"Free electrons" refers to negatively charged particles that are not bound to any atom or molecule. These electrons are not restricted to a specific location and are able to move freely within a material, carrying electric current. Full definition
However, getting strong pulses of x-rays is much harder than for low energy light, and required using the most modern sources, x-ray free electron lasers. (sciencedaily.com)
N - type materials contain an abundance of free electrons. (sciencedaily.com)
They calculate that a low - power laser aimed near the radioactive material could free electrons from the oxygen ions. (sciencedaily.com)