Electron capture from aligned p-state Rydberg atoms

Electron capture by Li⁺ ions from aligned Na(25p) Rydberg atoms was measured as a function of the approach angle between the alignment axis of the target-atom wave functions and the ion-beam direction, over a range of reduced velocity ṽ=1.0–2.0, where ṽ=v_ion/v_e. A Stark barrel was used to generate uniform rotatable electric fields over the collision volume and to align Rydberg atoms along the field at arbitrary angles over a full range of 360°. A mixture of Na(25p) fine-structure levels was prepared in the Stark barrel by two-step pulsed laser excitation in the presence of a moderate electric field (about 56 V/cm) followed by adiabatic switching and rotation to 1.8 V/cm. The relative capture cross section, which varied sinusoidally as a function of the alignment angle, had maxima at 0° and 180° and minima at 90° and 270° throughout this velocity range, showing a strong preference for capture from the endwise arrangement. The alignment effect weakened as the projectile velocity was increased. The Rydberg-state results differ in both magnitude and trend from those based on low-lying aligned states. The spin-independent asymmetry parameter was inferred from the angle-dependent capture cross sections after the spin-orbit depolarization effect in the mixed Rydberg p state was fully accounted for so that the results correspond purely to molecular Σ and Π collision geometries.