## 1 Introduction  
Photoexcited molecules channel the energy of light into different energetic degrees of freedom, such as vibrational energy, charge transfer and rearrangement of chemical bonds. This process is fast and complex, and often happens in a way that cannot be described within the framework of the Born-Oppenheimer approximation (BOA) [1, 2, 3, 4]. In fact, many relevant photoinduced molecular changes, such as retinal isomerization in vision [5], bacterial light harvesting, as well as nucleobase photoproduction [6, 7, 8, 9] happen on an ultrafast timescale as non-BOA processes. In order to better understand the molecular dynamics, a close comparison of simulations and experiments is extremely fruitful. Using small and isolated chromophores is advantageous for quantitative comparisons of experiments to simulations, the latter can be performed using the highest level of electronic structure methods under these constraints. Moreover, in the gas phase, experiments can use rather powerful spectroscopic methods for charged particles including highly differential coincidence experiments [10, 11]. In addition, quantum manipulation such as molecular nonadiabatic, field-freealignment [12; 13] and quantum state-selection [14] to prepare specific molecular target states are possible in the gas phase. X-ray probing of molecular dynamics offers substantial new opportunities complementary to well-established optical pump-probe methods. The X-ray-matter-interaction is element selective due to large differences of the inner shell binding energies between different elements and has been demonstrated to be site selective [15]. We have used these advantages in past time resolved UV pump--X-ray probe studies, to investigate the dynamics of isolated nucleobases. The molecular internal conversion from and to \(n\pi^{*}\) states leads to a strong pre-edge feature in the X-ray absorption spectrum of the element at which the \(n\) lone-pair orbital is localized [16]. Since both the core-level as well as the lone pair level of one particular atom are having a large overlap, the dipole matrix element and thus the absorption cross section is larger than cross sections involving delocalized molecular orbitals. The method is therefore ideally suited for detection of any \(n\pi^{*}\) states via the spectroscopy at the corresponding heteroatom [17]. Changes in the bond distance manifest themselves as shifts in the Auger kinetic energy of one of the atoms in this bond [18]. This is due to the strongly repulsive, Coulomb-repulsion shape of the final, dicationic states in the Auger decay. In addition, time-resolved Auger probing can also be used to detect the dissociation into a neutral and charged fragment in a time resolved way [19]. In this work, we describe the design and first tests of a new user instrument at the free-electron laser (FEL) facility FLASH (Free Electron Laser in Hamburg), called URSA-PQ (German for 'Ultraschnelle Rontgenspektroskopie zur Abrage der Photoenergiekonversion an Quantensystemen', Engl. 'ultrafast X-ray spectroscopy for probing photoenergy conversion in quantum systems'), for pump-probe experiments primarily on gas-phase targets. The new instrument is equipped with a'magnetic bottle' electron spectrometer (MBES) [20], a molecular source, as well as several tools for finding the spatial and temporal overlap of the optical and X-ray laser pulses. The MBES is characterized by a high solid angle collection, ideal for dilute targets. In addition, it is able to measure a large range of kinetic energies, and the resolution at a particular range can be optimized by an electrostatic retardation system. We, furthermore, equipped the present system with a capillary resistively heated oven source for the evaporation of condensed molecular samples [21]. This system works well for fairly small molecules like nucleobases, which can reach an appreciable vapor pressure around \(10^{-4}\) mbar, as for the example of thymine [22], without undergoing pyrolysis or tautomerization.