Measurements of the Attachment of Low-Energy Electrons to Oxygen Molecules

The attachment coefficients for low-energy electrons to oxygen are determined from the electron and negative-ion waveforms resulting when a pulse of photoelectrons traverses a drift tube. The pressure dependence of the attachment coefficient shows that for average electron energies below about 1 eV the electrons attach to oxygen in three-body collisions involving two oxygen molecules or an oxygen molecule and a foreign gas molecule. The three-body coefficient for pure oxygen increases with electron energy to a maximum of 5×${10}^{-30\mathrm{}}$ ${\mathrm{cm}}^6$/sec at an average energy of 0.09 eV and then decreases slowly. At gas temperatures of 370, 300, 195, and 77°K the three-body attachment coefficients for thermal electrons are, respectively, 3.1×${10}^{-30\mathrm{}}$, 2.8×${10}^{-30\mathrm{}}$, 2×${10}^{-30\mathrm{}}$, and less than 1×${10}^{-30\mathrm{}}$ ${\mathrm{cm}}^6$/sec. The efficiences of ${\mathrm{N}}_2$ and He as third bodies are approximately 50 and 100 times lower than for ${\mathrm{O}}_2$. These results predict attachment coefficients for the lower ionosphere which are one to two orders of magnitude lower than the previously accepted values. For average electron energies above about 1.5 eV the pressure dependence and the magnitude of the attachment coefficient are consistent with previous electron beam studies of the process of dissociative attachment.