Simulated variation in the maximum fragment kinetic energy of N<sub>2</sub> Coulomb explosion as a function of (a) the FEL pulse energy (assuming a 100 fs pulse duration and the focal conditions described in the experimental section), (b) the FEL pulse duration (assuming the experimental pulse energy distribution and the focal conditions described in the experimental section)

<p><strong>Figure 11.</strong> Simulated variation in the maximum fragment kinetic energy of N<sub>2</sub> Coulomb explosion as a function of (a) the FEL pulse energy (assuming a 100 fs pulse duration and the focal conditions described in the experimental section), (b) the FEL pulse duration (assuming the experimental pulse energy distribution and the focal conditions described in the experimental section). Large filled circles show experimental results at 48 µJ average pulse energy and 100 fs expected pulse duration. The discrepancies between the experiment and the simulation are discussed in the text.</p> <p><strong>Abstract</strong></p> <p>Single-shot time-of-flight spectra for Coulomb explosion of N<sub>2</sub> and I<sub>2</sub> 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 N<sup>4+</sup>–N<sup>5+</sup> channel for nitrogen and up to the I<sup>8+</sup>–I<sup>9+</sup> 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 N<sub>2</sub> 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.</p>