relevant for user applications but does not affect the formation of the spectro-temporal double peak.

We now give a quantitative description of the two-colour emission results at FERMI@Elettra.

Experimental results {#experiment}
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![FEL spectrum at the $6^{th}$ harmonic of the seed ($43.5\,$nm), as a function of the relative delay between the seed pulse and the electron beam. At the zero value of the delay, the seed pulse is approximately centered on the bunch of electrons. The dispersive section $R_{56}$ and seed energy were set, respectively, to $23\,\mu$m and $80\,\mu$J; see Table 1 for other parameters.[]{data-label="Figure_delay_experiment"}](Figure_delay_experiment){width="60.00000%"}

Figure \[Figure\_delay\_experiment\] shows the behaviour of the FEL spectrum at the $6^{th}$ harmonic of the seed (43.5 nm) as one sweeps the relative time delay between the seed and the electron beam. To the best of our knowledge, this picture provides the first direct *experimental* evidence of the temporal pulse splitting in an FEL due to local overbunching induced by an intense seed. At a delay $ \approx -0.8\,$ps, the seed is superimposed on the electron beam tail, so that the first peak (at larger wavelength, since the frequency chirp is positive) appears. Then, at a delay $\approx -0.6\,$ps, the whole seed pulse lies on 