High-resolution measurement of resonances ine−O2scattering by electron time-of-flight spectroscopy

Resonances in the $e-{\mathrm{O}}_2$ total cross section in the energy range 50-300 meV were studied with a high-resolution ($\Delta E<\mathrm{}10\mathrm{}$ meV) electron time-of-flight spectrometer. Each vibrational resonance was found to have a well-resolved doublet structure, which has not previously been observed, and which is due to spin-orbit coupling in the ${}_{}{}^2{}_{}{}^{}\Pi _g^{}{}_{}{}^{}$ ground state of ${\mathrm{O}}_2^{-}$. The spin-orbit coupling constant of ${\mathrm{O}}_2^{-}$ was measured to be $\mathrm{}\left|A\right|=20\mathrm{}\pm 2 \mathrm{meV}=161\mathrm{}\pm 16 {\mathrm{cm}}^{-1\mathrm{}}$, in agreement with theoretical estimates. On an energy scale established from the measured electron flight times, the energy of the center of the $v^{\prime }=4\mathrm{}$ resonance was measured to be 91 ± 5 meV. This value is in agreement with a recent photodetachment measurement of the electron affinity of ${\mathrm{O}}_2$, and with a previously observed accidental coincidence between the $v=3\mathrm{}$ level of ${\mathrm{O}}_2$ and the $v^{\prime }=8\mathrm{}$ level of ${\mathrm{O}}_2^{-}$. The electron time-of-flight spectrometer is described in detail.