# Strange behavior difference between light and heavy hydrogen atoms [closed]

While hydrogen atoms in fundamental state emit both in cosmic spaces and in hydrogen maser a monochromatic radiation whose frequency 1.4204057 GHz is equal to the hyperfine splitting of their 1S spectral term, deuterium atoms do not emit a similar radiation with the frequency 0.327384 GHz equal to the hyperfine splitting of their 1S spectral term, neither in cosmic spaces (where huge quantities of deuterium exist), nor in hydrogen maser.

This different behavior is at least unexpected, as long as hydrogen and deuterium atoms have almost identical spectra, with very small differences between their homologous spectral lines because of the different reduced masses of the two atoms. More, the nuclear magnetic field in deuterium atoms is over four times weaker than in hydrogen atoms, which ought to favor an electron spin inversion similar to that causing 1.420406 GHz radiation mentioned above. For all that, such an electron spin inversion does not happen inside the nuclear magnetic field in deuterium atoms.

Is there an explanation for this absence of 0.327384 GHz radiation emitted by deuterium atoms?

## closed as unclear what you're asking by Emilio Pisanty, stafusa, Jon Custer, Asher, Rory AlsopNov 25 '17 at 20:44

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• Deuterium masers can be made in the lab. Are you asking why they don't occur in space? – John Rennie Nov 19 '17 at 8:20
• The expectation that the hyperfine structure of hydrogen and deuterium will be similar is ridiculous. Hyperfine structure is determined by the nuclear spin and the nuclear magnetic moment, and those are nothing alike in H and D, so any expectation that the hyperfine structure will be similar can only be grounded in willful ignorance or wishful thinking. – Emilio Pisanty Nov 23 '17 at 11:11
• Moreover, the only clear question in this post regards the "absence" of deuterium, and the existing responses provide clear evidence of its existence in both laboratory and astronomical observations. As such, this question requires substantial restructuring in refining its premises and providing clear evidence for them. – Emilio Pisanty Nov 23 '17 at 11:14
• @Emilio Pisanty - I cannot understand where in my post you find my “expectation that the hyperfine structure of hydrogen and deuterium will be similar” or my belief in “the absence of deuterium” in both laboratory or cosmic space. These allegations are only your phantasms, so that your aggressive language and negative vote are entirely gratuitous. – Sorin Vlaicu Nov 25 '17 at 8:04
• Ah, that was a mistype - that's 'absence of deuterium radiation' in the second comment. The D=H claim is in your second paragraph. Both points stand. – Emilio Pisanty Nov 25 '17 at 10:15

327 MHz radiation from deuterium has been detected from atomic gas in the plane of the Milky Way (e.g. see Rogers et. al. 2006).

There is no problem in principle with using this transition for astronomy, it is just that the ratio of deuterium to standard hydrogen is about 25 parts per million.

The detectability of the emission will be proportional to the number of D atoms along the line of sight multiplied by the Einstein A-coefficient and a small numerical factor associated with the degeneracy factor of the upper energy state.

According to Deguchi & Watson (1985) the Einstein A coefficient of the deuterium hyperfine transitions is $4.7\times 10^{-17}$ s$^{-1}$, and is also much less than for the equivalent 1420MHz emission of hydrogen, of $2.9\times 10^{-15}$ s$^{-1}$. That is mainly because, all other things being equal (i.e. they are both "forbidden" magnetic dipole transitions), the transition probability scales as the cube of the frequency (e.g. see p.79 of Astrophysical Quantities [4th Ed.] by Allen, 1973).

So to sum up, the optically thin hyperfine deuterium emission from a cloud of hydrogen is likely to be a factor of $\sim 4\times 10^{-7}$ weaker than the 21 cm hydrogen radiation.

Edit: You ask why, nowithstanding the expected weaker emission and relatively low abundance of deuterium atoms, that gas out to the edge of the observable universe does not contribute to a stronger and more observable "92 cm line".

The reason is that although this optically thin emission does "add up" along the line of sight, the expansion of the universe ensures that this emission is only at 92cm for gas local to the Milky Way (cosmologically speaking). Gas from more distant hydrogen clouds is redshifted by the universal expansion and therefore does not contribute to an observable 92cm line in our rest frame.

• However, while H-microwaves 21 cm arrive at Earth from all spatial directions with the same intensity and can anywhere be detected even by an artisanal telescope, D-microwaves 92 cm are detected only by powerful antennas selectively directed. And this difference cannot be explained only by the average ratio D/H ~ 25 ppm, because even so there exist huge quantities of deuterium in universe, but rather by a much smaller probability of the electron spin-flip in unexcited D-atoms as against unexcited H-atoms. Or, in other words, why we have a universal radiation 21 cm, but not a similar one 92 cm. – Sorin Vlaicu Nov 21 '17 at 6:57
• @SorinVlaicu "And this difference cannot be explained only by the average ratio D/H ~ 25 ppm" do you have a link for this assertion? How many hydrogen atoms are in involved in the detected spectra?1 million would give a sharp line but 25 would not. – anna v Nov 21 '17 at 11:29
• @Rob Jeffries – Beyond all theoretical or speculative arguments two questions remain: 1. Ought the huge quantities of cosmic deuterium to bring forth a universal (i.e. equally detectable anywhere and from any spatial direction) radiation ν = 0.327384 GHz? 2. If hydrogen maser is fed with deuterium, are microwaves ν = 0.327384 GHz with no spectral width emitted inside the central bulb? Well, I have never read about a universal radiation ν = 0.327384 GHz (how weak is less important), or about microwaves ν = 0.327384 GHz emitted by deuterium in hydrogen maser. – Sorin Vlaicu Nov 23 '17 at 7:17
• @SorinVlaicu There is no mystery and there is no universal 21cm radiation as you claim. As soon as you move away from the vicinity of the Milky Way, the wavelengths are redshifted by the universal expansion. Therefore the contributions from optically thin gas out to the edge of the observable universe do not add to emission at a single wavelength. – Rob Jeffries Nov 23 '17 at 10:39
• @SorinVlaicu And please could you refer me to where I can find more information about astrophysical 21cm masers. AFAIK because the line is intrinsically so narrow and the stimulated emission coefficient is so low that such maser action in astrophysical scenarios is not possible because a long enough path length is not available. – Rob Jeffries Nov 23 '17 at 11:50