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(Color online) Experimental setup: Pulses from FLASH and a NIR laser are superimposed in a Kr gas target. The region of temporal and spatial overlap is imaged by an electron lens system with a high-pass energy filter.
(Color online) Area-integrated photoelectron signal from Kr atoms ionized with vuv pulses plotted against retarding voltage in the absence of an optical laser field. Inset: Derivative of the photoelectron signal above with the optical laser on. Every data point is averaged over 200 macropulses .
(Color online) Area-integrated photoelectron signal at plotted against relative vuv/NIR timing. Signal from spectral sidebands appears only for a temporal overlap of both pulses. Every data point corresponds to one macropulse .
Photoelectron images: (a) (FEL ionization path without optical laser), (b) residual signal at without optical laser, (c) same as (b) but with optical laser on, and (d) difference picture . Images are generated by summation of (a) 200 or [(b)–(d)] 1200 individual exposures, each illuminated by one macropulse .
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