ONSET OF DIPOLE-ALLOWED TRANSITIONS IN INELASTIC-COLLISIONS OF CHARGED-PARTICLES WITH NA(30S) RYDBERG ATOMS

An experimental study of the depopulation of Na(30s) Rydberg atoms und er He+ ion bombardment has been undertaken to identify the range of co llision velocities at which strongly coupled behaviour of the 30s and nearby levels at low velocity changes to predominantly dipole-allowed coupled at high velocity. Ion energies ranged from 700 to 3900 eV corr esponding to collision velocities of 2.4 to 5.6 times the mean orbital speed of the excited 30s electron. The ratio of the cross section for the dipole-allowed transitions (30s to 30p and 30s to 29p) to the tot al depopulation cross section was determined for each collision veloci ty. (Other dipole-allowed transitions were observed to be negligible.) This ratio was found to increase from only 20% at a reduced velocity of 2.4 to 65% at a reduced velocity of 5.6. The ratio of the two dipol e cross sections, sigma(30s-30p)/sigma(30s 29p), was also determined a nd compared to calculations based on the dipole approximation with fir st-order perturbation theory.