Interaction of clusters with ultra short X-ray free electron laser pulses



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Biomolecular imaging has become one of the most exciting potential applications of the Linear Coherent Light Source (LCLS), which is a source of intense femtosecond X-rays. It has been predicted that a highly intense pulse with pulse lengths on the order of a few femtoseconds should be sufficient to capture the image of a biomolecule before it is destroyed. However, the rate at which a large biomolecule explodes during exposure is a large unknown, and will likely be one of the major factors in determining if such imaging will succeed. Clusters were chosen as a size dependant model system, ideal to study the evolution of complex systems in X-ray fields. From earlier intense near-infrared (IR) experiments, it is known that depending on size and Z constitution, clusters explode by Coulomb or hydrodynamic forces. These two limits have very different cluster explosion times and signatures. Coulomb explosion is too fast to allow imaging, whereas a hydrodynamically expanding cluster is a much slower process. The ionization process leading to cluster explosion is strongly wavelength dependent as one passes from IR through XUV to the X-ray regime because the kinetic energy of the released electrons determines the charge imbalance within the cluster, and therefore, determines the explosion dynamics. Unlike in previous experiments performed with near IR or XUV pulses, irradiation by photons at the LCLS will lead to the ejection of energetic photo- and Auger- electrons which could easily escape from the cluster, leaving behind positive ions. The buildup of this charge during exposure can lead to a Coulomb explosion of the sample. On the other hand, if the charge accumulates, the photoelectrons will be held inside the cluster, where they could contribute to the cluster temperature and form a nanoplasma and expand hydrodynamically. The main goal of the thesis was to study the explosion dynamics of clusters generated due to their interaction with intense X-rays and look at its dependencies on the X-ray energy, photon fluence, absorption cross sections, sample constituency and sample size. This thesis also compares the results from X-rays with the corresponding results obtained using ultrashort XUV and Infrared lasers.