Production of fast highly excited atoms in proton collisions with atomic hydrogen and argon

The study of keV H⁺-H(1s) collisions has been extended to the production by electron transfer of highly excited states with principal quantum numbers between 13 and 28. A fast-proton beam was passed through thermally dissociated hydrogen gas, and the resultant fast excited atoms were observed using electric field ionization and proton-counting techniques. The incident proton energy range was from 7 to 60 keV. The observed cross section for production of such excited states possesses a broad maximum near 30 keV. The sum of cross sections over the observed principal quantum numbers is 0.00089 Ų at 7 keV, 0.0070 Ų at 30 keV, and drops again with energy to 0.0023 Ų at 60 keV. Possible error in our cross section scale is ± 30%. Thus the fraction of electron-transfer collisions producing these highly excited states varies from a maximum value of about 0.03% in the range 30-60 keV to the lower value of 0.01% at 7 keV, values not markedly different from those found for inert-gas target atoms; this is contrary to the predictions of the Brinkman-Kramers-Oppenheimer approximation. The data at 60 keV agree with recent calculations by Band using the coupled Born approximation. The present results for H⁺-Ar collisions and 13≤n≤28 are in agreement with previous investigations for 9≤n≤15, assuming that the Jackson-Schiff scaling rule is valid for this collision system.