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The recent development of short-wavelength Free-Electron Lasers (FELs) has triggered a wide range of experiments to study the dynamics of electronic and nuclear rearrangement in small quantum systems like atoms, molecules and nano-particles. In particular, a nanometer-sized object exposed to intense FEL pulses will be transformed to a highly excited non-equilibrium plasma within femtoseconds. Understanding the way that high intensity X-ray pulses interact with the electronic structure of nano-samples is challenging because it requires experiments that can follow the dynamics on an extremely short time-scale.
We have utilized the terahertz-(THz)-field-driven streak camera technique at the FEL in Hamburg (FLASH) to study the dynamics of nanoplasma formation inside of rare gas clusters. In this approach the electrons activated by the 92 eV soft-X-ray pulse are accelerated or decelerated by the electrical field of the THz pulse. By changing the time-delay between the soft-X-ray and THz pulses, i.e. by varying the phase of the THz streaking field with respect to the electron photoemission time, and measuring the electron kinetic energy spectra a streaking spectrogram is obtained. Results of our study show, that in clusters the direct photoemission from the 4d shell of Xe is temporally shifted towards the leading edge of the soft-X-ray pulse as compared to the atomic gas target. This effect becomes even more pronounced in Xe-core/Ar-shell clusters. The photoemission of secondary nanoplasma electrons, on the other hand, seems not to be influenced by the THz streaking field. This may indicate that the time at which the electron escapes from the cluster becomes uncorrelated from the electron activation time.
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