Production of Metastable Hydrogen Atoms in Proton—Rare-Gas Collisions

Cross sections for electron capture into the metastable hydrogen $2s$ level for proton---rare-gas collisions have been measured. The $2s$ state population is determined by application of an electric field in the reaction region leading to a mixing of the 2s1/2 and the 2p1/2 atomic substates. The increase in the detected Lyman---alpha signal over that observed in the field-free case is related to the ratio of the cross sections for capture into the $2s$ and the $2p$ states. The cross sections, measured for proton energies between 1.5 and 23 keV, exhibit structure similar to that observed in $2p$-state capture. Possible interpretations of the results are discussed.