## 1 Introduction  
The speed of a photoexcited chemical reaction is determined by gradients of potential energy surfaces and the mass of reaction products or precursors. Generally, the decisive steps--the making or breaking of chemical bonds--occur on a femtosecond timescale. Numerous experimental methods have been implemented to approach this topic. The development of femtosecond laser pulses brought an extreme wealth of ultrafast techniques to life, among them visible and ultraviolet pump-probe spectroscopy [1] and multidimensional spectroscopy [2]. For isolated molecules, photoelectron spectroscopy [3] and electron, as well as X-ray scattering [4; 5; 6], have developed as high fidelity tools that can be directly compared to highest level ab initio calculations on isolated molecular dynamics. In this article, we focus on investigating a specific type of reaction, photoion fragmentation as it occurs in the presence of ionizing radiation in the upper atmosphere or in space [7]. Organic molecules can be photoionized by absorbing either one or multiple photons, which combined can overcome the ionization potential (typically on the order of 9 eV [8]). When sufficient energy is deposited in the molecule, it is placed on a cationic potential energy surface with repulsive character in at least one internal degree of freedom. Thus, the nuclei are sped up towards rapid bond dissociation and fragmentation by a steep gradient in the Franck-Condon region. The positive charge localizes on one of the fragments. Charged and neutral species separate with considerable velocity. Due to strongly different ionization potentials in charged and neutral species, such a process can only be investigated by probing the parent photocation and the fragments with photon energies considerably beyond what is readily available from table-top laser systems in the vacuum ultraviolet and soft X-ray regime. Short wavelength pulses in the femtosecond domain with unprecedented pulse energy became available with the advent of X-ray free electron lasers [9; 10; 11; 12]. For isolated molecules, this allowed for X-ray scattering experiments in extremely diluted targets [13; 14; 15] as well as the development of spectroscopic methods in the soft X-ray domain [16; 17; 18; 19; 20; 21; 22]. Due to the deeply-bound core electrons, X-ray spectroscopy is element-, as well as site-, selective [23]. In the present study, we demonstrate that unique insight into ionic photofragmentation can be gained by probing the dynamics using Auger electron spectroscopy. Here, the photoion and its fragments are core-ionized by a femtosecond soft X-ray pulse. They undergo Auger decay by filling the core hole with a valence electron and simultaneously ejecting another valence electron for energy conservation reasons, which is schematically shown in Figure 1. We measured the kinetic energy spectrum of the ejected Auger electron as a function of the time delay between the start of the photofragmentation reaction by the optical UV pulse and the X-ray ionization. By choosing the non-resonant soft X-ray photon energy to be high enough, Auger spectra of charged and neutral fragments can be observed simultaneously and the method is insensitive to small fluctuations of the photon energy at free electron lasers starting from noise. Furthermore, as shown in [16], Auger spectroscopy methods provide the unique opportunity to follow fragmentation dynamics beyond the typical bond distances of 1-2 A, since molecular Auger decay channels, which involve valence electrons situated at different atoms, still function at considerably larger atomic distances. We show in the following that the Auger spectral features reveal signatures of fragmentation dynamics at even longer distances. This allows for determination of e.g., their kinetic energies and can, therefore, give important mechanistic information about the redistribution of energy during the fragmentation process. We demonstrate the concept on a specific example, the multi-photon induced fragmentation process of the nucleobase thymine (see Figure 1).