Speaker: Nirmala Kandadai, The University of Texas, Austin
Program Description
Biomolecular imaging has become one of the most exciting potential applications of the Linear
Coherent Light Source (LCLS). Neutze et al. [1] were the first to predict that a highly intense
pulse with pulse lengths of the order of a few femtoseconds should be sufficient to capture the
image of a biomolecule before it is destroyed. The LCLS at SLAC has provided a source of Xrays
that has a pulse length of a few femtoseconds, short enough to capture the desired instant
picture. 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 size dependant model systems, ideal to study the evolution of complex
systems in X-ray fields.
From intense near-infrared (IR) experiments it is known, that depending on size and Z
constitution, clusters explode by Coulomb or hydrodynamic forces [2]. These two limits have
very different cluster explosion times and signatures. The ionization process leading to cluster
explosion is strongly wavelength dependent as one passes from IR through XUV [3,4] to the Xray
regime because the kinetic energy of the released electrons determines the charge imbalance
within the cluster and therefore the explosion dynamics. Unlike in previous experiments in near
IR [2] or EUV [3,4] pulses, irradiation by energetic photons at the LCLS will lead to the ejection
of energetic photo and Auger electrons which will easily escape from the cluster, leaving behind
positive ions, and the buildup of this charge during exposure can lead to a Coulomb explosion of
the sample. On the other hand, once the charge accumulates, the photoelectrons will be held
inside the cluster where they contribute to the cluster temperature and form a nanoplasma and
expand hydrodynamically. Coulomb explosion is a fast process and will destroy the cluster
before 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 and
hydrodynamic explosion and it’s dependencies on the X-ray energy, photon fluence, absorption
cross sections, and on sample size. This talk will use xenon and methane clusters as model
systems to study the explosion dynamics and also show how these results compare with the
corresponding work using intense infra red and XUV lasers.
