$e^{\pm }-\mathrm{CO}$ and $e^{\pm }-\mathrm{C}{\mathrm{O}}_2$ total cross-section measurements

As part of a continuing series of investigations, total scattering cross sections have been measured in the same apparatus for positrons and electrons colliding with CO and C${\mathrm{O}}_2$ using a beam-transmission technique. The projectile impact energies are in the range 1-500 eV for $e^{\pm }-\mathrm{CO}$, 30-500 eV for $e^{+}-\mathrm{C}{\mathrm{O}}_2$, and 100-500 eV for $e^{-}-\mathrm{C}{\mathrm{O}}_2$. An important aspect of our work is to compare the corresponding positron and electron total cross section ($Q_T$) curves for a given target gas. For both CO and C${\mathrm{O}}_2$ the electron $Q_T$ values are generally larger than the positron results. For both gases at low energies there are relatively narrow shape resonances for electron scattering and noticeable increases in $Q_T$ after the positronium-formation thresholds for positron scattering. At the highest energies investigated there are indications of a tendency toward merging of the positron and electron curves for each gas. A striking similarity is found between the present $e^{\pm }-\mathrm{CO} Q_T$ curves and the $e^{\pm }-{\mathrm{N}}_2 Q_T$ curves obtained by Hoffman et al. [Phys. Rev. A 25, 1393 (1982)], in that the corresponding shapes and magnitudes are very nearly the same. Estimates of potential experimental errors, as well as the experimental discrimination against projectiles scattered at small forward angles, are made.