Inelastic interaction between coherent light with constant frequency and free electrons enables periodic phase modulation of electron wave packets leading to periodic sidebands in the electron energy spectra. In this Letter, we propose a generalization of the interaction by considering linearly chirped electron wave packets interacting with chirped optical fields. We theoretically demonstrate that when matching the chirp parameters of the electron and light waves, the interaction leads to partial monochromatization of the electron spectra in one of the energy sidebands. Depending on the coherence time of the electrons, the electron spectrum may be narrowed down by a factor of 5 times with 26% of the electron distribution in the monochromatized energy band. This approach will improve the spectral resolution and reduce color aberrations in ultrafast imaging experiments with free electrons.

Department of Chemical Physics and Optics Faculty of Mathematics and Physics <a href= https ror org 024d6js02 >Charles University< a> Ke Karlovu 3 Prague CZ 12116 Czech Republic

Department of Condensed Matter Physics Faculty of Mathematics and Physics <a href= https ror org 024d6js02 >Charles University< a> Ke Karlovu 5 Prague CZ 12116 Czech Republic

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