Dissociation of Carbon Dioxide in the Positive Column of a Glow Discharge

Abstract

The dissociation of CO₂ by electron impact was investigated in the positive column of a dc glow discharge for gas pressures between 0.3 and 4 mm Hg and a current of 0.6 mA in a tube of radius 1.25 cm. The reaction rate was determined by separating and measuring the noncondensable products formed. The effects of the cathode and anode regions of the discharge were eliminated by varying the interelectrode distance to obtain the axial electric field in the positive column and also the molecular dissociation rate per unit length of column. The number of dissociating collisions ${\alpha }_d\mathrm{\hspace{0.17em}/\hspace{0.17em}p}$ by an electron drifting unit length along the field direction of the positive column was measured for values of $\mathrm{E\hspace{0.17em}/\hspace{0.17em}p}$ between 11 and 32 V cm⁻¹·(mm Hg)⁻¹ showing corresponding values of ${\alpha }_d\mathrm{\hspace{0.17em}/\hspace{0.17em}p}$ of 0.15 and 0.67 at these limits. The measured rate coefficients show that dissociation proceeds via uncharged species. A reaction mechanism $${\mathrm{CO}}_2\mathrm{+e\to }{\displaystyle {lim}^{\mathrm{collision}}}\mathrm{CO}+\mathrm{O}$$ is consistent with the observed phenomena. It is suggested that the dissociative excitation is to a ³Π state of CO₂ about 6.1 eV above the ground state. Assuming an onset potential of 6.1 eV, a cross section that rises to a maximum of 3.5 × 10⁻¹⁷ cm² at 6.9 eV and then rapidly decreases with increasing electron energies, is compatible with the observed dissociation‐rate coefficients.