Selection rules for the intermolecular enhancement of spin forbidden transitions in molecular oxygen

An experimental examination of the spin forbidden transitions of molecular oxygen dissolved in various solvents and in the gas phase reveals the following selection rules governing the intermolecular enhancement of these transitions: (i) Nonheavy atom solvents selectively enhance the ${}^1{\Delta }_g{\mathrm{\leftarrow \hspace{0.17em}}}^3{\Sigma }_g^{-}$ transition (factor of 1000) but not the ${}^1{\Sigma }_g^{+}{\mathrm{\leftarrow \hspace{0.17em}}}^3{\Sigma }_g^{-}$ transition; (ii) the bimolecular oxygen enhancement of the ${}^1{\Delta }_g{\mathrm{\leftarrow \hspace{0.17em}}}^3{\Sigma }_g^{-}\text{(0,0)}$ transition is selectively suppressed in solution by a factor of ∼ 3; (iii) heavy atom solvents enhance the ${}^1{\Sigma }_g^{+}{\mathrm{\leftarrow \hspace{0.17em}}}^3{\Sigma }_g^{-}$ transition (factor of 10 in iodine containing solvents) but not the ${}^1{\Delta }_g{\mathrm{\leftarrow \hspace{0.17em}}}^3{\Sigma }_g^{-}$ transition. The possible origins of these different selection rules are discussed in terms of previous theoretical treatments of the interaction of oxygen with electronically excited molecules.