Angular-momentum distributions following impact of slow ions on Na(nd) Rydberg atoms

The degree of dipole-forbidden l change in Na(26d) and Na(28d) Rydberg atoms caused by slow ${\mathrm{Na}}^{+}$ ion impact at reduced velocity ṽ=0.1–0.9 has been measured. ${\mathrm{Na}}^{+}$ beam energies 7.3–684 eV were used. Previous measurements were greatly improved by reducing stray electric fields in the Rydberg collision region to about 10 mV/cm. The distributions of final-angular-momentum states were analyzed using directly measured diabatic selective-field-ionization profiles for l=3, 4, and 5. l-change results in Na(26d) and Na(28d) targets are very similar and clearly show an increasing amount of dipole-forbidden behavior for decreasing reduced velocities ṽ and partial agreement with the coupled-channel calculation of Beigman and Syrkin (Zh. Eksp. Teor. Fiz. 89, 400 (1985) [Sov. Phys. JETP 62, 226 (1985)]). The dipole-forbidden process is the main channel for angular-momentum change in ion–Rydberg-atom collisions at ṽ≤0.7. At very low energy (ṽ≤0.3) the measurements demonstrate that the final-state distributions are dominated by very high angular-momentum states.