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 ${\mathrm{H}}^{+}+\mathrm{Cs}\to \mathrm{H}\mathrm{}\left(2s\right)+{\mathrm{Cs}}^2$ is often used for this purpose. The fraction $f$ of metastable atoms relative to the number of neutral atoms in all $n=2\mathrm{}$ 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 $f=0.43\mathrm{}\pm 0.03$ at 0.5 keV is found. For energies above 0.75 keV, $f=0.25\mathrm{}\pm 0.01$, which would be expected if the $n=2\mathrm{}$ states of H are statistically populated. Previously reported values for ${\sigma }_{+m}$, the cross section for electron pickup in the metastable $2s$ state of hydrogen for ${\mathrm{H}}^{+}$ + Cs collisions, differ by two orders of magnitude for a given energy. It can be assumed that essentially all electron pickup is into the $n=2\mathrm{}$ states at low energy. Thus measured values of $f$ can be multiplied by reported values of ${\sigma }_{+0\mathrm{}}$, the cross section for electron pickup into any neutral state of the H atom, to find values for ${\sigma }_{+m}$. As this method is independent of calibration or normalization of the photon measurements, these values for ${\sigma }_{+m}$ are more reliable than those previously reported. Our results for ${\sigma }_{+m}$ are between 4.3×${10}^{-15\mathrm{}}$ and 1.5×${10}^{-15\mathrm{}}$ ${\mathrm{cm}}^2$ in the energy range 0.5-3.0 keV.