Charge asymmetry in collisional depopulation of Na 36s Rydberg atoms

Impact of low-velocity charged particles on laser-excited 36s Rydberg states of Na in externally applied static electric fields up to 28 V cm -1 drives transitions into the neighbouring Stark energy levels, and the final-state distributions may be inferred by state-selective field ionization (SFI). We have bombarded a Na(36s) target with three positive projectile ions (Ar + , K + and Na + ) and two negative projectile ions (S - and Cl - ) at identical velocities equal to the Bohr orbital velocity of the electron in the initial Rydberg state. The resulting final-state distributions of the sodium Rydberg atom differ significantly between positive and negative ion impact at intermediate values of the field, but differences among ions of the same charge are much smaller. We suggest an interference mechanism as the cause of this charge asymmetry.