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 x 10(-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 or final-state distributions was apparent.