Electron-collision-induced dipole transitions in Na Rydberg atoms: 30s-30p and 30s-29p absolute cross sections

An experimental study of the depopulation of Na 30s Rydberg atoms under electron bombardment has been carried out. Electron energies E ranged from 105 to 660 eV corresponding to collision velocities 80 to 200 times the mean orbital speed of the excited 30s electron. The collision-induced final states were identified by means of selective field ionization (SFI). 30p, 29p and a small trace of 28p were clearly observed in the adiabatic SFI distributions. Collision-induced diabatic SFI signals were absent, in contrast with a previous study that used slow ionic projectiles. 30s-30p and 30s-29p absolute cross sections were obtained from measurements of the collisional depopulation of the 30s state and measurements of the electron beam current-density-profiles. The cross sections ranged from 0.33 to 1.69 * l0-10 cm2. The results agreed with calculations based on the dipole approximation with first-order perturbation theory and the Born approximation. Cross sections were also measured in external electric fields up to 15.75 V cm-1 for the higher collision energies (422 to 660 eV), but unlike the previous study no field dependence of cross sections for final-state distributions was apparent.