Temperature Dependence of Dissociative Attachment inO2and CO2

The onset, the shape, and the absolute magnitude of dissociative-attachment cross sections in ${\mathrm{O}}_2$ and C${\mathrm{O}}_2$ have been determined as functions of gas temperature. The range of temperatures is 300-1000 ° K. In ${\mathrm{O}}_2$, the onset of dissociative attachment is lowered from 4.4 eV at 300 ° K to 3.5 eV at 1000 ° K, in good agreement with O'Malley's theory. In C${\mathrm{O}}_2$, the first onset of ${\mathrm{O}}^{-}$ production occurs at 3.85 eV for a gas temperature of 300 ° K and at 3.35 eV for a gas temperature of 950 ° K. The onset extrapolates to 4.1 eV at 0 ° K. The magnitude of the cross section of the first peak in C${\mathrm{O}}_2$ increases monotonically with temperature, whereas the magnitude of the second peak decreases in the range 300-500 ° K, and then increases. It is found that the control of gas temperature, i.e., the control of the distribution of rotational and vibrational excitations of the target molecules, is necessary for an understanding of dissociative-attachment processes, and for the determination of certain molecular constants from such experiments.