Spatial displacement of forward-diffracted X-ray beams by perfect crystals.

Area of Science:

• * Physics
• * Materials Science
• * Optics

Background:

• * Hard X-ray free-electron lasers (XFELs) utilize self-seeding for enhanced coherence.
• * Forward Bragg diffraction of X-ray pulses generates time-delayed, narrow-band echoes.
• * Theoretical models predict a correlation between echo pulse retardation and transverse displacement.

Purpose of the Study:

• * To experimentally observe and verify the predicted transverse displacement of time-delayed echo pulses.
• * To validate dynamical diffraction theory in the context of XFEL self-seeding.
• * To explore the potential of these echo signals as diagnostic tools.

Main Methods:

• * Generation of X-ray pulses from a hard X-ray free-electron laser.
• * Exploitation of forward Bragg diffraction from a perfect thin crystal.
• * Detection and characterization of time-delayed, narrow-band echo pulses.
• * Comparison of experimental displacements with simulations based on dynamical diffraction theory.

Main Results:

• * First experimental observation of displaced echo pulses.
• * Experimental displacements closely match predictions from dynamical diffraction simulations.
• * Echo signal characteristics are linked to Bragg reflection, structure factor, and interplane distance.

Conclusions:

• * The experimental observation confirms theoretical predictions of displaced echoes in XFELs.
• * The findings support the use of these echoes as an online diagnostic for XFEL seeding.
• * This technique offers a pathway for femtosecond-timescale dynamical diffraction studies of strain.