High-resolution and Elemental Contrast Fluorescence Imaging with X-ray Free-electron Laser Pulses *

Coherent diffractive imaging (CDI) with x-ray free-electron laser (XFEL) pulses holds the promise to probe structure and follow the dynamics of non-periodic entities with atomic resolution.  However, this approach remains challenging due to sample damage during the pulse.  Based on the Hanbury Brown and Twiss effect [1], Classen and coworkers proposed to used fluorescence intensity correlation (FIC) to image three-dimensional arrangement of heavy atoms with XFEL [2].  Our previous work shows that fluorescence dynamics in the nonlinear x-ray regime differs from that in weak x-ray fields and enables higher fluorescence yield and narrower emission time window [3].  Building on this, we theoretically investigate the FICs of Ar clusters and Mo-doped iron oxide nanoparticles exposed to intense, few-fs and sub-fs XFEL pulses for high-resolution and elemental contrast imaging. We present the FIC of Kα and Kα^H emission in Ar clusters and discuss the impact of sample damage on retrieving high-resolution structural information.  Furthermore, we show that the FICs computed from the fluorescence of Mo atoms in Mo-doped iron oxide nanoparticles can be used to image dopant distributions, which are directly connected to the catalytic functionality of these nanoparticles.

[1] R. Hanbury Brown and R. Q. Twiss, Nature 177, 27 (1956).

[2] A. Classen, et al., Phys. Rev. Lett. 119, 053401 (2017).

[3] P. J. Ho, et al., Phys. Rev. A 101, 043413 (2020).