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21 Cm Radioline.

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Posted by Alexander on February 21, 2001 17:35:08 UTC

No, np-1s transitions are called Lyman lines: En=13.6(1-n2), so they result in lyman alpha 10.2 eV photon, Lyman beta 12.1 eV photon and so on. Those are ultraviolet photons and thus the Lyman levels can be exited only in hydrogen plasma with the temperature at least 1-2 eV (How 1-2 eV plasma can exite 10-12 eV levels? - by small amount of 10-15 eV electrons on the tail of Maxwellian distribution).

Interstellar hydrogen is very cold, and those Lyman lines can not be exited by its thermal motion. But each level has so call fine structure and some have so-called hyperfine sructure (sublevels). Hyperfine structure results from different mutual orientation of spins of electron, electron orbit and proton. Hyperfine structure of 1s level results in splitting into 2 sublevels: so-called singlet level and so-called triplet level (triplet- when all 3 spins are oriented in one direction, and singlet - when two of them are directed opposite).

Distance between triplet and singlet sublevels is 5.6 microelectronvolt only. So, triplet state can easily be exited from singlet state by thermal motion of atoms or electrons with temperatures in microelectronvolt range. Returning back to the singlet level electron radiate 21 cm radiowave narrow line radiation, which radiotelescopes easily detect due to abundance of cold hydrogen in space.

And vice versa, exitation of triplet state from singlet (lowest state) may result from the absorption of 21 cm photon. So this 21 cm line is a good tool to diagnose cold clouds of neutral hydrogen (their density, temperature and size), to trace their motion and dynamics, etc. Also because this line is located where there is not much of other natural sources of blending radiation, it was suggested as a possible "alien CB radio band" - the line other civilizations may use to communicate. Thus the CETI telescopes listen around this line.

(In new new generation of "Star Trek" show, by the way, they use 6.7 cm radiowaves for communication - 3.14 times the frequency of 21 cm line, in order not to mess with the blending 21 cm hydrogen radiation itself as the old "Star Trek" crew "did").

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