Absolute partial cross sections from threshold to 1000 eV are reported for electron-impact ionization of ${\mathrm{H}}_2$, ${\mathrm{N}}_2$, and ${\mathrm{O}}_2$. Data are presented for the production of ${\mathrm{H}}_2^{\mathrm{}+}$ and ${\mathrm{H}}^{+}$ from ${\mathrm{H}}_2$; the production of ${\mathrm{N}}_2^{\mathrm{}+}$, ${\mathrm{N}}^{+}$+${\mathrm{N}}_2^{\mathrm{}2+}$, and ${\mathrm{N}}^{2+}$ from ${\mathrm{N}}_2$; and the production of ${\mathrm{O}}_2^{\mathrm{}+}$, ${\mathrm{O}}^{+}$+${\mathrm{O}}_2^{\mathrm{}2+}$, and ${\mathrm{O}}^{2+}$ from ${\mathrm{O}}_2$. The product ions are mass analyzed with a time-of-flight mass spectrometer and detected with a position-sensitive detector whose output provides clear evidence that all energetic fragment ions are collected. The overall uncertainty in the absolute cross-section values for singly charged parent ions is ±3.5% and is marginally higher for fragment ions. Previous cross-section measurements are compared to the present results. © 1996 The American Physical Society.

• Received 27 December 1995

DOI:https://doi.org/10.1103/PhysRevA.54.2146