# Molecular Dynamics of XFEL-Induced Photo-Dissociation, Revealed by Ion-Ion Coincidence Measurements
## 1 Introduction  
Nuclear motion in molecules and clusters takes place in a timescale starting from a few femtoseconds. Photoinduced nuclear dynamics--isomerization, bond breakage and dissociation, Coulomb explosion of molecules and clusters, chemical reactions--have long been of particular interest from both the fundamental and applied points of view [1; 2; 3]. In order to accurately observe such motion and trace its development, a sufficiently short initiating pulse of radiation is needed as not to obscure the real dynamics. Lasers have provided such an excitation source. The advent of free electron lasers (FELs) operating in the UV- and X-ray regime, however, opened up completely new avenues, providing access to tunable atomic inner-shell multiphoton excitation and ionization in femtosecond timescales [4; 5; 6; 7; 8; 9; 10; 11]. In the present paper, we focus on the unimolecular dissociation reactions induced by intense femtosecond-range X-ray pulses of FEL radiation. Specifically, we present examples on how to extract detailed information about the nuclear, and also the related electron dynamics by using coincident ion spectroscopy and momentum filtering analysis methods. The full results of these experiments and their interpretation are published or submitted for publication elsewhere, in [12] for CH\({}_{3}\)I, in [13] for CH\({}_{3}\)I\({}_{2}\), and in [14; 15] for 5-iodouracil. In order to reconstruct the dissociation process, as much information as possible should be gathered from each dissociation event; ideally this means detecting all atomic and molecular fragments (of all charges), as well as emitted electrons, and measuring their momenta. In practice, it would be a formidable task for all but the simplest systems and processes. Various experimental set-ups, as well as analysis methods, have been developed towards this goal. Most usable at FEL sources so far have been electron and ion time-of-flight (TOF) spectrometers and their various developments, such as COLTRIMS for ion-electron coincidences [16; 17], and magnetic bottle spectrometers for electrons [18; 19; 20; 21; 22; 23]. The results presented in this article are based on experiments performed at SACLA XFEL in Japan, using momentum-resolving ion TOF spectrometers with multi-ion detection capabilities per FEL pulse [24]. The multi-ion detection offers the opportunity for performing _coincidence_ (specifically ion-ion or multi-ion coincidence) analysis. In coincidence methods, each event is analyzed separately, tracing back the detected ions to a single source--an ionized and subsequently dissociating molecule, cluster, or nanoparticle. By its nature, it is one of the most accurate approaches in experimentally reconstructing the quantum mechanical event [25; 26; 27; 28; 29]. Coincidence experiments at FEL sources also face major challenges. Firstly, the repetition rates of FEL sources have been relatively low, even as low as 10 Hz for SACLA, which means that collecting sufficient coincidence data is a lengthy process during the extremely valuable beamtime (once referred to as a \"heroic experiment\" by a referee). This limitation is gradually becoming less severe with, for example, the Linac Coherent Light Source (LCLS) operating at 120 Hz and the European XFEL shortly starting operations at the planned 27,000 pulses per second. The second limitation is more fundamental, namely that for efficient coincidence analysis it must be ensured that all detected fragment ions indeed originate from the same event. Otherwise, the combinations of ions from different sources would be meaningless and create erroneous data, often referred to as \"false coincidences\". It can be a particularly severe problem at FEL sources, since the pulses have very high photon density (often desirable for a specific process to occur), creating many ionization events per pulse while, for a coincidence experiment, the ideal would be a single ionized particle per pulse.