Why are helium resonance lines called "resonance lines"?

Question

Examples of the use of the term:

• Formation of the helium extreme-UV resonance lines
• On the Formation of the Resonance Lines of Helium in the Sun (unpaywalled)
• Formation of the helium EUV resonance lines.

When I hear of a resonant state I think of a free particle incident on a potential where it spends a lot of time then leaves, but the electrons responsible for these lines (HeI 584 Å, HeII 304 Å) start in bound states below their binding energy and are just transitions from one bound state to another.

Since most emission lines from atomic transitions are not called resonance lines:

1. Why are these lines different?
2. What is particularly resonant about them?

Answer 1

It appears to be a conventional label that is applied to transitions between the ground state and another energy level (some definitions specify the first excited level) of an atom and is used in all the physical sciences, not just astrophysics.

e.g. He I (58.4 nm) is a transition from $^1$P to the $^1$S ground state.

In fact all atomic/ionic transitions can be considered resonant phenomena, but the term "resonance line" is applied only to these particular types of transition, perhaps because they are usually the strongest lines in the spectrum from that species, since in most cases, the ground state is the most populated. So there is nothing "particularly resonant" about them

A "resonance line" is:

A spectral line caused by an electron jumping between the ground state and the first energy level in an atom or ion. It is the longest-wavelength line produced by a jump to or from the ground state. Because the majority of electrons are in the ground state in many astrophysical environments, and because the energy required to reach the first level is the least needed for any transition, resonance lines are the strongest lines in the spectrum for any given atom or ion.

https://www.oxfordreference.com/view/10.1093/oi/authority.20110803100415843

And here it is used in that context in astrophysics: (Morton 2013)

The tabulation emphasizes resonance lines, i.e., lines whose lower level is the ground state.

Or in Chemistry: From https://goldbook.iupac.org/terms/view/R05341

The radiative decay of an excitation level may proceed to the neutral ground state and would thus occur at the same energy as the corresponding line in the absorption spectrum. Such a line is called a resonance line and the process is called resonance emission.

Or from the Basic Atomic Spectroscopic Handbook:(https://www.nist.gov/pml/basic-atomic-spectroscopic-data-handbook)

The strongest persistent lines usually include one or more resonance lines, i.e., transitions to the ground level or term.

Answer 2

I can not assure that this terminology is different in other fields than in solar physics, but when a line is called to be "resonant" is because the lower level of the transition is the ground level of the atom (or ion).

EDIT: after some research about this terminology, I think I've found the origin of "resonant" lines. The first mention of this phenomenon, called at that epoch as "resonance radiation" was done by Robert W. Wood at 1903. The phenomenon was described, briefly, as the property of certain atoms and molecules to emit light at the same wavelength as the incident beam exciting them. Wood called these lines as "resonance radiation lines", and he gave an explanation of this arguing that the atom/molecule is excited to a "resonant" state in which the deexcitation produces light at the same frequency. Most of the experiments were done using sodium vapor and assume (because I didn't read the experimental papers by lack of time) that the vapor was in laboratory conditions (i.e. in the fundamental state).

The first link that I've found in the literature between this phenomena and the Quantum Theory is a publication made in Nature at 1917 be T. K. Chinmayam, where the author suggest that the spectral structure of the resonance radiation emitted by the vapour can be represented following the Bohr's theory.

My bibliographic search stops here, but I guess that due to the development on the Quantum Theory, the concept of "resonance radiation lines" evolves to the well-established terminology of "resonant lines" for the lines where the lower level is the ground level.