Ultrafast X-ray-induced dephasing of valence electrons
Diffract-before-destroy schemes for atomic-scale electron mapping assume that ultrashort X-ray pulses outrun radiation damage. Although atomic cores move over tens of femtoseconds, outer electrons respond much faster, compromising their locations. We test this limit in elemental silicon using broadband, energy-chirped X-ray pulses to encode time with few-femtosecond resolution. Comparing valence-sensitive structurally forbidden reflections with simultaneously measured core-dominated Bragg peaks reveals valence-bond dephasing on sub-20-fs timescales even at pulse energies where direct ionization effects are negligible. Our findings demonstrate that crystallography where outer electrons contribute substantially to Bragg peaks requires pulses shorter than today's standard $\sim50\,fs$ standard. They also establish a path to disentangle valence electron and lattice contributions to order and dynamics with hard X-rays, enabling studies across molecular biology, quantum matter and devices.