# Molecular Dynamics of XFEL-Induced Photo-Dissociation, Revealed by Ion-Ion Coincidence Measurements
## 3 Results and Discussion  
### lodomethane  
CH\({}_{3}\)I molecules were irradiated with ultrashort (\(\sim 10\) fs) XFEL pulses of 5.5 keV energy. The experimental results, as well as the Coulomb explosion modeling, was published in [12]; here we summarize them as far as necessary for the coincidence analysis' background. The molecules absorb X-rays predominantly in the iodine atomic site, where electrons from inner shells down to I 2p are emitted. Multiphoton absorption occurs during the pulse and the charge is also further increased by Auger cascades, as demonstrated in the case of xenon--an atom with a very similar electronic structure [36; 37]. The Auger cascades are also the most likely mechanism of transferring charge to the methyl group of the molecule. The highly-charged molecule undergoes Coulomb explosion, producing the P\({}^{+}\), C\({}^{m+}\), and H\({}^{0,1+}\) fragments. Ion-ion coincidence analysis of these fragments was detected by the momentum-resolving ion TOF spectrometer, which then provided the data to reconstruct the dynamics of the dissociation events. The ion count per XFEL pulse was quite high, with the average of 7.25 heavy (excluding H\({}^{+}\)) ions/pulse. Assuming an ion collection and detection efficiency of roughly 50% means that in average of about 7 molecules/pulse were ionized; quite unfavorable conditions for coincidence analysis. Under such conditions, the most informative approach was, mainly, to perform a two-particle ion-ion coincidence (PIPICO) analysis with the help of MF. A possible pitfall in the MF-enhanced PIPICO lies in carrying over the filtering conditions to the result and erroneously interpreting the apparent correlations as coincidence information, while they were just created by the filtering process. To avoid this, we used several strategies. First, the MF can be performed using only two momentum vector components on the detector plane perpendicular to the TOF spectrometer's axis (See Figure 2). That leaves the third axial component undisturbed and, if a clear PIPICO pattern emerges in the first ion's TOF versus the second ion's TOF plot, it is a confirmation that true coincidence events were found. Second, the measured events can be \"scrambled\" by redistributing the individual ions between the events. Scrambling ensures that there are no true coincident ion pairs left in the data and, thus, all remaining apparent correlations, if any, must be assigned to the MF action and discarded. We first present an example of the analysis of a coincident pair (I\({}^{5+}\), C\({}^{2+}\)) that has the best counting statistics. At first, ion pairs are formed from all events with no filtering nor bias--if, for example, one iodine and two carbon ions are detected in a pulse, a pair is formed by randomly choosing one of the carbons. A momentum sum plot is then generated and is shown in Figure 3a, where the direction of the iodine's momentum is always pointing along the \(x\)-axis. It is seen from Figure 3a, that there is a diffuse background of uncorrelated momenta and a bright spot that corresponds to well-correlated momenta. However, the bright spot is not centered at zero; its shift indicates that the iodine's momentum is larger than the carbon's. This is easily understood from the molecular geometry: both the H\({}^{+}\) and C\({}^{2+}\) ions balance the momentum of I\({}^{5+}\), but the H\({}^{+}\) momenta are not included in the sum. This \"dark\" momentum is manifested as the shift of the correlated spot. However, it is also seen that the spot is well defined in the \(y\)-direction, meaning that the three hydrogens carry a concerted momentum that is directed closely along the C-I bond.