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 Na+ ion impact at reduced velocity upsilon=0.1-0.9 has been measured. 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 upsilon 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 upsilon less-than-or-equal-to 0.7. At very low energy (upsilon less-than-or-equal-to 0.3) the measurements demonstrate that the final-state distributions are dominated by very high angular-momentum states.