IONIZATION AND DISSOCIATION OF FAST H$sub 2$ MOLECULES INCIDENT ON H$sub 2$ GAS

The yield cross sections ${\ensuremath{\sigma}}_{{{\mathrm{H}}_{2}}^{+}}$, ${\ensuremath{\sigma}}_{{\mathrm{H}}^{+}}$, and ${\ensuremath{\sigma}}_{\mathrm{H}}$ for production of fast ${\mathrm{H}}_{2}^{+}$ and ${\mathrm{H}}^{+}$ ions and H atoms resulting from the ionization and dissociation of fast ${\mathrm{H}}_{2}$ molecules in single collisions with ${\mathrm{H}}_{2}$ gas molecules have been measured in the 6- to 120-keV energy range (collision velocity 0.7 to 3.2\ifmmode\times\else\texttimes\fi{}${10}^{8}$ cm/sec). The neutral primary beam contained a fraction of H atoms which was determined by proportional counter analysis. Secondary particles produced by the H atoms in the primary beam were determined in separate H beam experiments and subtracted from the composite (H,${\mathrm{H}}_{2}$) neutral beam data to yield the production of secondaries due to ${\mathrm{H}}_{2}$ primaries alone. Cross section ${\ensuremath{\sigma}}_{{{\mathrm{H}}_{2}}^{+}}$ increases monatonically from 0.5 to 2.0 ${\mathrm{\AA{}}}^{2}$ in the energy range 6 to 120 keV while ${\ensuremath{\sigma}}_{{\mathrm{H}}^{+}}$ increases from 0.28 ${\mathrm{\AA{}}}^{2}$ at 7.5 keV to a maximum of 0.70 ${\mathrm{\AA{}}}^{2}$ at 50 keV. Cross section ${\ensuremath{\sigma}}_{\mathrm{H}}$, measured only at 10 keV, was found to have an approximate value of 2.5 ${\mathrm{\AA{}}}^{2}$ based on an assumed shape of the H atom angular distribution at small angles. The results are compared with data on a number of collision processes in the same velocity range involving various species of hydrogen ions and H atoms incident on ${\mathrm{H}}_{2}$ gas molecules.