We report on the dynamics of ultrafast heating in cryogenic hydrogen initiated by a ≲300 fs, 92 eV free electron laser x-ray burst. The rise of the x-ray scattering amplitude from a second x-ray pulse probes the transition from dense cryogenic molecular hydrogen to a nearly uncorrelated plasmalike structure, indicating an electron-ion equilibration time of ∼0.9 ps. The rise time agrees with radiation hydrodynamics simulations based on a conductivity model for partially ionized plasma that is validated by two-temperature density-functional theory.
The pulse duration of soft x-ray free-electron laser ͑FEL͒ radiation is directly measured by time-resolved observation of doubly charged helium ions at 51.8 eV. A wave front splitting autocorrelator produces two correlated FEL pulses with a resolution of better than a femtosecond. In the interesting intensity range from 10 13 to 10 16 W / cm 2 direct and sequential double ionization contribute to the ion yield which has significant influence on the correlation width, being a general feature at high photon energies. Here, a duration of L = ͑29Ϯ 5͒ fs is derived for the soft x-ray pulses at FLASH.
The average temporal (longitudinal) and spatial (transverse) coherence of free-electron-laser pulses in the extreme ultraviolet at FLASH is measured by interfering two time-delayed partial beams directly on a CCD camera. Wavelengths between ¼ 32 nm and ¼ 8 nm are examined. A decrease of the coherence time for the fundamental wavelengths from c ¼ ð6 AE 0:5Þ fs at 32 nm to c ¼ ð2:9 AE 0:5Þ fs at 8 nm is measured. At ¼ 8 nm the fundamental wavelength and the third harmonic of 24 nm are compared to each other. For 8 nm radiation as third harmonic of 24 nm a coherence time of c ¼ ð2:5 AE 0:5Þ fs is observed. The spatial coherence of 24 and 8 nm fundamental pulses are found to be very similar. The visibility decreases to 50% of the maximum visibility at about 3.2 mm overlap of the partial beams, which corresponds to 42% of the beam diameter at a distance of 90 m from the exit of the undulator. These results are analyzed in terms of the Gaussian Schell model resulting in six contributing modes to the total radiation. In addition, the correlation of the visibility between the fundamental radiation at 24 nm and its third harmonic at ¼ 8 nm is investigated for identical shots.
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