Enhancement of X-ray image brightness through transient ionic resonances

Most of our high-resolution imaging methods compromise between temporal or spatial resolutions akin to a pinhole camera. This still limits our capabilities to observe fast processes at the nanoscale, especially as the required brightness often damages the sample. Examples of such processes include chemical and catalytic reactions, nucleation dynamics and growth of nanoparticles, as well as other fragile/intermediate states of matter. One idea to overcome this obstacle is to use Free Electron Lasers (FELs), such as the LCLS at SLAC Nat. Lab., which are capable of producing very bright bursts of coherent X-rays within a few femtoseconds. X-ray FELs help to visualize transient processes in “frozen” time steps via single shot coherent X-ray diffractive imaging (CDI). Currently, the resolution of CDI images is close to tens of nanometers, which is limited by the brightness of the images [1-3]. Our recent study at LCLS suggests that transient ionic resonances (TIR) above an absorption edge can enhance diffraction efficiency before significant structural damage can occur. This is surprising as TIRs are usually regarded as a signature of increased X-ray absorption/damage and thus, detrimental to image quality [4-6]. We recorded a large data set of single exposure diffraction patterns with different FEL pulse durations from Xe nanoparticles by scanning the FEL energy in the vicinity of the 3d absorption edge. TIRs seem to increase the scattering efficiency above the edge even for sub-fs pulses. Our experimental results and a theoretical simulation suggest, that TIRs may provide a pathway to increase the quality of CDI with soft and hard X-ray FELs.

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[2] Aquila, A., et al. Struct. Dyn. 2.4 (2015): 041701.
[3] Gorkhover, T, et al. Nat. Phot. 12.3 (2018): 150.
[4] Kanter, E. P., et al. Phys. Rev. Lett. 107.23 (2011): 233001.
[5] Rudek, B, et al. Nat. Phot. 6.12 (2012): 858.
[6] Bostedt, Ch, et al. Phys. Rev. Lett. 108.9 (2012): 093401.