Energy Spectra of Metastable Oxygen Atoms Produced by Electron-Impact Dissociation ofO2

Kinetic energies of metastable O atoms formed in dissociative excitation of ${\mathrm{O}}_2$ were measured by a time-of-flight experiment. Metastable $\mathrm{O}\left({}_{}{}^5{}_{}{}^{}S_{}^0{}_{}{}^{}\right)$ atoms (9.14 eV) as well as high-lying states near the ionization limit of O (13.6 eV) were detected by means of Auger ejection of secondary electrons from a Cu-Be surface. The energy spectra of metastable fragments showed distinct structure with maxima near 0.3 and 2eV. Repulsive-potential-energy curves of ${\mathrm{O}}_2$ were constructed from the measured energy spectra and time-resolved excitation functions. The excitation function for the direct excitation of the $\mathrm{O}\left({}_{}{}^5{}_{}{}^{}S_{}^0{}_{}{}^{}\right)$ state had a threshold of 14.3 ± 0.2 eV which is equal to the minimum energy for dissociatively exciting the $\mathrm{O}\left({}_{}{}^5{}_{}{}^{}S_{}^0{}_{}{}^{}\right)$ state. Using selection rules and symmetry arguments concerning the angular variation of dissociation products, the molecular state reached near 14.3 eV was identified as a ${\Pi }_u$ state with a probable multiplicity of 3. Other molecular states reached at higher energies were not identified. It seems, however, that there was a sizable contribution due to O atoms in high-lying Rydberg states excited in dissociative excitation. The differential cross section for the direct formation of $\mathrm{O}\left({}_{}{}^5{}_{}{}^{}S_{}^0{}_{}{}^{}\right)$ was found to be $\frac{d\sigma }{d\Omega }\simeq 1\times {10}^{-19\mathrm{}}$ ${\mathrm{cm}}^2$/sr within a factor of 2 at an electron energy of 30 eV and an angle of 90° with respect to the electron beam. Several atmospheric applications of the measurements concerning the heating of the atmosphere, radiative transfer of resonance radiation, and chemical reactions are mentioned briefly.