Radiation damage behavior of soft matter in ultrafast cryo-electron microscopy (cryo-UEM)

Whether time-modulated pulsed-electron imaging in ultrafast electron microscopy (UEM) can mitigate the electron radiation damage that occurs to samples, is still controversial. The effectiveness of such mitigation effect and relevant potential application in cryo-EM remain to be explored. Herein, we built an ultrafast cryo-EM (cryo-UEM) device based on an ultrafast laser system. Using such equipment and the saturated aliphatic hydrocarbon compounds (C44H90), the fading curves of diffraction intensity and corresponding critical electron doses (Ne) of the samples were carefully measured under different imaging modes, temperatures, imaging dose rates and pulsed repetition rates. Our experimental results show that, the fading curves and Ne values of the C44H90 crystals are uncorrelated with the imaging electron dose rates and do not show dependence on the dose-rate effect. As the temperature decreased, the Ne values of the sample increased, indicating the cryoprotective effect on radiation damage to the samples. Surprisingly, at a constant temperature, the fading curves and Ne values of the sample in multi-electrons-packet and near-single-electron-packet pulsed modes are all approximately the same as those in conventional continuous electron-beam mode, even when the results are obtained at different pulsed repetition rates. These results show that the time-modulated pulsed electron beam does not seem to mitigate the electron radiation damage that occurs on samples. Our findings offer new insights and experimental basis for the radiation damage behavior of samples under electron beams, and provide guidance and inspiration for elucidating the fundamental principles of radiation damage.