TOF spectrum of ions from Coulomb explosion of N2, averaged over all recorded FEL shots (red) and a single-shot spectrum (blue, shifted by −10 mV) showing individual ion hits (marked with arrows) where the signal is low
Figure 2. TOF spectrum of ions from Coulomb explosion of N2, averaged over all recorded FEL shots (red) and a single-shot spectrum (blue, shifted by −10 mV) showing individual ion hits (marked with arrows) where the signal is low. In the regions of high ion signal the individual pulses overlap, which excludes the use of coincidence techniques. The Coulomb explosion leads to a broadening of the Nn+ atomic ion islands and, consequently, a mutual overlap of these islands. Several relatively sharp peaks resulting from the ionization of background gas can be seen as well. The combination of both observations effectively prevents a measurement of the kinetic energy of the Nn+ atomic ion peaks on the basis of this TOF spectrum.
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.