## 3 Results and Discussion
### Temporal Resolution  
We now investigate the temporal resolution in the optical pump--these are X-ray probe experiments. As mentioned above, we initially determined the coarse temporal overlap by monitoring a fast-diode response, induced by UV and X-ray pulses, on an oscilloscope. The setup had an accuracy in the sub-100 ps domain. As 100 ps resolution is not sufficient for the experiments, more accurate information on the temporal overlap, as well as the temporal resolution achievable by this setup, we performed measurements of photoelectron signals of molecules using the URSA instrument. We used the molecule 2-thiouracil (C\({}_{4}\)H\({}_{4}\)N\({}_{2}\)OS), as this has been the molecule of interest for our first beamtime utilizing this instrument. Thionucleobases are interesting as they show an efficient relaxation to long-lasting triplet states after UV excitation, thereby showing different behavior compared to canonical nucleobases, which relax relatively quickly to the ground state [31]. Figure 5a shows a core level photoelectron spectrum of 2-thiouracil illuminated by X-ray photons of 272 eV mean-photon-energy. The pulses have less than 1% mean- photon-energy-jitter and a relative bandwidth of 1-1.5%. Shown in this particular part of the spectrum is the 2p-photoline of sulfur at 103.5 eV kinetic energy. This line should be spin-orbit split resulting in the j = 3/2 and 1/2 components with an energy spacing of 1.2 eV [32], however already the photon energy resolution is not sufficient to resolve that splitting. The small shoulder visible on the lower kinetic energy side at 95-97 eV is a satellite structure that accompanies the main 2p photoline (as, for instance, previously documented for sulfur on surfaces [33]). Upon UV excitation, the main photoline shifts towards lower kinetic energies, due to the molecular dynamics setting in immediately. The molecular origin for this observation will be discussed in detail in a separate paper. Here, we use this feature to determine the temporal overlap with sub-ps precision, and the overall time resolution of the experiment itself. We take here the difference spectrum (not shown), calculate its absolute value \(\left|\text{UV}_{\text{on}}-\text{UV}_{\text{off}}\right|\) as a measure for the UV induced change in the spectrum. This observable as a function of relative delay between UV-pump and X-ray-probe pulse is shown by the blue dots in Figure 5b, together with a theoretical fit shown by the orange line. The fit function includes a Gaussian describing the time-resolution, convoluted with two exponential decays (see for instance Equation (3) in Reference [34]). The second exponential decay is longer than 100 ps and has limited influence on the data in the delay window shown. The faster exponential decay of (230 \(\pm\) 30) fs describes some molecular dynamics. Most interesting in the context here is the Gaussian time-resolution function. We actually determined the relative time-overlap by the maximum of this Gaussian, which can be done with a sub-10 fs accuracy. The Gaussian time resolution when correcting data using the beam arrival monitor [35] has a FWHM of \(\sigma_{TR}\) = (190 \(\pm\) 10) fs. There are different contributions to the origin of the time-resolution known in the literature which sum up geometrically to the final time resolution \(\sigma_{TR}\) = \(\sqrt{\sigma_{UVL}{}^{2}+\sigma_{FEL}{}^{2}+\sigma_{FELtrain}{}^{2}+\sigma_{UVL train}{}^{2}+\sigma_{FEL-UVLift}{}^{2}}\), where \(\sigma_{UVL}\) is the UV pulse length, \(\sigma_{FEL}\) the X-ray pulse length, \(\sigma_{FELtrain}\) the intra-train jitter of the X-ray pulses within the macro-bunch, \(\sigma_{UVLtrain}\) the intra-train jitter of the UV pulses within the macrobunch and \(\sigma_{FEL-UVLift}\) the relative timing jitter of X-ray and UV laser pulses [36]. The UV pulse duration has been determined to be \(\sigma_{UVL}\) = 80 fs by a frequency resolved optical gating (FROG) measurement [37]. Subtracting the UV pulse duration geometrically yields 170 fs Gaussian FWHM total for all other components.