Laser-induced streaking of MeV electrons in liquid medium

Free-electron–laser interactions in liquids constitute a fundamental process underpinning a broad range of ultrafast spectroscopic and diffraction techniques, including time-resolved photoelectron spectroscopy, attosecond spectroscopy, and ultrafast electron diffraction. Despite their central role, the interaction between free electrons and optical fields in a liquid environment remains largely unexplored. Here, we present a comprehensive study of laser-induced streaking (LIS) of MeV electrons in liquids using a state-of-the-art liquid-phase ultrafast electron diffraction (LUED) setup. We identify a streaking mechanism distinct from the gas phase: instead of plasma lensing, liquid-phase LIS arises from a direct electron-light interaction mediated by the liquid's dielectric response, regardless of whether the laser intensity is above or below the ionization threshold. This conclusion is supported by the dependencies of the streaking on laser incident angle, laser field strength, liquid thickness, and the resulting distortion of scattering patterns. Furthermore, we demonstrate that LIS can be completely suppressed at near-colinear electron-laser incidence, even above the ionization threshold. Our findings elucidate a previously unknown interaction mechanism and provide critical guidance for preventing scattering pattern corruption in future LUED experiments.