Absolute and total electron-capture cross sections in slowArq+−C60collisions

Abstract

We present experimental absolute and total electron-capture cross sections for ${\mathrm{Ar}}^{q+}\ensuremath{-}{\mathrm{C}}_{60}$ $(q=4,$ 6, and 8--18) collisions at $3.3q \mathrm{keV}.$ The absolute scale is based on the ${\mathrm{C}}_{60}$ vapor pressure by Abrefah et al. [Appl. Phys. Lett. 60, 1313 (1992)], which together with the results of Mathews et al. [J. Phys. Chem. 96, 3566, (1992)] are the only possible choices among the many, widely dispersed, vapor pressures reported in the literature. In order to support this statement, we calculate total electron-capture cross sections modeling ${\mathrm{C}}_{60}$ as a pointlike object and as an infinitely conducting sphere (ICS). These model results are shown to differ little, since polarization and finite-size effects are relatively unimportant for the distant collisions at which the outermost (first) electrons are transferred. We are thus able to use the semiempirical formula for absolute ion-atom cross sections by Selberg et al. [Phys. Rev. A 54, 4127 (1996)], treating ${\mathrm{C}}_{60}$ as a hypothetical atom with ionization potential ${I}_{1}=7.6 \mathrm{eV},$ and the ICS model to define lower and upper bounds for the absolute cross-section scale. We find significant oscillations in the total electron-capture cross sections as functions of q.