## 2 Materials and Methods  
### Overview and Vacuum System  
Figure 1 shows an overview of the URSA-PQ instrument and its functionalities. In the current version, the sample is introduced by either backfilling the vacuum vessel or using a capillary oven. In this paper, the former is used to characterize the MBES, and the latter is used with 2-thiouracil to evaluate the time resolution. The interaction region is defined by the crossing point of the optical  
and X-ray beams overlapping with the region of highest sensitivity of the MBES. Molecules in the interaction region are excited by an optical laser pulse, in this case a 266 nm UV pulse, and subsequently probed by an X-ray pulse. Optical and X-ray pulses are focused by separate mirrors and combined inside the beamline by a mirror having a hole for X-ray transmission. The UV beam is set on side or top of the hole to avoid power losses. The tunable X-rays can induce a resonant core-to-valence excitation and/or non-resonant ionization. The resulting photo- and Auger electrons are then efficiently guided by the MBES's magnetic field arrangement along a flight tube of 1.7 m length to a micro-channel-plate (MCP) detector. The time-of-flight of the electrons is measured and converted to electron kinetic energy. The apparatus is designed in a modular way that also allows upgrades at later times, the usage of different measurement devices from the FLASH environment and other types of sample delivery systems. The desired high flexibility is achieved by designing the central vacuum vessel as a customized six-way cross (see Figure 2), following the most critical dimensions of the permanent CAMP instrument situated at FLASH 1. An overview of the instrument is shown in Figure 2. A central cross serves as the light-matter interaction chamber. A manipulator is mounted vertically (z-direction), holding the capillary oven molecular source and diagnostics tools on top of that source. Either one two of the devices (oven or diagnostics tools) can be brought into the center and thus the interaction region of the instrument. Shown in the positive y-direction is the MBES spectrometer flight tube with the MCP detector at the end. The manipulator holding a permanent magnet in the negative y-direction provides the up to 1 Tesla strong B-field of the MBES. The optical and X-ray beams travel in the x-direction, entering by the center flange, through the interaction region in the center of the cross to the center flange at the end. Figure 1: Diagrammatic representation of the URSA-PQ (German for ’Ultraschnelle Röntgenspektroskopie zur Abfrage der Photoenergiekonversion an Quantensystemen’, Engl. ’ultrafast X-ray spectroscopy for probing photoenergy conversion in quantum systems’) instrument with its functionalities. The molecular sample is evaporated and guided into the interaction region by a capillary oven. An optical laser pulse excites the sample before a delayed X-ray pulse from the FLASH FEL, focused by a Kirkpatrick-Baez (KB) mirror set, probes the excited molecular ensemble. Photo- and Auger-electrons created by the light-matter interaction are guided by the magnetic field of the magnetic bottle electron spectrometer (MBES) towards a detector. The diagnostic tools for spatial and time-overlap can we driven into the interaction region. The central cross of the instrument has three flanges (top in z-direction and left, right in \(\pm\) y direction) that adhere to the CAMP instrument dimensions concerning flange size (DN 250 CF) and distance of flanges from the interaction region. For the front port where the FEL enters, the chamber is kept at short distance to the interaction region (280 mm) to support short focal lengths of the beamline. Breadboards are welded to the bottom of this port and the rear port to allow the incorporation of in-vacuum equipment such as incoupling optics and diagnosis tools. The floor facing port (\(-\)z direction) can be used for a LN\({}_{2}\) cold trap (not shown here). A 1300 l/s turbomolecular pump is attached to a flange facing 45 degrees downwards.