to generate two independent pulses spectrally and temporally separated in an externally controllable fashion. As previously shown in [@PRLDP], this opens the possibility of using a chirped-seeded FEL as a self-standing source for pump-probe experiments in the vacuum ultraviolet and X-ray spectral domain.

The paper is divided into the following parts: in Section \[formation\] we introduce the principle upon which a CHG FEL is based, with specific reference to the case of the FERMI@Elettra FEL [@NaturePhot]. We then qualitatively describe the formation of the double spectral peak, when the seed laser carries a linear frequency chirp. In Section \[description\], we give a description of the effects due to both a laser frequency chirp and an electron beam energy chirp. Then in Section \[experiment\], we present the results obtained on the FERMI@Elettra FEL and, in Section \[simulations\], compare them to numerical simulations. Finally, we summarize our conclusions and provide some perspectives for future experimental studies.

Formation of the spectral double peak {#formation}
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A CHG FEL such as FERMI@Elettra typically employs a layout similar to that shown in Fig. \[Figure\_FERMI\]. A relativistic electron beam first propagates through the periodic field generated by an undulator, called the modulator, simultaneously interacting with a 