Pump–probe experiments in the femtosecond regime, combining first and third harmonics of SASE FEL radiation

Two-color pump–probe experiments combining optical femtosecond lasers with short wavelength radiation from a free electron laser (FEL) are very attractive for sub-picosecond time-resolved studies. Since the synchronization between the two light sources to an accuracy of 100 fs is not yet solved, it is proposed to derive both radiation pulses from the same electron bunch. In the present work we focus on the special case where pump and probe beams are generated by the same electron bunch in the same insertion device. Specifically we propose to combine GW-level VUV FEL pulses between 150 and 90 nm wavelength and 10 MW-level third-harmonic radiation between 50 and 30 nm. This scheme does not require any special synchronization nor additional FEL hardware components since the nonlinear third-harmonic generation occurs naturally in the planar FEL undulator. Reflection optics are used for beam splitting and tunable delay, and the two harmonics are separated by using notch filters. The effect on the pulse duration is negligible.