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

Abstract

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