Explosions of Clusters Driven by Intense X-Ray FEL Pulses

Biomolecular imaging has become one of the most exciting potential applications of the LinearCoherent Light Source (LCLS). Neutze et al. [1] were the first to predict that a highly intensepulse with pulse lengths of the order of a few femtoseconds should be sufficient to capture theimage of a biomolecule before it is destroyed. The LCLS at SLAC has provided a source of Xraysthat has a pulse length of a few femtoseconds, short enough to capture the desired instantpicture. However, the rate at which a large biomolecule explodes during exposure is a largeunknown and will likely be one of the major factors in determining if such imaging will succeed.Clusters were chosen as size dependant model systems, ideal to study the evolution of complexsystems in X-ray fields.

From intense near-infrared (IR) experiments it is known, that depending on size and Zconstitution, clusters explode by Coulomb or hydrodynamic forces [2]. These two limits havevery different cluster explosion times and signatures. The ionization process leading to clusterexplosion is strongly wavelength dependent as one passes from IR through XUV [3,4] to the Xrayregime because the kinetic energy of the released electrons determines the charge imbalancewithin the cluster and therefore the explosion dynamics. Unlike in previous experiments in nearIR [2] or EUV [3,4] pulses, irradiation by energetic photons at the LCLS will lead to the ejectionof energetic photo and Auger electrons which will easily escape from the cluster, leaving behindpositive ions, and the buildup of this charge during exposure can lead to a Coulomb explosion ofthe sample. On the other hand, once the charge accumulates, the photoelectrons will be heldinside the cluster where they contribute to the cluster temperature and form a nanoplasma andexpand hydrodynamically. Coulomb explosion is a fast process and will destroy the clusterbefore it can be imaged whereas a hydrodynamically expanding cluster is a much slower process.

The main goal of the experiment was to explore the transition between Coulomb andhydrodynamic explosion and it’s dependencies on the X-ray energy, photon fluence, absorptioncross sections, and on sample size. This talk will use xenon and methane clusters as modelsystems to study the explosion dynamics and also show how these results compare with thecorresponding work using intense infra red and XUV lasers.