Rapid X-ray pulses enable 100-fold efficiency boost for photoionization

Speed matters. When an X-ray photon excites an atom or ion, making a core electron jump onto a higher energy level, a short-lived window of opportunity opens. For just a few femtoseconds, before an electron fills the void in the lower energy level, a second photon has the chance to be absorbed by another core electron, creating a doubly excited state.