(a) Covariance map of Ne atoms obtained for >500 000 LCLS shots at the central photon energy of ~ 1062 eV; panels (b) and (c) show details of panel (a) emphasizing pair correlations of several K-shell electrons or K-shell electrons with valence electrons, respectively; panel (d) shows a coincidence evaluation (〈<em>XY</em>〉 = Coinc(<em>X</em>, <em>Y</em>)<em>N</em><sub>shots</sub>) of the experimental data for the energy region corresponding to electrons emitted from inner shells

<p><strong>Figure 2.</strong> (a) Covariance map of Ne atoms obtained for >500 000 LCLS shots at the central photon energy of ~ 1062 eV; panels (b) and (c) show details of panel (a) emphasizing pair correlations of several K-shell electrons or K-shell electrons with valence electrons, respectively; panel (d) shows a coincidence evaluation (〈<em>XY</em>〉 = Coinc(<em>X</em>, <em>Y</em>)<em>N</em><sub>shots</sub>) of the experimental data for the energy region corresponding to electrons emitted from inner shells.</p> <p><strong>Abstract</strong></p> <p>We report on a detailed investigation into the electron emission processes of Ne atoms exposed to intense femtosecond x-ray pulses, provided by the Linac Coherent Light Source Free Electron Laser (FEL) at Stanford. The covariance mapping technique is applied to analyse the data, and the capability of this approach to disentangle both linear and nonlinear correlation features which may be hidden on coincidence maps of the same data set is demonstrated. Different correction techniques which enable improvements on the quality of the spectral features extracted from the covariance maps are explored. Finally, a method for deriving characteristics of the x-ray FEL pulses based on covariance mapping in combination with model simulations is presented.</p>