FORMATION OF METASTABLE HYDROGEN-ATOMS BY CHARGE-EXCHANGE OF FAST PROTONS IN MAGNESIUM AND BARIUM VAPORS

The cross sections for the formation of H(2s) metastable atoms in H+ collisions with Mg and Ba alkaline-earth-metal vapors have been measured over the energy range 1.5-90 keV. The absolute value of the cross sections has been obtained by normalizing the present results to the cross section for H(2s) formation in Ar gas at 30 keV. In Ba, the cross section reaches a maximum value of 5.6 × 10-16 cm2 at 3 keV. In Mg, the maximum value is 3.2 × 10-16 cm2 and occurs at 7 keV. The measured cross sections exhibit structure at high energies which is attributed to inner-shell capture. A comparison of the present results with previous n=6 electron-capture data in Mg indicates that the n-3 capture law is not obeyed. An analysis based on data available for 16 different collisions between H+ ions and ns1 and ns2 targets shows that the scaling law for the velocity vm at which the single-electron-capture cross section is a maximum is not consistent with the Massey adiabatic criterion. Instead, the data are consistent with the relation vm|ΔE|12 where |ΔE|12 is the energy defect of the collision. The fraction of the neutral beam in the 2s state formed by collisions of H+ ions with thin targets of Mg and Ba is also reported. In Ba, the fraction is equal to 0.25 ± 0.05 at 2.0 keV and is rising at low energies. In Mg, the maximum value of the fraction is 0.13 ± 0.03 and occurs at ∼ 7 keV. © 1979 The American Physical Society.