A partial covariance map of I₂ fragmentation constructed from TOF data recorded at FLASH

Figure 5. A partial covariance map of I₂ fragmentation constructed from TOF data recorded at FLASH. The FEL pulse has an average energy of 27 µJ and an expected duration of approximately 100 fs. The inset shows correlations of the high charges recorded with the acceleration field of the spectrometer reduced by a factor of 4. The inset TOF range is 2.0–3.4 µs. Fragmentation channels 7–8, 7–9, 8–8 and 8–9 are indicated by arrows. For better visibility, the inset is plotted with a nonlinear colour scale.

Abstract

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.