Dynamics of Samples Irradiated by Free-electron-laser Radiation

Ultrashort pulses of intense radiation emitted from free-electron-lasers (FELs) are powerful tools for the investigation of the static and dynamic properties of matter down to atomic length scales. For applications of FELs we need to understand quantitatively how the FEL radiation interacts with matter, including the extreme case of the focussed laser beam. Our theoretical group investigates the interaction of FEL radiation at VUV and X-ray wavelengths with atomic clusters, large biomolecules, and solids. The tools used are kinetic Boltzmann equations and molecular dynamics simulations. Our results are important for understanding existing experimental results and for planning future experiments with FELs, in particular for planning experiments on single particle diffraction imaging.

Two theoretical methods can be used to describe the evolution of irradiated samples: (i) particle approach, where trajectories of single particles (atoms and ions) are followed, and (ii) transport approach, where particle density is evolved using kinetic equations.

Ion fractions, Ni/N, within the irradiated xenon clusters at the end of the ionization phase. The number, Ni/N, denotes here the number of Xe+i ions, and the number, N, is the total number of ions. At t=0 fs these clusters contained: a) 20 atoms, b) 70 atoms, c) 2500 atoms and d) 90000 atoms. The experimental results are in agreement with the ion fractions obtained from the model.

Below we give an overview of some of our projects:

  i) Evolution of atomic clusters irradiated with VUV and X-rays:
The results of the first cluster experiments with FLASH could not be understood within the standard approach [1]. Several novel models [2-6] could explain various aspects of the observed ionization dynamics. However, there are still some controversies about the role of individual processes. A statistical Boltzmann model, applicable also for large samples (N>1000), was useful in evaluating the contributions of different mechanisms to the ionization dynamics [8, 9]. The results were cross-checked with an independent molecular dynamics approach. In Fig. 2 we show the results obtained with our model for xenon clusters of various size irradiated with a VUV FEL pulse of a fixed fluence, F=0.4 J/cm2.

ii) Analysis of some processes contributing to the ionization dynamics within irradiated samples: (a) inverse bremsstrahlung estimated using effective atomic potentials [10], (b) many-body recombination.

iii) Dynamics of irradiated large biomolecules and crystals: Time-scales of damage processes within irradiated biomolecules and crystals are important for planning future experiments on single particle diffraction imaging.

iv) Classification of diffraction images: Diffraction images of a randomly oriented biomolecule are classified in respect to the orientation of the biomolecule. Classes of equivalent images are then identified. Averaging over images within one class of images increases the signal-to-noise ratio of the averaged images.


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