Dissociation of Molecular Hydrogen Ions Enhanced by Ion-Source Operation

The cross section for the dissociation reaction ${\mathrm{H}}_3^{+}$+${\mathrm{H}}_2$→ ${\mathrm{H}}_2^{+}$+H+${\mathrm{H}}_2$ increased by a factor of about 4 when the rf ion-source magnet field was raised. This increase is attributed to ${\mathrm{H}}_3^{+}$ ions in high vibrational states. The energy dependence of the cross section was measured. Excited ions increased from 6 ${\mathrm{\AA }}^2$ at 5 keV to 10 ${\mathrm{\AA }}^2$ at 50 keV, whereas the normal ions increased from 1.5 to 2.5 ${\mathrm{\AA }}^2$ over the same energy range. The companion reaction cross section, for ${\mathrm{H}}_3^{+}$ producing ${\mathrm{H}}^{+}$, was 0.5 ${\mathrm{\AA }}^2$ at 5 keV increasing to 1.6 Å at 35 keV. Cross sections were about 0.25 ${\mathrm{\AA }}^2$ higher for excited ions than for normal ions. The cross section for dissociation of ${\mathrm{H}}_2^{+}$ to ${\mathrm{H}}^{+}$ was about 1.6${\mathrm{\AA }}^2$ from 5 to 45 keV. ${\mathrm{H}}_5^{+}$ was detected; its dissociation to ${\mathrm{H}}^{+}$ with one-fifth the energy gave a cross section of 3.2 ${\mathrm{\AA }}^2$ at 20 keV, independent of source condition. Excited ${\mathrm{H}}_3^{+}$ may be a useful ion for transverse injection into a magnetically confined plasma.