In a deuterium spectrum tube there is a thin capillary in the middle (see picture) that glows purplish sort of. Now, I wonder what this glowing part actually consist of? Is it a completely dissociated plasma (deuterons and electrons) or atomic deuterium, where the atoms are excited to a higher energy level?

And is there a glow discharge in the tube or is it an arc discharge? One can see some striations in the picture (is this the "positive column"?); that should be a hint that it is the former...

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This purplish glow emitted from the central capillary is the light from the Balmer series of the hydrogen spectrum.

Balmer series
The visible hydrogen emission spectrum lines in the Balmer series. $H_\alpha$ is the red line at the right.
(from Wikipedia:Balmer series)

These spectral lines ($H_\alpha$, $H_\beta$, $H_\gamma$, $H_\delta$, $H_\epsilon$) are emitted by excited hydrogen atoms, when the electron falls from a higher level ($n = 3, 4, 5, 6, 7$) down to the $n = 2$ level.

The fact that your tube is filled with deuterium (instead of normal hydrogen) makes no big difference, since the wavelengths depend on the reduced mass of electron and nucleus $\left( \mu = \frac{m_e m_N}{m_e + m_N} \right)$ which is very close to the electron mass $m_e$ in any case. See also: What is the difference between the Balmer series of hydrogen and deuterium?

  • $\begingroup$ Oh yes, it does make a difference. Maybe most because the pressure is also different for such tubes. oceanoptics.com/product/dh-mini/#tab-graphs $\endgroup$ – Pieter Mar 27 '19 at 23:03
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    $\begingroup$ @Pieter Ok, deuterium may give significantly higher intensity than hydrogen. But the wavelengths are nearly the same for both. $\endgroup$ – Thomas Fritsch Mar 28 '19 at 2:07

The glowing gas in the capillary must be mostly atomic excited deuterium. Some free electrons and some D2+ ions, probably. The stream of electrons excite the D atoms, and they emit the Balmer lines, which is the purple glow.


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