Measurement of the Total Cross Section for Charge Transfer into the Metastable StateH(2s)for Protons Incident onH2and Ar Gases

The energy dependence of the total cross section for ${\mathrm{H}}^{+}+{\mathrm{H}}_2\to \mathrm{H}\mathrm{}\left(2s\right)+{\mathrm{H}}_2^{+}$ exhibits two broad maxima in the range 2-70 keV. The lower maximum occurs at an energy where the total charge-transfer cross section is largest, and thus may be due to coupling with the ground-state scattering channel. The observed energy dependence is similar to that for ${\mathrm{H}}^{+}$ + ${\mathrm{H}}_2$→Ly $\alpha$, but with the positions of both maxima shifted to higher energies by about 5 keV. The cross section for ${\mathrm{H}}^{+}+\mathrm{Ar}\to \mathrm{H}\mathrm{}\left(2s\right)+{\mathrm{Ar}}^{+}$ over the extended energy range 2-70 keV also exhibits only two maxima. Internal and external quenching measurements of this cross section are in agreement. Its absolute magnitude at 20 keV has now been independently measured by three investigators, with results consistent with ± 20%.