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Wikipedia's Astrophysical_maser#; comets mentions some anecdotes of notable masers associated with comets, and Maser emissions from comets begins:

The 18-cm lines of the OH radical are the only well-documented masers in comets. They have been observed in more than 65 comets since 1973. The good knowledge of their excitation mechanisms and their linear regime allow us to estimate the water production rate and its variation with heliocentric distance from radio observations of OH in comets…

I'm guessing that OH around comets starts as water and is dissociated by UV light from the Sun. Either it left in a metastable excited state or it's excited (pumped) up to one by other photons from the Sun.

What is it about the 18-cm lines that allows one to conclude that this is masing; that there is microwave amplification by stimulated emission rather than just strong fluorescence?

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  • $\begingroup$ The same Wikipedia article list the properties of astrophysical master emission, including very high brightness and polarisation. $\endgroup$ – Rob Jeffries Apr 8 at 20:12
  • $\begingroup$ @RobJeffries okay those are aspects associated with masers, but in terms of a real "How do we know..." is there some specific threshold on brightness temperature that can never be reached by pumping, cascades and spontaneous emission alone and can only be the result of amplification by stimulated emission, and comets exceed this threshold? And are there constraints on the type and degree of polarization that can not be produced by any other mechanism and can only occur via amplification by stimulated emission? $\endgroup$ – uhoh Apr 8 at 23:17
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I'm not sure if this is what you're asking but:

fluorescence: excited electrons randomly emit photons.

Superfluorescence: excited electrons emit photons, and there is some self-stimulation, leading to a strong short pulse but little coherence.

Superradiance: strong, shorter pulse, much self-stimulation , high coherence in output.

Lasing/Masing: almost always requires a cavity of some sort, leading to directional coherent output.

I cannot tell exactly what the authors of the article consider qualifying as Maser vs. Superradiance. possibly the material volume in question (on the comet) is elliptical enough to lead to a more-or-less directed beam.

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    $\begingroup$ Astrophysical masers do not involve a cavity. $\endgroup$ – Rob Jeffries May 10 at 9:43

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