期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2015
卷号：112
期号：3
页码：696-701
DOI：10.1073/pnas.1407771112
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：SignificanceExposure to high-energy X-rays, experienced during X-ray diffraction experiments for determining atomic structures, can potentially create large local electric fields from photoelectron ejection. Under cryogenic conditions, these fields may persist indefinitely. Mapping of electric fields is important for a complete understanding of damage mechanisms involved in X-ray measurements, for potential effects on the quality and interpretation of X-ray data, and for possible direct impact on diffraction resolution through the piezoelectric effect. Mapping of electric fields may also be useful in retaining a record of locations of X-ray exposure. In this paper, electric-field induced second harmonic generation imaging is explored in simulation and experiment. It provides insight into the position and distribution of local electric fields within an X-ray exposed sample. Electron-hole separation following hard X-ray absorption during diffraction analysis of soft materials under cryogenic conditions produces substantial local electric fields visualizable by second harmonic generation (SHG) microscopy. Monte Carlo simulations of X-ray photoelectron trajectories suggest the formation of substantial local electric fields in the regions adjacent to those exposed to X-rays, indicating a possible electric-field-induced SHG (EFISH) mechanism for generating the observed signal. In studies of amorphous vitreous solvents, analysis of the SHG spatial profiles following X-ray microbeam exposure was consistent with an EFISH mechanism. Within protein crystals, exposure to 12-keV (1.033-[IMG]f1.gif" ALT="A" BORDER="0">) X-rays resulted in increased SHG in the region extending [~]3 m beyond the borders of the X-ray beam. Moderate X-ray exposures typical of those used for crystal centering by raster scanning through an X-ray beam were sufficient to produce static electric fields easily detectable by SHG. The X-ray-induced SHG activity was observed with no measurable loss for longer than 2 wk while maintained under cryogenic conditions, but disappeared if annealed to room temperature for a few seconds. These results provide direct experimental observables capable of validating simulations of X-ray-induced damage within soft materials. In addition, X-ray-induced local fields may potentially impact diffraction resolution through localized piezoelectric distortions of the lattice.
