Measurement of the Total Cross Section for Charge Transfer into the Metastable StateH(2s)for Proton Collisions with Atomic Hydrogen

The total cross section ${\sigma }_{2s}\left(\mathrm{H}\right)$ for ${\mathrm{H}}^{+}+\mathrm{H}\mathrm{}\left(1s\right)\mathrm{}\to \mathrm{H}\mathrm{}\left(2s\right)+{\mathrm{H}}^{+}$ has been measured over the energy range 3-70 keV. Above 10 keV, absolute cross-section values are believed accurate to within ±35%. Only one maximum is found in the energy dependence of ${\sigma }_{2s}\left(\mathrm{H}\right)$. The position of this maximum lies at an energy consistent with the Massey criterion. A comparison with theoretical cross sections suggests that intermediate-state coupling through the resonant charge-transfer channel ${\mathrm{H}}^{+}+\mathrm{H}\mathrm{}\left(1s\right)\mathrm{}\to \mathrm{H}\mathrm{}\left(1s\right)+{\mathrm{H}}^{+}$ is unimportant at low energies. The low-energy data is consistent with a molecular picture of the collision. Calculations utilizing Sturmian wave functions are not in agreement with the data. The Born approximation cross section lies significantly below experiment between 20 and 70 keV.