. It is emitted when the atomic [[Physics:Electron|electron]] transitions from an ''n''&nbsp;=&nbsp;2 [[Physics:Atomic orbital|orbital]] to the [[Physics:Ground state|ground state]] (''n''&nbsp;=&nbsp;1), where ''n'' is the [[Physics:Principal quantum number|principal quantum number]]. In hydrogen, its [[Wavelength|wavelength]] of 1215.67 [[Angstrom|angstrom]]s ({{val|121.567|u=nm}} or {{val|1.21567e-7|u=m}}), corresponding to a [[Physics:Frequency|frequency]] of about {{val|2.47e15|u=Hz}}, places Lyman-alpha in the [[Physics:Ultraviolet|ultraviolet]] (UV) part of the electromagnetic spectrum. More specifically, Ly-α lies in vacuum UV (VUV), characterized by a strong absorption in the air.

==Fine structure==
thumb|The Lyman-alpha doublet.
Because of the [[Physics:Spin–orbit interaction|spin–orbit interaction]], the Lyman-alpha line splits into a [[Physics: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]] [[Physics:Hamiltonian (quantum mechanics)|Hamiltonian]] are labeled by the ''total'' [[Physics:Angular momentum#Angular momentum in quantum mechanics|angular momentum]] ''j'' of the electron, not just the [[Physics: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 [[Astronomy:Redshift|redshift]]s allow the line to penetrate the [[Astronomy:Earth|Earth]] atmosphere.

The line was also observed in [[Physics:Antihydrogen|antihydrogen]].<ref>{{cite journal |title=Observation of the 1S–2P Lyman-α transition in antihydrogen |first1=M. |last1=Ahmadi |display-authors=0 |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 [[Physics:Quantum electrodynamics|quantum electrodynamics]].

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

== References ==
{{Reflist}}

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

{{Sourceattribution|Lyman-alpha line|1}}