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Hyperfine transitions, like the (astronomically) famous 21cm line, involve just spin flips, right?

And the electron jumps we first learn about in school, that release visible or near-visible photons, involve jumps between 'principal' quantum orbitals ($K$ to $L$, a.k.a. 1 to 2, e.g.), right?

So, do the intermediate 'fine transition' leaps involve no change in principal quantum number/level, but a transition between $s$ and $p$ sublevels within the $L$ (#2) level of an atom or molecule?

P.S.: just to be clear, do low-frequency electron jumps like the 21-cm hydrogen line involve just a spin flip, from spin-up to spin-down, and no change in principal level (K to L, e.g.) or sublevel (s to p, e.g.)?

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  • $\begingroup$ Your edit is unclear. What does "low frequency" mean, objectively? Keep in mind that transitions between Rydberg states can happen (at least in principle) at arbitrarily low energy differences. As an example, for hydrogen, transitions between the $n=170$ $s$ state and the $n+1$ $p$ state are at wavelength $\approx 22.5\:\rm cm$. $\endgroup$ Mar 4 '21 at 10:34
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The term "fine transition" is not widely used. (As an example, see this search on Google Scholar $-$ basically all the results are line-wrapped versions of "hyperfine".) The meaning you suggest is quite natural, but if you wanted to use it in a publication then it would be advisable to be clear about precisely what you mean.

In generic atomic physics, there is usually no distinction between transitions that involve fine-structure physics versus transitions that don't, and indeed in larger atoms this classification would be quite hard to set up. (For an example of transitions in action, see the Lines section of the NIST ASD.)

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