σ(ν) ∝ ν⁻⁸/³ above threshold — quantum mechanics of bound-free transitions
Photoionization removes a bound electron when photon energy exceeds the ionization threshold E_I = hν_th. For hydrogen, the Kramers cross section σ(ν) = σ₀(ν/ν_th)⁻⁸/³ (n=1 ground state) drops steeply above threshold. Multi-electron atoms show additional edges when inner-shell electrons are reached. The cross section peaks at threshold, reaching ~6.3 Mb for hydrogen at 13.6 eV.