## 3 Results and Discussion
The appearance of narrow lines within 600 fs, therefore, directly reflects the formation of neutral HNCO fragments. The transient shift in the peak positions stems from the interaction of the outgoing Auger electron of HNCO with the positive charge of the C\({}_{4}\)H\({}_{5}\)NO\({}^{+}\) fragment. If the Auger electron is emitted in the vicinity of a positive fragment, it has to overcome the Coulomb potential of that charge. It is, thus, detected with lower kinetic energy compared to a HNCO Auger electron being emitted far away from the charged fragment. With the assumption of the Coulomb interaction resulting in a shift in atomic units being directly 1/distance to be responsible for the peak shifts, we can directly determine distances between charged and neutral fragments from the energy shifts. The distances are listed in Table 1. The evaluated distances for different peaks in part differ considerably, which can be mainly attributed to the limited accuracy to which the peak positions could be extracted from the spectra. We confirm the validity of our interpretation by comparing the values in Table 1 with time-dependent distances calculated under the model assumption that the entire excess energy from the UV photoionization is transferred into kinetic energy. It is clear that three UV photons of 4.65 eV are needed to overcome the appearance energy of 10.7 eV for HNCO fragments in thymine [24]. This leaves approximately 3 eV of excess kinetic energy. The masses of HNCO and the C\({}_{4}\)H\({}_{5}\)NO\({}^{+}\) fragment are \(m_{1}=43\) a.u. and \(m_{2}=83\) a.u., respectively, and the velocity of their separation approximately 4600 m/s. The distance of the two fragments \(x\) as a function of time \(t\) is given in Equation (1):  
\[x(t)=v_{2}t+v_{1}t=t\left(\sqrt{\frac{2E_{kin}}{m_{2}\Big{(}\frac{m_{2}}{m_{1} }+1\Big{)}}}+\sqrt{\frac{2E_{kin}}{m_{1}\Big{(}\frac{m_{1}}{m_{2}}+1\Big{)}}}\right) \tag{1}\]  
Time-dependent distances from this model agree reasonably well with those from evaluating the peak shifts, which strongly supports our assignment to the effect of Coulomb interactions with the cationic fragment. We are using the expression for time-dependent distances to model the evolution of the HNCO Auger spectra. We are describing the HNCO peaks as Gaussian functions with 1.5 eV full width at half maximum as extracted from the steady-state HNCO spectra. We simulate the time-dependent shifts by assuming a pure Coulombic 1/x potential (using atomic units). A trivial explanation for the peak sharpening would be that it is due to the limited time resolution of 330 fs. Since the gradient of the Coulomb potential has an inverse dependence on the distance to the charged fragment, sampling over the 330 fs time evolution would broaden the peaks more strongly at earlier times than at later times. Convolving our modeled time-dependent spectra, however, does not completely account for the peak sharpening. To achieve good agreement with experimental spectra, we had to include an additional time-dependent broadening to the Gaussian functions, which decreases from 4.7 to 1.5 eV during the first picosecond. The Gaussian peaks are normalized according to their area and scaled by the same universal factor for all time delays. The resulting simulated spectra are given as the grey thick lines in Figure 3b. The shift as well as the position fits well within the noise on the experimental Auger spectra. The additional broadening most likely stems from the finite width of the nuclear wavepacket. From [24], we know that the dissociating nuclear wavepacket has to overcome two potential barriers which will broaden it in the dissociation coordinate. The broad wavepacket in this degree of freedom will have a similar effect as the time resolution on the Auger electron peak width.