The notation (which I found in the abstract of this paper) is

$$\mathrm{O}_{2} \ \ a\,{}^{1} Δ_{g} ← X\,{}^{3}Σ^{-}_{g}. $$

Any help with this?

I understand it's talking about quantum states of the molecule but I can't find a concise source that explains all this notation. The paper it's from is talking about oxygen absorption bands.