# Molecular Dynamics of XFEL-Induced Photo-Dissociation, Revealed by Ion-Ion Coincidence Measurements
## 3 Results and Discussion
### 5-lodouracil  
The last molecule in the series studied at SACLA is 5-iodouracil, a cyclic planar molecule. Extracting high-quality coincidence datasets by MF is, on one hand, more difficult in this case because no clear axis of momentum distribution exists and it is shared by a number of ions. On the other hand, the ionization rate/pulse was relatively low in this experiment [14; 15]. Here, again, preliminary Coulomb explosion simulations are valuable in deciding the strategy of applying MF. Modeling suggested, first, that the ion trajectories are significantly affected by their initial vibrational motion, which should, therefore, be added to the model. Second, the ion momenta are not expected to remain planar (due to vibrations) but strong deviations can occur. Planarity is, therefore, not the best criterion for the MF. Third, the carbon ions, being surrounded by heavier elements, have very variable trajectories that bear little resemblance to the original geometry and, therefore, are not the best candidates to be included in MF. On the other hand, oxygen ions, being on the outer positions, have trajectories that reflect well the geometry of the ring and also have sufficiently high momenta for efficient filtering. However, the lack of axial symmetry means that the filtering should be done based on the _angle_ between the momenta, rather than the length of their sum. We will demonstrate this type of MF on the cases of triple coincidence sets containing the two oxygen ions. Figure 11 shows the angular correlations between the momenta of the ions in the (O\({}^{+}\),O\({}^{2+}\),H\({}^{+}\)) triple coincidence set and confirms the predictions of the simulations: the O\({}^{+}\),O\({}^{2+}\) angle has a strong correlation maximum at around 115\({}^{\circ}\), while the planarity of the three vectors is not sharply defined (although the dissociation is preferentially planar). Next, we extract triple coincidence sets with angular correlation MF for the two oxygen ions. Figure 12 shows a Newton plot for the (O\({}^{+}\), O\({}^{+}\), N\({}^{+}\)) dataset. There are two possible origins for the oxygen defining the \(x\)-axis; these molecular orientations are depicted next to the figure. Then there are four initial positions for the N\({}^{+}\) ions, two of them degenerate. Indeed, the N\({}^{+}\) island in the figure corresponding to the degenerate position shows approximately twice the intensity of the other two. In extracting this plot, the background was slightly improved by subtracting a Newton plot generated from a scrambled coincidence set (not showing any defined islands). The conical sharp limits of the O\({}^{2+}\) island is the result of applying the angular MF. Figure 10: (C\({}^{2+}\), I\({}^{3+}\)) PIPICO plot from the (I\({}^{3+}\), I\({}^{3+}\), C\({}^{2+}\)) coincidence set in CH\({}_{2}\)I\({}_{2}\). Color map: experiment, red dots: model. The second example is the (O\({}^{+}\), O\({}^{+}\), H\({}^{+}\)) dataset. Due to the relatively low total charge in this experiment and the larger size of the molecule, the kinetic energies of the H\({}^{+}\) ions were sufficiently low for efficient detection together with the heavier ions. After applying angular MF for the two oxygens and subtracting the scrambled coincidence background, again a clear island structure emerges for the third (H\({}^{+}\)) ion in the dataset, as shown in Figure 13. Indeed, all different initial positions of the hydrogens are discernible, indicating that the Coulomb explosion involving these ions has a common center of force in a good approximation. In contrast, we also generated Newton plots for (O\({}^{+}\),O\({}^{+}\),C\({}^{+}\)) datasets, which showed no visible island structures for the carbon ions, in agreement with the simulations. Interestingly, hydrogens emitted to the directions 1,4 appear to have larger momenta than the rest.