Formation of metastable hydrogen atoms by charge exchange of fast protons in magnesium and barium vapors

Abstract

The cross sections for the formation of $\mathrm{H}\mathrm{}\left(2s\right)\mathrm{}$ metastable atoms in ${\mathrm{H}}^{+}$ 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 $\mathrm{H}\mathrm{}\left(2s\right)\mathrm{}$ formation in Ar gas at 30 keV. In Ba, the cross section reaches a maximum value of 5.6 × ${10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$ at 3 keV. In Mg, the maximum value is 3.2 × ${10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$ 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 $n=6\mathrm{}$ electron-capture data in Mg indicates that the $n^{-3\mathrm{}}$ capture law is not obeyed. An analysis based on data available for 16 different collisions between ${\mathrm{H}}^{+}$ ions and $n{s}^1$ and $n{s}^2$ targets shows that the scaling law for the velocity $v_m$ 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 $v_m\propto {\left|\Delta E_{\infty }\right|}^{\frac{1}{2}}$ where ${\left|\Delta E_{\infty }\right|}^{\frac{1}{2}}$ is the energy defect of the collision. The fraction of the neutral beam in the $2s$ state formed by collisions of ${\mathrm{H}}^{+}$ 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.