Quantitive determination of the fraction of Rydberg electrons in a convoy electron cusp

Convoy electron energy spectra were measured using an electrostatic electron energy analyzer and CEM detector. Recently it was suggested that the origin of a significant fraction of the measured convoy electron yields is due to field ionization of high Rydberg states inside the electron energy analyzer.

Using the ORNL EN Tandem Van De Graaff accelerator, beams of 18 MeV 0⁺³ and 32 MeV O⁺⁵ ions were passed either through a foil target in the focus of the analyzer or through a foil target out of focus and then through a series of field ionization and deflection plates, before entering the electrostatic electron energy analyzer. Various voltages were applied across the different plates. Yields of energy analyzed electrons were obtained for different geometric arrangements of the plates and for different combinations of the electric fields.

The data indicate that the contribution of field ionized Rydbergs electrons to convoy yields is small, less than 3% in traditional experiments designed to study convoy electrons. The measured yields as a function of the applied electric field between the ionization plates are consistent with a quantum state population P(n) of highly excited states varying as n^-3 No indication of strong alignment of these highly excited states is found when comparing the effectiveness in field ionization of a transverse vs a longitudinal field.