Secondary-electron spectra compared to some electron excitation cross sections for singlet B, C, B′, and D and triplet a states as a function of the energy (Scarlett et al. 2023). The green-scale shaded regions enclose the secondary-electron spectra calculated at N = 10²⁰ cm⁻² (upper boundary) and 10²⁴ cm⁻² (lower boundary) for the three Galactic CR proton models (ℒ, ℋ, 𝒰). The five cross sections shown in black solid lines are the strongest ones for the excitation of the B, C, B′, and D singlet states and the triplet a state from the ground state X(0, 0). The inset shows the electron excitation cross section of atomic hydrogen leading to Lyman and Balmer lines (Möhlmann et al. 1977; Karolis & Harting 1978; van Zyl et al. 1989; Ajello et al. 1991, 1996).