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Description
Dimethylsulfoxide (DMSO) is a widespread aprotic solvent proposed for use in lithium-air batteries [1]. However, the individual steps of electron transfer from the cathode into the solvated oxygen remain unknown. Using a model system of DMSO films on Cu(111) we investigate the fundamental steps in electron transfer using time- and angle-resolved two-photon photoelectron spectroscopy. On an ultrafast timescale we observe the formation and localization of a small polaron located at the surface of the second monolayer within 200 fs. Coverages with $>$ 2 monolayer equivalents additionally exhibit two surface-bound electronic states at 2.41 $\pm$ 0.05 eV and 2.30 $\pm$ 0.05 eV above the Fermi level whose lifetimes are on the order of several seconds. Detailed analysis reveals that both are formed from the same precursor state, most probably the small polaron. Upon exposure to O$_2$, the long-lived surface-bound electronic states are immediately quenched and a new state 2.53 $\pm$ 0.05 eV above the Fermi level appears with laser illumination. Moreover, the small polaron dynamics are shifted down in energy, approaching the energy of the oxygen-related state at times $>$ 200 fs implying that the small polaron forms the latter. Comparison to the gas phase O$_2$ [2] suggests that we are observing the formation of O$_2^-$ at this model electrode/electrolyte interface.
[1] K. M. Abraham, J. Electrochem. Soc. 162, A3021 (2015).
[2] K. M. Ervin et al., J. Phys. Chem. A 107, 8521 (2003).
