lchange in sodium Rydberg atoms induced by ion collisions near the matching velocity

Cross-section measurements for inelastic collisions of rare-gas ions with Na Rydberg atoms that change the orbital angular momentum l are reported. He⁺, Ne⁺, and Ar⁺ ions at 400-2000 eV collide with laser-excited Na(nd) atoms in a crossed-beam experiment. Time-resolved field ionization is used as a selective detection method. The l-changing cross section is found to vary with n as n^β, where β=5.12±0.20 when projectile and bound-electron velocities are comparable. The velocity dependence is given in the range ̃ \~v=0.6-4.0, where reduced velocity ̃ \~v is the ratio of ion and electronic orbital velocities. Evidence is offered that the induced transitions are predominantly dipole in character under single-collision conditions but that rapid equilibration of higher l's occurs by successive ion impacts. Initial Na(ns) states yield an l-changing cross section at least an order of magnitude smaller than d states and appear to go most readily to (n-1)l≥2 states in endothermic nondipole-allowed Δn=1 processes. The cross-section magnitude for Ne⁺+Na(28d)→Ne⁺+Na(28,l≥3) at 1000 eV is ≈5×10^-8 cm², in agreement with existing theories. This exceeds the geometric cross section n⁴πa₀² by a factor of 10³ and indicates that passage of an ion closer to the Rydberg atom than 30 orbital radii n²a₀ suffices to induce l change in the velocity-matching region.