Excess energy partitioning between electrons departing at and in the ionization of helium near threshold

Abstract

Partitioning of excess energy between electrons departing at and after electron-impact ionization of helium has been measured. A recently developed scheme, using a magnetically collimated electron-impact spectrometer, pulsed incident electron beam and time-of-flight detection was used to detect the electrons departing in the forward and backward directions. The ratios of the forward and backward doubly differential cross sections (DDCS) for an incident electron energy 3 eV above threshold and secondary electron energies of 0.7, 1.5 and 2.9 eV were measured. The relative DDCS at and are also presented as a function of the secondary electron energy, for incident electron energies 3 and 5 eV above threshold. Our measurements show that the distribution of the secondary electrons is, in contrast to electrons detected at , not flat for electrons departing at and at the present low primary energies. The yield of the faster secondary electrons is forward peaked, the yield of the slower secondary electrons is backward peaked. The sum of the and DDCS is, however, flat within the experimental uncertainty limits.