URSA-PQ: A Mobile and Flexible Pump-Probe Instrument for Gas Phase Samples at the FLASH Free Electron Laser

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.

Core-Level Spectroscopy of 2-Thiouracil at the Sulfur L1- and L2,3-Edges Utilizing a SASE Free-Electron Laser

“…The poor modulation of the FLASH2 valence photoelectron spectrum in Figures 1 and 3 is a combined effect of the photon energy bandwidth of 4 eV and the reduced resolution of the magnetic bottle spectrometer at these comparatively high kinetic energies. The magnetic bottle was operated at retardation of only 5 eV; taking the measured 1/40 resolution [25], we arrive at a feature width of about 4 eV at Ekin = 150 eV. The valence features disperse with a slope of one in hυ per eV in Ekin throughout the whole measurement range, confirming the use of fundamental undulator radiation.…”

“…The data were obtained at the FL24 beamline of the FLASH2 free-electron laser (FEL) [37,38] as part of a more extensive investigation of the dynamics following UV excita-tion [20,21]. An instrument containing sample injection and a magnetic bottle-type electron spectrometer (URSA-PQ, German: Ultraschnelle Röntgenspektroskopie zur Abfrage der Photoenergiekonversion in Quantensystemen, English: Ultrafast X-ray spectroscopy for probing photoenergy conversion in quantum systems)-designed and built by the group at University of Potsdam-was connected to the beamline [25].…”

“…The FEL delivers the radiation in trains of pulses with a repetition rate of 10 Hz [40]. Each train consists of 50 pulses at 200 kHz internal repetition rate, with an estimated pulse duration of 150 fs [25]. Pulse energy scans were carried out to avoid X-ray saturation of the obtained electron spectra.…”

“…An electrostatic lens was used to apply a retardation potential to the electrons as they enter the flight tube, slowing them down and thus increasing the time-of-flight resolution. Calibration of the spectrometer was carried out using Kr-MNN Auger-Meitner electrons [25]. Using the calibration data together with a geometric model of the flight tube, a time-of-flight to kinetic energy conversion function was developed.…”

“…Specifically, we used recorded krypton spectra to construct a model of the spectrometer and extrapolated it to other retardation settings/energy ranges. The resolution (E/∆E) was found to be around 40 under the chosen settings [25]. For each X-ray pulse, FLASH provides a shot-to-shot measurement of the pulse energy (photon number) through the use of gas-monitor detectors (GMD).…”