Caption: Grotrian diagram of Li I (i.e., neutral lithium).

Features:

1. The labeled atomic line wavelengths are in angstroms (Å).

2. The transition line from the ground state labeled 6707 is in fact a doublet (e.g., Allen 1973, p. 53) consisting of two lines Li I 670.776 Å A_ki=3.6891e+07 s**(-1), 670.791 Å A_ki=3.6890e+07 s**(-1) (scroll ∼ 40%), where A_ki stands for Einstein A coefficient (which is linear measure of line strength among other things).

3. The Li I doublet is the strongest Li I transition line because arises from ground state and its Einstein A coefficients are larger by a factor of ∼ 30 than the other ground state doublets labeled 2741 and 3232 in the Grotrian diagram (see NIST: Li I lines).

4. Because the Li I doublet is the strongest Li I transition line and it occurs in visible band (fiducial range 0.4--0.7 μm), it is almost certainly the most obvious Li I transition line in astrophysical spectra. Since the occupation number of an ionization stage ground state is almost always overwhelmingly dominant, it is usually almost the same as the of the ionization stage occupation number. This makes observations of the Li I doublet highly usefull in determining lithium abundance from spectroscopy and modeling.

5. There is, in fact, a problem (or anomaly in Kuhnian paradigm jargon) with the cosmic composition abundance of lithium called cosmological lithium problem. Standard Big Bang nucleosynthesis (cosmic time ∼ 10--1200 s ≅ 0.17--20 m) predicts ∼ 3 times more lithium-7 (Li-7) than is observed. Since the other predictions Big Bang nucleosynthesis are in very good agreement with observations, one believes that the discrepancy with respect to Li-7 must have a solution without discarding standard Big Bang nucleosynthesis. But no accepted solution has materialized after decades of searching (see, e.g., Hou et al. 2017). It's possible that the solution will non-revolutionary, but, on the other hand, it could cause a paradigm shift.