Cross sections and energy loss function for electrons in helium. Upper panel: Momentum transfer cross section (‘m.t.’; Pinto & Galli 2008), excitation cross sections (Ralchenko et al. 2008), and ionisation cross section (‘ion’; Kim et al. 2000) for electrons colliding with He atoms. Lower panel: Energy loss function for electrons colliding with He (solid black line), momentum transfer loss (‘m.t.’; solid blue line), total excitation loss (solid orange line), ionisation loss (solid cyan line), and bremsstrahlung loss (solid grey line, from Blumenthal & Gould 1970). For comparison, the loss function for electrons colliding with H₂ (dashed black line) is shown.