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X-RAY LASERS, PHOTON SCIENCE, AND STRUCTURAL BIOLOGY

Final Report Summary - XLASERS (X-RAY LASERS, PHOTON SCIENCE, AND STRUCTURAL BIOLOGY)

The act of observation changes the object that is being observed. This effect can be quite dramatic, and for instance in imaging, fundamental limits to observation appear long before quantum limits. We set up a European research network to explore this problem in 1996, and in 2000 our results turned into the scientific case that assured funding for the construction of the first hard X-ray free-electron lasers. X-ray lasers are new and their science does not have decades of history like crystallography or electron microscopy. The first lasing was only achieved in 2009. Over the past five years (i.e. during this project), results from X-ray lasers have made remarkable advances in physics, chemistry, materials science, and biology. At the moment, these lasers can produce X-ray pulses from ~ 500 attosecond to ~ 300 femtosecond duration at a repetition rate of 120 Hz with up to 10^12 photons per pulse at keV photon energies. A dramatic improvement in these parameters can be expected in 2017 when the European XFEL begins user operations. XFEL will be capable of producing more than a billion shots in a single day, and deliver about 100-times more intense pulses than pulses available today.

Theoretical studies and simulations agree with each other, and predict that with a very short and very intense coherent X-ray pulse, a single diffraction pattern may be recorded from a large macromolecule, a virus, or a cell, before the sample explodes and turns into plasma from the energy deposited into it by the X-ray pulse.

The principle of "diffraction before destruction" has been experimentally demonstrated at Ångström resolution on micro- and nanocrystals and at nanometer resolution on single biological objects. However, the goal of near-atomic resolution in single particle imaging with X-ray lasers has not yet been reached. A detailed analysis of parameters shows that pulses should contain ~ 10^14 photons to produce good signal levels at high resolution on single macromolecules. Presently, only about 10^12 photons per pulse are available and this is less than the desired target value. This will change later in 2017, when the European XFEL will start user operations. Preliminary analysis of one of our LCLS experiment from June 2016 points to diffraction signal to a few nanometer resolution from a single Rubisco molecule.

At the length scales currently accessible to us in experiments, we find no measurable damage to samples during ultra-short illumination. Existing free-electron lasers have not yet reached any physical limits. We may expect frontier science to emerge from the superluminous new machines.

In addition to experiments, we have initiated a programme on the development of open source software packages for research with X-ray lasers. Data from the experiments are archived in the Coherent X-ray Imaging Data Bank (CXIDB) that was established during the project. Data banks with experimental data are crucial for education and research, aiding the development and validation of new theories and techniques.