Extraction of the charge state distributions of N<em>ⁿ</em>⁺ fragments from the average TOF spectra of specific pulses in the 30 pulse long FEL bunch

<p><strong>Figure 7.</strong> Extraction of the charge state distributions of N<em>ⁿ</em>⁺ fragments from the average TOF spectra of specific pulses in the 30 pulse long FEL bunch. The average TOF spectrum of the tenth pulse (blue) is fitted to a model distribution (red). Below are model curves corresponding to charge fragments N⁺–N⁵⁺ (black).</p> <p><strong>Abstract</strong></p> <p>Single-shot time-of-flight spectra for Coulomb explosion of N₂ and I₂ molecules have been recorded at the Free Electron LASer in Hamburg (FLASH) and have been analysed using a partial covariance mapping technique. The partial covariance analysis unravels a detailed picture of all significant Coulomb explosion pathways, extending up to the N⁴⁺–N⁵⁺ channel for nitrogen and up to the I⁸⁺–I⁹⁺ channel for iodine. The observation of the latter channel is unexpected if only sequential ionization processes from the ground state ions are considered. The maximum kinetic energy release extracted from the covariance maps for each dissociation channel shows that Coulomb explosion of nitrogen molecules proceeds much faster than that of the iodine. The N₂ ionization dynamics is modelled using classical trajectory simulations in good agreement with the outcome of the experiments. The results suggest that covariance mapping of the Coulomb explosion can be used to measure the intensity and pulse duration of free-electron lasers.