# Molecular Dynamics of XFEL-Induced Photo-Dissociation, Revealed by Ion-Ion Coincidence Measurements
## 3 Results and Discussion
### Diiodomethane  
In diiodomethane that contains three heavier atoms, one does not expect to find two-ion coincidences that can be well-approximated by a two-body dissociation process. However, the geometry suggests that, at least in the cases where the charge distribution between the two iodine atoms is not very unbalanced, a similar approach than for CH\({}_{3}\)I could be useful. We, therefore, assume that the C ion is well momentum correlated with the _combined_ momenta of the two I ions, but that some momentum is removed by the hydrogens along the direction of the carbon momentum. This is supported by point-charge Coulomb explosion simulations. Figure 7: Kinetic energy distributions of ions in the (I\({}^{11+}\), C\({}^{3+}\)) coincidences. Top panel: I\({}^{11+}\), bottom panel: C\({}^{3+}\). A test of this approach is presented on the example of a triple-coincidence set, (I\({}^{3+}\), I\({}^{3+}\), C\({}^{2+}\)). In a dark momentum coefficient scan, such as in CH\({}_{3}\)I, the triple coincident count reaches maximum at around \(k\) = 0.75, although in this case the maximum is not as sharply defined as in CH\({}_{3}\)I. The triple momentum sum plot, after applying the correction coefficient to the carbon momentum, is shown in Figure 8. An alternative would be to apply the dark momentum correction to the combined momentum of the two iodine atoms, reducing it instead of increasing the carbon momentum. Both approaches gave very similar results. In Figure 8 one can see a rather well defined true coincidence region (again there is a slight displacement of it from the origin, for the same reason as discussed above). For triple coincidences, a more informative representation than PIPICO plots is, for example, a Newton plot, as shown in Figure 9. The plot was obtained by applying MF with the strength as indicated by the circle in Figure 9, but in order to further reduce the false coincidence contributions, the filter was applied in all three dimensions (restricting the full vector sum to within a sphere around zero). The MF, itself, does not define any structures in the Newton plot, as it only restricts the three-vector sum values not, for example, the angle between the iodine ions. In generating Figure 9, all momenta of the first I\({}^{3+}\) ion in the set were rescaled to 1 \(\times\) 10\({}^{-21}\) kg\(\cdot\)m/s and rotated onto the \(x\)-axis. The red spot is the distribution of the second iodine ion's momenta and the blue spot shows the momenta of carbon ions. The black dots are the calculated momentum vectors from a point-charge Coulomb explosion simulation. For the simulation, the initial positions of the charges were fixed according to the ground state equilibrium geometry of diiodomethane, full charges were assigned instantaneously to the atoms and then their classical trajectories were followed. The comparison with the experimental momentum islands in Figure 9 shows a very good agreement in the angular distribution, but a clearly shorter vector length for the carbon ions, indicating that in the Coulomb explosion they obtain a lesser share of the kinetic energy release than predicted by the simplest model. Detailed investigations of the Coulomb explosion's dynamics in CH\({}_{2}\)I\({}_{2}\) can be found in [13]. Figure 8: Momentum sum plot of (I\({}^{3+}\), I\({}^{3+}\), C\({}^{2+}\)) triple coincidences. The circle illustrates the strength of the three-dimensional MF applied afterwards. The sum of the iodine momenta is directed along the \(x\)-axis. Figure 9: Newton plot of the (I\({}^{3+}\),I\({}^{3+}\),C\({}^{2+}\)) triple coincidences, with the first I\({}^{3+}\) ion’s momentum in the \(x\)-direction and normalized to 1 \(\times\) 10\({}^{-21}\) kg\(\cdot\)m/s. The red-coloured area marks the second I\({}^{3+}\) ion’s momenta and the blue-colored area the C\({}^{2+}\) ion’s momenta. Black dots are from point-charge Coulomb explosion simulation.