title: Imaging local electric fields produced upon synchrotron X-ray exposure.
abstract: 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-Å) X-rays resulted in increased SHG in the region extending ∼ 3 μm 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.
authors.affiliation: Department of Chemistry, Purdue University, West Lafayette, IN 47907; and.
authors.forename: Christopher M
authors.initials: CM
authors.lastname: Dettmar
authors.affiliation: Department of Chemistry, Purdue University, West Lafayette, IN 47907; and.
authors.forename: Justin A
authors.initials: JA
authors.lastname: Newman
authors.affiliation: Department of Chemistry, Purdue University, West Lafayette, IN 47907; and.
authors.forename: Scott J
authors.initials: SJ
authors.lastname: Toth
authors.affiliation: The General Medical Sciences and Cancer Institutes Structural Biology Facility at Advanced Photon Source, Argonne National Laboratory, Lemont, IL 60439.
authors.forename: Michael
authors.initials: M
authors.lastname: Becker
authors.affiliation: The General Medical Sciences and Cancer Institutes Structural Biology Facility at Advanced Photon Source, Argonne National Laboratory, Lemont, IL 60439.
authors.forename: Robert F
authors.initials: RF
authors.lastname: Fischetti
authors.affiliation: Department of Chemistry, Purdue University, West Lafayette, IN 47907; and gsimpson@purdue.edu.
authors.forename: Garth J
authors.initials: GJ
authors.lastname: Simpson
mesh_terms: D004560:Electricity; D007700:Kinetics; D008962:Models, Theoretical; D009010:Monte Carlo Method; D017356:Synchrotrons
keywords: EFISH; X-ray damage; piezoelectric; structural biology; synchrotron