Vibrational Excitation Effects on Charge-Transfer Processes InvolvingH2+andD2+Between 70 and 1000 eV

Abstract

Charge-transfer cross sections have been measured for ${\mathrm{H}}_2^{+}$ in ${\mathrm{H}}_2$, ${\mathrm{D}}_2$, and Ar, and for ${\mathrm{D}}_2^{+}$ in ${\mathrm{H}}_2$ as a function of both incident ion energy and ion-source electron energy. The ion energy range was 70-1000 eV for the ${\mathrm{H}}_2^{+}$ +${\mathrm{H}}_2$ case and 250-1000 eV for the processes ${\mathrm{H}}_2^{+}$ + ${\mathrm{D}}_2$, ${\mathrm{H}}_2^{+}$ + Ar, and ${\mathrm{D}}_2^{+}$ + ${\mathrm{H}}_2$. Ionizing electron energy was varied from 16 to 21 eV. The primary ${\mathrm{H}}_2^{+}$ ions were magnetically separated from other beam constituents. Charge-transfer cross sections were deduced from slow-ion currents measured in a charge-transfer cell employing a cylindrical slow-ion energy analyzer. Measured cross sections for ${\mathrm{H}}_2^{+}$ in ${\mathrm{H}}_2$ decreased 5 to 10% as the electron energy was increased over the 5-eV interval. The minimum in the ${\mathrm{H}}_2^{+}$ + ${\mathrm{H}}_2$ cross-section-versus-energy curve observed by other investigators was found to persist at low electron energies, thus ruling out the possibility of contributing exothermic effects by highly excited incident ions. The lack of agreement between experimental results and theory is discussed. Sizable differences were observed at low electron energies between the ${\mathrm{D}}_2^{+}$ + ${\mathrm{H}}_2$ and the ${\mathrm{H}}_2^{+}$ + ${\mathrm{D}}_2$ cross sections. Both results fell below those for the symmetric ${\mathrm{H}}_2^{+}$ + ${\mathrm{H}}_2$ case. At higher electron energies, these differences were smaller. An overall change of 20% in the ${\mathrm{H}}_2^{+}$ + Ar cross section was observed over the given electron-energy interval with 700-eV ions.