Nonlinear X-ray scattering
by
PSI Auditorium
WHGA/001
X-ray crystallography is essential for determining protein structures, but challenges persist in achieving sub-Ångström resolution, such as growing high-quality crystals, radiation damage, and the phase problem. Serial femtosecond crystallography at XFELs addresses the first two issues, but obtaining model-free atomic structures remains difficult. Single-wavelength anomalous dispersion helps to enhance phase sensitivity, but has limitations due to the addition of heavy atoms to the protein. A solution might involve enhancing the scattering properties and phase sensitivity of lighter atoms by elastic resonant scattering of hidden, transient resonances. In a first proof-of-principle experiment we demonstrate enhanced resonant scattering at the Cu L-edge of a superlattice of a Cu-containing multilayer mirror. The XFEL creates warm-dense copper and opens transient resonances by efficient creation of 3d holes through electron collisions. The polarization response includes a non-resonant term defined by the atomic scattering factor of Cu and a resonant elastic scattering contribution from 2p-3d transitions. In addition to the enhancement and phase sensitivity, the spectrally resolved scattering signal encodes electronic structure information of the warm-dense copper, in agreement to a recent transient absorption experiment of XFEL created warm-dense copper (L. Mercadier et al., Nature Physics 2024, https://doi.org/10.1038/s41567-024-02587-w). Ideally, enhanced resonant X-ray scattering involves a two-color attosecond X-ray pump-probe scheme: The pump creates population inversion in atoms via ultrafast 1s ionization, while the probe pulse targets the transient 1s-2p (Kα) resonance to scatter elastically with a significant dipole-matrix element. In high-intensity probes, the resonant part may enter the nonlinear regime, with a significant contribution to the effective scattering factor for Cu of f‘‘~-i·2500·ρinv. I will present our theoretical estimates for fs pump-probe enhanced resonant X-ray scattering in copper, along with recent experimental results in CuSO4 single crystals. Experimental challenges, in particular the stochastic fluctuations of the scattering strength due to the limited transverse coherence of XFELs, will be discussed, that also have implications for high-resolution X-ray scattering for valence-electronic structure determination.
PSI Center for Scientific Computing, Theory and Data (CSD)