The notation (which I found in [this paper](https://dx.doi.org/10.1029/97JD03141)) is 

$$O_{2} a^{1} Δ_{g} ← X^{3}Σ^{-}_{g} $$

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.