Question: Is the spectral line signal for hydrogen different for
hydrogen in hydrogen gas compared with hydrogen in water?

In article ,
Martin Brown writes:
I think the answer is yes it will alter the frequencies that the bound
hydrogen atoms emit due to the oxygen distorting the previously perfect
spherically symmetric distribution of the electron wavefunction for an
isolated hydrogen atom. There will also be a bunch of new possible
emission frequencies from a polyatomic molecule too.

That last is hugely important. Water has a vast number of spectral
lines because of the multiple ways the molecule can vibrate and
rotate. These lines are mostly at infrared through millimeter
wavelengths. If the pressure or water abundance is high, there are
so many lines that they blend together in bands. Even at low
pressure and abundance, observations with low spectral resolution
will see bands, not lines.

I suspect the hydrogen can only emit the neutral hydrogen line when it
is in the form of isolated neutral atoms of hydrogen.

If this means the hyperfine structure line at 21 cm, I don't think
water has any such line because there are an even number of
electrons. A water ion with an odd number of electrons would have a
line coming from the same physics, but the energy level separation
and hence the line frequency would surely shift.

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