Nonlinear X-ray scattering

by Prof. Nina Rohringer (DESY and Uni Hamburg)

Monday 21 Oct 2024, 11:00 → 12:00 Europe/Zurich

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·ρ_inv. 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.