Limitations of Structural Insight into Ultrafast Melting of Solid Materials with X-ray Diffraction Imaging

Abstract

In this work, we analyze the application of X-ray diffraction imaging techniques to follow ultrafast structural transitions in solid materials using the example of an X-ray pump–X-ray probe experiment with a single-crystal silicon performed at a Linac Coherent Light Source. Due to the spatially non-uniform profile of the X-ray beam, the diffractive signal recorded in this experiment included contributions from crystal parts experiencing different fluences from the peak fluence down to zero. With our theoretical model, we could identify specific processes contributing to the silicon melting in those crystal regions, i.e., the non-thermal and thermal melting whose occurrences depended on the locally absorbed X-ray doses. We then constructed the total volume-integrated signal by summing up the coherent signal contributions (amplitudes) from the various crystal regions and found that this significantly differed from the signals obtained for a few selected uniform fluence values, including the peak fluence. This shows that the diffraction imaging signal obtained for a structurally damaged material after an impact of a non-uniform X-ray pump pulse cannot be always interpreted as the material’s response to a pulse of a specific (e.g., peak) fluence as it is sometimes believed. This observation has to be taken into account in planning and interpreting future experiments investigating structural changes in materials with X-ray diffraction imaging.

Delayed Onset and Directionality of X-Ray-Induced Atomic Displacements Observed on Subatomic Length Scales

“…Although the simulation for 100% fluence reproduces the trend of the time dependence of the structure parameters observed in the experiment, the predicted shift of the atomic positions and degree of the atomic disorder are larger than those evaluated by the experiment. This discrepancy can be explained by the nonuniformity of the pump fluence [55]. Since the focal spots of the pump and probe pulses had Gaussian shapes in the present experiment [45], the probe diffraction signals originated from various sample areas with different pump fluence.…”

Delayed onset and directionality of x-ray-induced atomic displacements observed on subatomic length scales

“…Although the simulation for 100% fluence reproduces the trend of the time-dependence of the structure parameters observed in the experiment, the predicted shift of the atomic positions and degree of the atomic disorder are larger than those evaluated by the experiment. This discrepancy can be explained by the non-uniformity of the pump fluence [55]. Since the focal spots of the pump and probe pulses had Gaussian shapes in the present experiment [45], the probe diffraction signals originated from various sample areas with different pump fluence.…”

Advantages of Using Hard X-ray Photons for Ultrafast Diffraction Measurements

“…In Ref. [30], we showed how one could take the spatial pulse profile into account and perform a rigorous volume integration of the simulated diffraction signal. Such computationally expensive research is not necessary for the scope of the current work; we aim to advertise here a general idea of using hard X-rays for imaging without attempting to perform a comparison with any concrete experimental result.…”