## 3 Results and Discussion
The relative error of the spectral intensity on is on the order of 5-10%, which was determined in another paper on data from the same lincac coherent light source experiment using bootstrap statistical analysis [17]. (**b**) The same transient spectra as in (**a**), corrected by a background (see text). For large delays, the transient spectrum fits the reference HNCO spectrum taken at the synchrotron (red dots). The relative intensity error in the significant range is on the order of a few percent. The calculation indicated in blue reproduces all the essential features of the HNCO reference spectrum, although with slightly different peak distances. The thick grey lines model the transient spectra by shifting and broadening three Gaussian lines according to a one-dimensional fragmentation process as described in the text. reproduced by the ADC(2) calculation. The upper energy maximum is only poorly modulated in the simulation. In addition, we observe energy shifts in the four lower maxima. Shifts on the order of 1 eV are typical for the ADC(2) method and the deviations will be the topic of future investigations. In order to discern the details of the sharp features in the time-resolved spectra we model a background from a scaled spectrum at a 200 fs delay and subtract it from the spectra at all delays. The scaling factor is fitted such that the low and high energy wings of the spectra after this delay essentially have zero background. We plot the background-corrected spectra in Figure 3b. The Auger spectra of UV-photoionized thymine for delays 1150, 1340, and 1530 fs show a very good agreement with the steady-state Auger spectrum of HNCO. The three lower peaks at positions of 495.6, 497.3 and 500.4 eV (in the remainder of the paper labeled as Peak 1-3 in ascending Auger electron kinetic energy) are perfectly reproduced both in position and width. The highest energy peak at 505.2 eV in the steady-state spectrum is not well reproduced in the time-dependent spectra. This can be attributed to additional contributions from excited state dynamics in the neutral thymine and the ground state bleach, which are known to both appear in this regime [17]. Inspecting the transient trend in these peaks, one realizes that the peak width as well as position follows a similar trend for all three features. The peaks sharpen after the UV excitation to reach their stationary shape at about 1 ps. In addition, all peaks experience a shift towards higher kinetic energies. Around 400 fs, peak 3 appears at 499.7 eV, which is 0.7 eV shifted from the steady state. Around 770 fs, this shift diminishes to 0.3 eV. At the same delay, peak 1 is also red-shifted by about 0.3 eV from its steady state value of 495.6 eV. Peak 2 needs a longer time to fully form and also has a larger shift than the high and low energy features. At 770 fs, it is shifted by 0.6 eV lower from the steady state value of 497.3 eV. In the following, we provide an explanation of the three main observations in the experimental data, the appearance of sharp new features, their spectroscopic shift, and their sharpening. From our calculations of the Auger spectra of thymine, we know that both oxygen atoms of thymine give rise to very dense spectral features, resulting from the high density of final states (DOFS) in this rather large molecule [17]. The high DOFS is reflected in a broad and unstructured Auger spectrum even for only one of the two oxygen atoms, demonstrated by calculations in the supporting material in [17]. From photoion mass spectrometry, it was suggested that photoionized thymine fragments into neutral HNCO and a positively-charged residual with mass 83 [24]. As the neutral HNCO fragment splits off the positively-charged residual, the electronic overlap between the two constituents gets smaller. This in turn reduces the density of dicationic states, in particular for the small HNCO fragment with only 22 electrons compared to the 43 electrons of the C\({}_{4}\)H\({}_{5}\)NO\({}^{+}\) fragment.