Abstract
We present a highly flexible and portable instrument to perform pump-probe spectroscopy with an optical and an X-ray pulse in the gas phase. The so-called 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 is equipped with a magnetic bottle electron spectrometer (MBES) and tools to characterize the spatial and temporal overlap of optical and X-ray laser pulses. Its adherence to the CAMP instrument dimensions allows for a wide range of sample sources as well as other spectrometers to be included in the setup. We present the main design and technical features of the instrument. The MBES performance was evaluated using Kr M4,5NN Auger lines using backfilled Kr gas, with an energy resolution ΔE/E ≅ 1/40 in the integrating operative mode. The time resolution of the setup at FLASH 2 FL 24 has been characterized with the help of an experiment on 2-thiouracil that is inserted via the instruments’ capillary oven. We find a time resolution of 190 fs using the molecular 2p photoline shift and attribute this to different origins in the UV-pump—the X-ray probe setup.
Highlights
Photoexcited molecules channel the energy of light into different energetic degrees of freedom, such as vibrational energy, charge transfer and rearrangement of chemical bonds
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
The capillary reduces the angular width of the beam compared to usual effusive sources and we have measured the full width at half maximum (FWHM) of 10 degrees when operating in the 1011 molecules/cm3 density regime [21]
Summary
Photoexcited molecules channel the energy of light into different energetic degrees of freedom, such as vibrational energy, charge transfer and rearrangement of chemical bonds. 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 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. 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. X-ray-probe measurements by using the example of 2-thiouracil
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