  In order to develop a better understanding of electrochemical $\mathrm{O_2}$
reduction in non-aqueous solvents, we apply two-photon photoelectron
spectroscopy to probe the dynamics of $\mathrm{O_2}$ reduction at a
DMSO/Cu(111) model battery interface. By analyzing the temporal evolution of
the photoemission signal, we observe the formation of $\mathrm{O_2^-}$ from a
trapped electron state at the DMSO/vacuum interface. We find the vertical
binding energy of $\mathrm{O_2^-}$ to be 3.80 $\pm$ 0.05 eV, in good agreement
with previous results from electrochemical measurements, but with improved
accuracy, potentially serving as a basis for future calculations on the
kinetics of electron transfer at electrode interfaces. Modelling the
$\mathrm{O_2}$ diffusion through the DMSO layer enables us to quantify the
activation energy of diffusion (31 $\pm$ 6 meV), the diffusion constant (1
$\pm$ 1$\cdot 10^{-8}$ cm$^2$/s), and the reaction quenching distance for
electron transfer to $\mathrm{O_2}$ in DMSO (12.4 $\pm$ 0.4 $\unicode{x212B}$),
a critical value for evaluating possible mechanisms for electrochemical side
reactions. These results ultimately will inform the development and
optimization of metal-air batteries in non-aqueous solvents.
