Chirped seeded free-electron lasers: self-standing light sources for two-color pump-probe experiments.

We demonstrate the possibility of running a single-pass free electron laser (FEL) in a dynamical regime, which can be exploited to perform two-color pump-probe experiments in the vacuum ultraviolet or x-ray domain, using the free-electron laser emission both as a pump and as a probe. The studied regime is induced by triggering the free-electron laser process with a powerful laser pulse, carrying a significant and adjustable frequency chirp. As a result, the output FEL radiation is split in two pulses, separated in time (as previously observed by different authors), and having different central wavelengths. We show that both the spectral and temporal distances between FEL pulses can be independently controlled. We also provide a theoretical description of this phenomenon, which is found in good agreement with experiments performed on the FERMI@Elettra free-electron laser.