Secondary ionization and heating by fast electrons

Understand

We examine the fate of fast electrons (with energies E>10 eV) in a thermal gas of primordial composition.

• To follow their interactions with the background gas, we construct a Monte Carlo model that includes: (1) electron-electron scattering (which transforms the electron kinetic energy into heat), (2) collisional ionization of hydrogen and helium (which produces secondary electrons that themselves scatter through the medium), and (3) collisional excitation (which produces secondary photons, whose fates we also follow approximately).
• For the last process, we explicitly include all transitions to upper levels n<=4, together with a well-motivated extrapolation to higher levels.
• In all cases, we use recent calculated cross-sections at E<1 keV and the Bethe approximation to extrapolate to higher energies.