Speckle contrast of interfering fluorescence X-rays

With the development of X-ray free-electron lasers (XFELs), producing pulses of femtosecond durations comparable with the coherence times of X-ray fluorescence, it has become possible to observe intensity – intensity correlations due to the interference of emission from independent atoms. This has been used to compare durations of X-ray pulses and to measure the size of a focusedX-ray   beam, for example. Here it is shown that it is also possible to observe the interference of fluorescence photons through the measurement of the speckle contrast of angle-resolved fluorescence patterns. Speckle contrast is often used as a measure of the degree of coherence of the incident beam or the fluctuations of the illuminated sample as determined from X-ray diffraction patterns formed by elastic scattering, rather than from fluorescence patterns as addressed here. Commonly used approaches to estimate speckle contrast were found to suffer when applied to XFEL-generated fluorescence patterns due to low photon counts and a significant variation of the excitation pulse energy from shot to shot. A new method to reliably estimate speckle contrast under such conditions, using a weighting scheme, is introduced. The method is demonstrated by comparing the speckle contrast of fluorescence observed with pulses of 3   fs to 15   fs duration.

Source: Journal of Synchrotron Radiation - November 18, 2022 Category: Physics Authors: Trost, F. Ayyer, K. Oberthuer, D. Yefanov, O. Bajt, S. Caleman, C. Weimer, A. Feld, A. Weller, H. Boutet, S. Koglin, J. Timneanu, N. von Zanthier, J. R ö hlsberger, R. Chapman, H.N. Tags: speckle contrast estimation X-ray fluorescence incoherent diffraction imaging XPCS research papers Source Type: research