Electron-capture and ionization cross sections for collisions ofHe2+with Li: Production ofHe+(3l)at low velocities

Abstract

Single-electron capture and ionization cross sections have been calculated for collisions of ${\mathrm{He}}^{2+}$ with Li over the relative collision velocity range 0.1-10.0 × ${10}^8$ cm/sec. For low collision velocities (i.e., $v=0.1-0.6\mathrm{}\times {10}^8$ cm/sec), a molecular approach has been used to determine the electron-capture cross section. Ab initio potential-energy curves and coupling matrix elements have been computed for the ${\mathrm{}\left(\mathrm{HeLi}\right)\mathrm{}}^{2+}$ system and used in a cross-section evaluation based on the perturbed-stationary-state method. For the low velocities, the electron-capture reaction ${\mathrm{He}}^{2+}+\mathrm{Li}\to {\mathrm{He}}^{+}\mathrm{}\left(3l\right)+{\mathrm{Li}}^{+}$ is found to dominate the collision process, and proceeds with a large cross section $\sigma \approx {10}^{-15\mathrm{}}$ ${\mathrm{cm}}^2$, thus providing the possibility for population inversion and the subsequent emission of Lyman-$\alpha$ (304 Å) and Lyman-$\beta$ (256 Å) soft-x-ray photons. At the higher velocities (i.e., $v=1.4-10.0\mathrm{}\times {10}^8$ cm/sec), a classical-trajectory Monte Carlo method has been used to estimate both the electron-capture and ionization cross sections. At 2.2 × ${10}^8$ cm/sec, both cross sections are found to be equal, while one ionization dominates the collisions process at higher velocities and electron-capture dominates at lower velocities.