## 1 Introduction
Ultraviolet light of 4.65 eV photon energy induces a resonant three photon transition to an excited ionic state, from where the molecule is predicted to fragment into neutral isocyanic acid (HNCO) and a charged C\({}_{4}\)H\({}_{5}\)NO\({}^{+}\) fragment. Charged fragments have been observed in classical mass spectroscopy under high energy electron or photon excitation. The onset of the C\({}_{4}\)H\({}_{5}\)NO\({}^{+}\) fragment channel has been determined by threshold spectroscopy [24] to be above 10 eV. Thus, three photons of 4.65 eV photon energy need to be absorbed to observe the described fragmentation. The fragmentation of cationic thymine into HNCO and C\({}_{4}\)H\({}_{5}\)NO\({}^{+}\) has been theoretically investigated [25]. Although the reaction can be formally understood as a retro-Diels-Alder reaction, which should be concerted, a stepwise mechanism cleaving the two involved C-N bonds, as sketched in Figure 1, was found. Large molecular systems possess a relatively unstructured Auger spectrum after creation of 1s core holes in carbon, nitrogen, or oxygen. Many different decay paths are energetically allowed and broad bands originating from the 2s2s, 2s2p, and 2p2p valence electron orbitals are involved in the Auger decay [17; 26; 27]. In terms of potential energy surfaces, one can rephrase this assertion by stating that the density of final states of the Auger decay process is high and the Auger lines overlap to form large bands. After fragmentation and sufficient separation, however, electrons from the cationic fragment cannot take part in Auger decays on the neutral fragments, because such decay processes have distance dependences of \(r^{-6}\)[28]. Small molecules show relatively narrow lines with a width on the few 100 meV scale [29] because the density of final states is much lower as the number of electrons in the molecule is smaller. Different fragments can therefore be easily distinguished in the Auger spectra by characteristic sharp fingerprints. It is energetically more favorable to delocalize a positive charge on a large fragment than on a small one. Therefore, as a rule of thumb, neutral fragments will be observable as sharper  
Figure 1: Probing molecular photofragmentation by soft X-ray-induced Auger spectra. We exemplify the method on the thymine molecular cation, which breaks apart into neutral isocyanic acid (HNCO) and singly-charged C\({}_{4}\)H\({}_{5}\)NO\({}^{+}\) after three-photon UV ionization (bottom). The distance of the HNCO fragment is interrogated by a time-delayed soft X-ray pulse creating core ionized molecules. The subsequent Auger decay leads to the emission of an additional electron, which does not only have to overcome the ionization potential intrinsic to the molecule, but also the external Coulomb potential (red) of the nearby cationic fragment. The Auger emission from the small neutral HNCO in the vicinity of the positively-charged large fragment leads to a red-shifted Auger kinetic energy with respect to the Auger electron spectrum of the isolated molecule. As the dissociation coordinate increases, the redshift asymptotically disappears. lines on top of a background from the cationic system with higher density of final states and, thus, less structural. The electrons, which are emitted during Auger decay of the neutral fragment, still experience the Coulomb potential of the cationic fragment with considerably longer-range characteristic (\(r^{-1}\)) than the above mentioned joint neutral-charged fragment Auger channels (\(r^{-6}\)). The effect is observable by a shift of the Auger spectrum to lower kinetic energies. Probing the fragmentation process just after the optical excitation still interrogates the large molecular ion. The Auger spectrum will be showing a broad unstructured band. After some time, however, fragmentation has taken place and one can treat the molecular fragments as small molecules with a reduced dicationic state density. The resulting spectrum will be structured in much sharper lines.