(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>_shots) 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>_shots) 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

Multi-photon processes in Ne and associated electron kinetic energies

<p><strong>Figure 1.</strong> Multi-photon processes in Ne and associated electron kinetic energies. The left-hand diagram shows the kinetic energies of electrons created upon ionization by 1062 eV photons, labelled according to the chronological sequence of the following processes: core photoionization (P), valence photoionization (V), single-photon core-valence double ionization (D) and Auger decay (A). Four crucial multi-photon processes are schematically presented in the right-hand diagram using the same notation. Arrows indicate the ejected electrons and are numbered in chronological order.</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

Location of the correlation islands deduced from figure 2

<p><b>Table 1.</b> Location of the correlation islands deduced from figure <a href="http://iopscience.iop.org/0953-4075/46/16/164034/article#jpb470112f2" target="_blank">2</a>. Their assignments are confirmed by comparison with theoretical predictions. The <em>E</em>_kin values given correspond to specific sequences of the multi-ionization processes as labelled in bold in the third column. To denote the Ne 1s^<em>r</em>2s^<em>t</em>2p^<em>k</em> electron configurations, the notation <em>rtk</em> is used.</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

Covariance maps of Ne for the electron kinetic energy range of 10–220 eV obtained from (a) raw experimental data; the white curve can be traced back to an additional signal associated with a reflection in the high-voltage cables of the detector appearing in the TOF spectra 320 ns away from every electron peak

<p><strong>Figure 3.</strong> Covariance maps of Ne for the electron kinetic energy range of 10–220 eV obtained from (a) raw experimental data; the white curve can be traced back to an additional signal associated with a reflection in the high-voltage cables of the detector appearing in the TOF spectra 320 ns away from every electron peak. (b) Discriminated data. (c) Energy shifted data compensating for the photon energy jitter. (d) Fourier deconvoluted data (cf figure <a href="http://iopscience.iop.org/0953-4075/46/16/164034/article#jpb470112f2" target="_blank">2</a>(b)).</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

Integrated intensities of our model calculations and our experimental results associated with different features of the covariance maps relative to the intensity of the feature associated with the PAP process

<p><b>Table 2.</b> Integrated intensities of our model calculations and our experimental results associated with different features of the covariance maps relative to the intensity of the feature associated with the PAP process. The experimental values were corrected for the collection–detection efficiency of the spectrometer used, taking into account a decrease of about 50% for energetic valence and Auger electrons.</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

Evolution of linear and nonlinear processes in time

<p><strong>Figure 4.</strong> Evolution of linear and nonlinear processes in time. As reference, the pulse intensity profile is shown in blue.</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