Figure 2. Model for X-ray-induced transmission and absorption.

a, Ideal semiconductor density of states, with a filled valence band separated from an empty conduction band by energy gap E_g. Photons with energy less than E_g are transmitted, and are absorbed by promoting an electron from the valence to the conduction band if the energy exceeds E_g. b, Intense X-ray excitation fills the states at the bottom of the conduction band, blocking absorption of photons with energies just above the bandgap energy. Induced absorption for photons with energy just below the bandgap could arise from transient localized states at the band edges, as seen with doped semiconductors.