Collisional formation and destruction of fast negative hydrogen ions in He, Ne, and Ar targets

Measurements are reported of the cross sections for collisional destruction (single- and double-electron loss) of ${\mathrm{H}}^{\mathrm{-}}$ and for its formation through single- and double-electron capture by H and ${\mathrm{H}}^{+}$, respectively. Three different targets were employed (He, Ne, and Ar) and the velocity (in a.u.) of the projectiles was 2.4≤v≤12.6 for ${\mathrm{H}}^{+}$ and 4<v<9 for H and ${\mathrm{H}}^{\mathrm{-}}$. The charge states emerging from the target were simultaneously detected and the data were analyzed by the growth rate and attenuation methods from which individual cross sections could be deduced. Special attention was paid to the double-capture process in Ar where a very pronounced shell effect was firmly established on experimental grounds and was successfully described by theoretical estimates based on independent-electron probabilities of capture and simple scaling rules. Difficulties for obtaining an overall description of the cross sections for either destruction or formation of ${\mathrm{H}}^{\mathrm{-}}$ based on simple models are discussed.