Secondary-electron emission from metals impacted by high-velocity particles: molecular-effect deviations from a single-particle picture

Experiments have been performed to study the influence of projectile molecular effects on the production of secondary electrons in the MeV/amu energy regime. Secondary-electron yields from Au, Ta and A1 2O 3 surfaces were measured using charged-particle projectiles provided by Van de Graaff accelerators (H + at 1.5–6 MeV, H 2 + at 2–6 MeV, H 3 + at 3–6 MeV, 3He + at 13–22 MeV, 3He 2+ at 13–33 MeV, 4He + at 18–22 MeV, and 4He 2+ at 18–33 MeV) and scanning electron microscopes (electrons at 0.8–30 keV). For low projectile velocities, the total electron yield due to a composite projectile is less than the sum of the yields of the individual components of that projectile. This “molecular effect” is attributed to interference effects between the individual components of the projectile which lessen each component's ability to produce secondary electrons. As the projectile velocity is increased, these interference effects are reduced and the projectile as a whole becomes more effective in producing secondary electrons. Secondary electrons produced by backscattered electrons were also studied and found to constitute 60–80% of the total number of secondary electrons emitted from Au and Ta and ~ 40% of those emitted from Al 2O 3. These results are interpreted in terms of existing theories of secondary-electron emission. Ongoing computer simulations are contributing to an understanding of the particle dynamics of the secondary-electron emission process for composite projectiles and are showing qualitative agreement with experiment.