# Molecular Dynamics of XFEL-Induced Photo-Dissociation, Revealed by Ion-Ion Coincidence Measurements
## 2 Materials and Methods  
The experiments were carried out at beam line 3 (BL3) of the SACLA X-ray free electron laser (XFEL) facility [35] and are described in detail in [12; 13; 14; 15]. The XFEL beam is focused by the Kirkpatrick-Baez (KB) mirror system to a focal size of about 1 \(\upmu\)m (FWHM) diameter. The photon energy was set to 5.5 keV and the photon bandwidth was about 40 eV (FWHM). The repetition rate of the XFEL pulses was 10 (for CH3I and 5-iodouracil) or 30 Hz (for CH2I2). The pulse duration was estimated to be about 10 fs. The peak fluence was estimated to be 26 \(\upmu\)J/\(\upmu\)m\({}^{2}\) in the CH3I and 5-iodouracil experiment, and 26 \(\upmu\)J/\(\upmu\)m\({}^{2}\) in the CH2I2 experiment.  
The gas phase samples were introduced to the focal point of the XFEL pulses as a pulsed supersonic gas jet seeded by helium gas. The ions were detected by a multi-coincidence recoil ion momentum spectrometer to measure three-dimensional momenta of each fragment ion (see Figure 2).  
The molecular beam was crossed with the focused XFEL beam at the reaction point and the emitted ions were projected by electric fields onto a microchannel plate (MCP) detector in front of a delay-line anode (Roentdek HEX80, by RoentDek Handels GmbH, Kelkheim, Germany). We used velocity-map-imaging (VMI) electric field conditions. Signals from the delay-line anode and MCP were recorded by a digitizer and analyzed by a software discriminator. The arrival time and the arrival position of each ion were determined and allowed us to extract the three-dimensional momentum of each ion.  
An essential element in interpreting the observations in all three cases was a comparison with a rather simple Coulomb explosion model, predicting the fragments' momentum vector correlations, magnitudes and geometry. Such quick, empirical models proved an invaluable aid in data analysis and we, thus, summarize its main properties. The classical point charge model assumes the source of the total charge creation to be the heavy (in this case iodine) atom, due to its much higher photoionization cross-section at around 5 keV. The charge build-up is described statistically as a smooth function:  
\[Q_{tot}(t)=Q_{tot,final}\bigg{(}1-\exp\bigg{(}-\frac{t}{\tau}\bigg{)}\bigg{)}, \tag{1}\]  
Figure 1: The molecules studied: (**a**) iodomethane, (**b**) diiodomethane, and (**c**) 5-iodouracil. Pink—iodoine, blue—nitrogen, red—oxygen, gray—carbon, and light gray—hydrogen.  
Figure 2: Sketch of the experimental setup at SPring-8 Angstrom Compact free electron LASer (SACLA).  
where \(\tau\) is the empirical, adjustable charge build-up constant. In comparison with the experiments, we found \(\tau\) close to 10 fs in all cases. The positive charge is multiplied by additional absorption and by Auger cascades, transferring charge from iodine (\(Q_{l}\)) to the rest of the molecule (\(Q_{X}\)):  
\[\frac{dQ_{X}(t)}{dt}=R\times Q_{l}(t), \tag{2}\]  
where \(R\) is the charge transfer rate--another empirical constant in the model. The fragment momenta and their correlations is a sensitive function of the charge dynamics and the above-given model parameters.