Electron Capture and Ionization Cross Sections for Collisions of He(++) with Li: Production of He(+)(3l) at Low Velocities

Single electron capture and ionization cross sections have been calculated for collisions of He(++) with Li over the relative collision velocity range 0.1-10.0 x 10 to the 8th power cm/sec. For low collision velocities (i.e., v = 0.1-0.6 x 10 to the 8th power 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 (HeLi)++ system and used in a cross section evaluation based on the perturbed-stationary-state method. For the low velocities, the electron capture reaction He(++) + Li yields He+(3l) + Li(+) is found to dominate the collision process and proceeds with a large cross section, sigma approximately 10 to the minus 15th power sq cm, thus providing the possibility for population inversion and the subsequent emission of Lyman-alpha (304 A) and Lyman-beta (256 A) soft x-ray photons. At the higher velocities (i.e., v=1.4-10.0 x 10 to the 8th power cm/sec), a classical-trajectory Monte-Carlo method has been used to estimate both the electron capture and ionization cross sections. At 2.2 x 10 to the 8th power cm/sec, both cross sections are found to be equal, while one ionization dominates the collision process at higher velocities and electron capture dominates at lower velocities.
Prepared in cooperation with SRI International, Menlo Park, CA. Molecular Physics Lab., Rept. no. MP-78-02.