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The low $1s2s$ levels of a helium atom lack an allowed direct electric dipole decay to the ground $1s^2{}^1{S}_0$ level. The triplet is excluded by the spin selection rule, and the singlet-to-singlet $S$ transition is excluded by parity and orbital selection. Weaker radiative channels and collisions can still de-excite these states; metastable does not mean infinitely lived.

Molecular excitations in organic semiconductors often retain strong local character. Weak screening supports Frenkel excitons or intermolecular charge-transfer admixtures, so a photon can create a bound neutral excitation rather than free carriers. Transition dipoles, spin selection rules, molecular packing and Franck-Condon factors set the bright spectral features. Disorder and thermal vibrations broaden them. Electronic overlap can also produce more delocalized excitons; the molecular limit is a useful model, not a definition covering every organic solid.

Measure each energy level by the wavenumber $\tilde{E}=(E-E_g)/(hc)$ in ${\mathrm{m}}^{-1}$. An upward electronic transition of vacuum wavelength $\lambda$ adds $1/\lambda$. Treat the quoted wavelengths as vacuum values to their limited accuracy; air corrections would be smaller than the precision justified here.

The strong ground-state absorption must be $5s^2{}^1{S}_0\to 5s5p{}^1{P}_1$ at $461\mathrm{nm}$. The weak ground-state electronic transition at $689\mathrm{nm}$ must reach $5s5p{}^3{P}_1$: the electric-dipole selection rule on $J$ excludes the other two triplet state energy levels, while weak spin-orbit coupling admixes singlet state character and relaxes the approximate spin selection rule.

Assume the usual ordering $E({}^3{P}_0)<E({}^3{P}_1)<E({}^3{P}_2)$, as expected from a positive effective spin-orbit coupling constant. The three nearby strong spectral lines then join these energy levels to a common $5s6s{}^3{S}_1$ energy level. Larger lower-state energy means a smaller gap and longer wavelength, so $679,688,707\mathrm{nm}$ correspond to $J=0,1,2$ respectively. The remaining $1.12\mu \mathrm{m}$ spectral line joins $5s5p{}^1{P}_1$ and $5s6s{}^1{S}_0$. Assume the latter is the upper energy level. These assignments and orderings agree with independently tabulated strontium energy levels.^[ref]

Starting with the two ground-state gaps, the construction isThe resulting energy levels, rounded to avoid suggesting spectroscopic precision, are

Both $461\mathrm{nm}$ and $1.12\mu \mathrm{m}$ exhibit the normal Zeeman effect. Each connects two singlet state energy levels, with Landé g-factor $g_J=1$ for the $J=1$ energy level and no first-order shift for $J=0$. Their frequency shifts are $0,\pm {\mu }_{B}B/h$. The weak $689\mathrm{nm}$ spectral line also has three components, but $g({}^3{P}_1)=3/2$ gives shifts $0,\pm 3{\mu }_{B}B/(2h)$; a three-component pattern alone does not establish the normal Zeeman effect. The $679\mathrm{nm}$ spectral line likewise has three components with $g({}^3{S}_1)=2$.

Here ${\Delta }_o/B=20$, so the ground term is ${}^6{A}_{1g}$. There are no spin-allowed d–d bands in the displayed manifold under the spin selection rule: the excited terms shown have doublet or quartet rather than sextet multiplicity. Very weak spin-forbidden absorptions can still gain intensity through spin–orbit coupling, and even spin-allowed d–d transitions would be forbidden by the Laporte rule in ideal centrosymmetric coordination. Thus a strong ordinary d–d spectrum should not be predicted simply by multiplying the splitting by a numerical factor. For scale, the lowest quartet branches read roughly $E/B\simeq 17,23,33$ at this abscissa, corresponding to weak features around $1.4\times 10^4$, $1.8\times 10^4$ and $2.6\times 10^4\mathrm{c}{\mathrm{m}}^{-1}$ if such forbidden intensity is detectable. These are graphical estimates, not precision assignments of every weak line.