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Abstract
We explain an analysis strategy for ultrafast X-ray photon correlation spectroscopy, a technique enabled by X-ray free electron lasers to probe nano- and atomic-scale dynamics in complex systems on nanosecond timescales or faster. Central to the technique is the methodology for extracting contrast in coherent X-ray diffraction patterns, known as speckle patterns. Guided by simulations, we analyze common challenges and examine errors encountered in contrast extraction. A method for accurately determining contrast amid shot noise across a wide range of count rates is presented.Graphical abstract
Highlights
We show the strategy for contrast extraction from speckle patterns in this sparse photon limit
The simulations presented in this paper analyzed contrast extraction across various scenarios
Our analysis shows that an RMS error below 7% in the intensity monitor is sufficient to achieve contrast accuracy better than 0.01
Summary
Most condensed matter systems exhibit a hierarchy of length and time scales in their dynamic properties. Using Eq 9 with averaged probabilities provides an accurate estimation, as indicated in blue overlapping with the red crosses This is because averaging over a large number of frames enables precise count rate determination. We note that with knowledge of R from the additional intensity measurement, the calculated contrast βcalc using Eq 9 from the averaged probabilities can be corrected, in the extremely low count rate regime. This indicates that by applying Eqs. 9 and 11, knowledge of R, the normalized variance of intensity fluctuations, is sufficient to accurately extract contrast values, irrespective of the intensity distribution This method is widely applicable for different beam operation schemes, including SASE and the increasingly utilized seeded FELs.[33–36]
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