Observation of Site-Specific Electron Emission in the Decay of Superexcited $O_2$

The photodissociation of superexcited ${\mathrm{O}}_2^{*}$ to $\mathrm{O}{(}^{3}P)$ and ${\mathrm{O}}^{*}\left({[}^2{P}^0\right]{3d}^{\prime \prime })$ atoms, with subsequent autoionization to ${\mathrm{O}}^{+}{(}^2{D}^0\left){+e}^{-}\right(0.16\mathrm{eV})$ at 22.36 eV photon energy, was studied by a new method: angle resolved photoelectron-photoion coincidence spectroscopy. A position-sensitive detector was used to measure the three components of the fragment ion velocity. When the excited atom autoionizes it emits an electron preferentially along the photodissociation axis, with a 12% greater probability to be emitted away from the recessing ground-state atom than towards the latter, pointing to possible intramolecular scattering of the electron on its way out.