For almost 50 years, attempts have been made to account for the pronounced solvent effect on the lifetime of singlet molecular oxygen, O 2 (a 1 Δ g ). This process is dominated by the O 2 (a 1 Δ g ) → O 2 (X 3 Σ g − ) nonradiative transition. Given the comparatively low O 2 (a 1 Δ g ) excitation energy of ∼7880 cm −1 , existing models have been built upon a foundation of electronic-to-vibrational (e-to-v) energy transfer in which C−H and O−H stretching modes in the solvent act as the dominant energy sink. The latter accounts for large H/D solvent isotope effects on the O 2 (a 1 Δ g ) lifetime. However, recent experiments showing a pronounced temperature effect on the O 2 (a 1 Δ g ) lifetime in some solvents reveal limitations in these models. We have developed a general and computationally tenable model that accounts for both temperature and H/D solvent isotope effects on the O 2 (a 1 Δ g ) lifetime. A key feature of our approach is the need to strike a balance in the oxygen−solvent interaction between weak and strong coupling. In the weak coupling limit, the O 2 (a 1 Δ g ) → O 2 (X 3 Σ g − ) transition probability is determined by the overlap of vibrational wave functions, and this is the main component defining the H/D isotope effects. In the strong coupling limit, the transition probability is determined by an activated process and thus accounts for the observed temperature dependence. In addition to resolving a long-standing oxygen-dependent problem, our model may provide useful insights into a wide range of bimolecular interactions that involve e-to-v energy transfer.

The carbon disulphide (CS_{2}) dimer is formed inside He nanodroplets and identified using fs laser-induced Coulomb explosion, by observing the CS_{2}^{+} ion recoil velocity. It is then shown that a 160 ps moderately intense laser pulse can align the dimer in advantageous spatial orientations which allow us to determine the cross-shaped structure of the dimer by analysis of the correlations between the emission angles of the nascent CS_{2}^{+} and S^{+} ions, following the explosion process. Our method will enable fs time-resolved structural imaging of weakly bound molecular complexes during conformational isomerization, including formation of exciplexes.

Studies of the interactions between molecular oxygen and a perturbing species, such as an organic solvent, have been an active research area for at least 70 years. In particular, interaction with a neighboring molecule or atom may perturb the electronic states of oxygen to such an extent that the O 2 (a 1 Δ g ) → O 2 (X 3 Σ g − ) transition, formally forbidden as an electric dipole process, achieves significant transition probability. We present a computational study of how the geometry of complexes consisting of molecular oxygen and different perturbing species influences the magnitude of spin−orbit coupling that facilitates the O 2 (a 1 Δ g ) → O 2 (X 3 Σ g − ) transition. We rationalize our results using a model based on orbital interactions: a non-zero spin− orbit coupling matrix element results from asymmetric transfer of charge to or from the 1π g orbitals on oxygen. Our results indicate that the atoms in a perturbing species closest to oxygen are responsible for the majority of the spin−orbit interactions, suggesting that large systems can be simplified appreciably. Furthermore, we infer and confirm that an estimate of the spin−orbit coupling matrix element can be obtained from the magnitude of the induced energy splitting of oxygen's 1π g orbitals. These results should provide further momentum in the long-standing issue of understanding phenomena that influence the O 2 (a 1 Δ g ) → O 2 (X 3 Σ g − ) transition.

The collision complex between the ground electronic state of an organic molecule, M, and ground state oxygen, O2(X3Σg-), can absorb light to produce an intermolecular charge transfer (CT) state, often...