Absolute doubly differential cross sections for ejection of secondary electrons from gases by electron impact. I. 100- and 200-eV electrons on helium

We have measured absolute values of the cross sections for ejection of electrons from helium gas by 100- and 200-eV electrons. These cross sections were measured for emission angles from 10° to 150° and for electron energies from 3 eV to a value equal to the primary energy minus the ionization potential. The measurements were made using a static-gas target and an electrostatic analyzer. The resulting cross sections agree reasonably well with those of Opal, Peterson, and Beaty near 90°, but show the need for an adjustment of their angular distributions. Comparison with Born-approximation calculations of Manson et al. at 2 keV indicate that the cross sections of Opal et al. should be divided by $0.53+0.47sin\theta$. A somewhat larger correction is needed to bring their data into agreement with the results of this experiment. The adjustment in the angular distributions also affects the cross sections integrated over all angles.