European XFEL Seminar | Novel Gas Phase Targets for the Study of Physical Sciences using X-ray Free Electron Lasers

by Klaus von Haeften (Leicester University)

Monday, April 10, 2017 from to (Europe/Berlin)
at European XFEL ( E1.173, Schenefeld Campus )
The choice of appropriate targets is paramount for successful and meaningful experiments with Xray free electron lasers. For example, the high intensities associated with a high number of photons concentrated in ultrashort pulses mean that samples will inevitably be destroyed. Supersonic beams play therefore an important role as targets. They allow the investigation of single atoms, molecules and clusters in vacuum, free of external interaction. While this is an excellent advantage for the study of intrinsic properties, it may be seen as a downside when it is required to study molecules or clusters in a liquid environment. Liquid jets enjoy increasing popularity for the study of molecular interactions. They also establish excellent targets for experiments with X-ray free electron lasers because of the high collimation of the beam, the continuous sample renewal and the wide choice of solvents that can be studied. However, a disadvantage is that pressure and temperature can hardly be changed. High pressure gas cells offer just this advantage of variability of pressure and temperature, including over very wide ranges. It is therefore possible to change the thermodynamic phases. Of particular interest is the supercritical phase because very large density changes can be achieved. For gas mixtures this means that chemical reactions including nucleation and growth of nanoclusters can be studied. High pressure cells nevertheless require windows which limit such experiments to the hard X-ray regime. Here, I will present an experiment by which the nucleation and growth of nanoparticles in liquids can be studied. At the University of Leicester, my research group has vaporised metals in vacuum and deposited the vapour onto a liquid jet. We found that the atoms become trapped in the liquid and grow to nanoparticles. This scheme mimics a variety of wet-chemical synthesis protocols, but provides much better purity and control over the reaction partners, whose structural and energetic changes can be measured using X-ray pulses. Furthermore, it is highly promising as a method to form supported metal nanoparticles in one step, something that is not possible with competing new technologies developed elsewhere. I will present a similar proposal for experiments in high pressure cells. We found that atoms nucleate on ions in supercritical gas and grow to clusters when pressure is increased. Both methods, atomic deposition onto liquid jets and nanoparticle growth in supercritical gases have scope for a range of applications and commercial spin-off, for example in the chemical industries and in the security sectors.
Organised by Joachim Schulz