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In "How is it possible some substances burn with an invisible flame" it is stated that light emitted from burning hydrogen is almost invisible, more precisely:

You say that hydrogen flames are visible, but in fact a flame of hydrogen burning in air emits almost no light in the visible region. There is an emission band in the near ultraviolet at around 300nm and strong emission in the infrared due to vibrational excitations of the water produced, but almost no visible light.

In another process using hydrogen gas in a discharge tube outcome is that there is a significant red color as well (at 656nm, see the Balmers series).

Why is there this difference in emission from hydrogen in the two different processes? I'm guessing it has something to do with hydrogen molecules not being excited in the same way, i.e. they are being excited with very different energy levels? Is this correct? Can it be quantified further?

mijen67
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    This must be a good question since nobody answered?! ;o) – mijen67 Mar 05 '20 at 15:24
  • Your intuition is correct. In the combustion of hydrogen molecules, very few excited hydrogen atoms are produced, so there is very little atomic emission, e.g., the Balmer lines, as shown in my answer here: https://physics.stackexchange.com/a/768678/313612. In contrast, the electrically energized hydrogen discharge tube has both hydrogen molecules and H atoms that came from hydrogen molecules that were dissociated. Some H atoms are excited and de-excite by emitting the characteristic hydrogen atomic emission lines. And some excited hydrogen molecules emit light, e.g., the Fulcher alpha bands. – Ed V Jul 20 '23 at 13:24

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