Single x-ray photon excitaton of two and three electrons in nickel

Cooperative single-photon excitations of two ($K+L$) and three ($K+L+M$) electrons in nickel were studied using x-ray absorption spectroscopy. Each of the edges detected has an energy higher by about 164 ± 3 eV than the sum of those required for exciting the electrons involved independently. The probabilities for the two- and three-electron excitation are found to be 0.25% and 0.08%, respectively, of that for a single $K$-electron excitation.