Formation of metastable hydrogen atoms by charge exchange of protons in cesium vapor

Formation of an intense beam of hydrogen atoms in the metastable $2s$ state is not only of basic interest, but is also useful for polarized-ion sources of the Lamb-shift type. The nearly resonant reaction H⁺+Cs→H(2s)+Cs² is often used for this purpose. The fraction f of metastable atoms relative to the number of neutral atoms in all $n=2$ states formed in the collision of a proton in a thin Cs-vapor target is reported here for the energy range 0.4-3.0 keV. An apparent maximum of f=0.43±0.03 at 0.5 keV is found. For energies above 0.75 keV, f=0.25±0.01, which would be expected if the $n=2$ states of H are statistically populated. Previously reported values for σ₊ₘ, the cross section for electron pickup in the metastable $2s$ state of hydrogen for H⁺ + Cs collisions, differ by two orders of magnitude for a given energy. It can be assumed that essentially all electron pickup is into the $n=2$ states at low energy. Thus measured values of f can be multiplied by reported values of σ₊₀, the cross section for electron pickup into any neutral state of the H atom, to find values for σ₊ₘ. As this method is independent of calibration or normalization of the photon measurements, these values for σ₊ₘ are more reliable than those previously reported. Our results for σ₊ₘ are between 4.3×{}10^-15 and 1.5×{}10^-15 cm² in the energy range 0.5-3.0 keV.