Total-cross-section measurements for positrons and electrons colliding with ${\mathrm{H}}_2$, ${\mathrm{N}}_2$, and C${\mathrm{O}}_2$

Total scattering cross sections have been measured in the same apparatus for positrons and electrons colliding with ${\mathrm{H}}_2$, ${\mathrm{N}}_2$, and C${\mathrm{O}}_2$ using a beam transmission technique. The projectile impact energies range from 1 - 500 eV for $e^{+}-{\mathrm{H}}_2,2-500\mathrm{}$ eV for $e^{-}-{\mathrm{H}}_2,0.5-750\mathrm{}$ eV for $e^{+}-{\mathrm{N}}_2,2.2-700\mathrm{}$ eV for $e^{-}-{\mathrm{N}}_2,0.5-60\mathrm{}$ eV for $e^{+}-{\mathrm{CO}}_2$, and $2-50$ eV for $e^{-}-{\mathrm{CO}}_2$. The onset of positronium formation is clearly seen by an abrupt rise in the total cross sections for positrons colliding with each of the molecules at the respective positronium-formation thresholds. The positron measurements are compared with the electron measurements at intermediate energies for ${\mathrm{H}}_2$ and ${\mathrm{N}}_2$. This comparison reveals a merging of the cross sections for ${\mathrm{H}}_2$ at energies above 200 eV, while for ${\mathrm{N}}_2$ the electron results remain higher than the positron results at all energies. Estimates are made of potential experimental errors, as well as the experimental resolution for discrimination against projectiles scattered at small forward angles.