Observation of non-linear resonances of inner-shell electrons by X-ray free electron laser

Recently, a very stimulating paper has been published discussing experimentally the fundamental processes of photo-absorption and excitation of electrons by using extremely high-fluence, ultra-short X-ray pulses. The research was done for the electron system in inert Ne gas at LCLS (Linac Coherent Light Source), Stanford, USA, which is the world's first hard X-ray free-electron laser facility. The scheme is as follows: an intense single X-ray pulse of sub-10-fs duration at 848 eV first strips a 2p electron from Ne and, at this stage, since the X-ray energy is below the binding energy of a 1s electron in neutral neon, 870 eV, a 1s hole cannot be produced, but because of the above 2p hole, the next pulse can excite the 1s electron, leading to 1s-2p resonance in the Ne⁺ ion and, finally, stimulated emission (2p-1s) competes with Auger decay to refill the 1s hole. The results have indicated that intense X-ray pulses of sub-10-fs duration can modify and even control the Auger decay process. For more information, see the paper, "Unveiling and Driving Hidden Resonances with High-Fluence, High-Intensity X-Ray Pulses", E. P. Kanter et al., Phys. Rev. Lett. 107, 233001 (2011).