O. Hoidn, G. Seidler, K. Akli, K. George, H. J. Lee, A. Marinello, J. Rehr, S. Trickey

California Institute of Technology

XFEL-based studies of the electronic and lattice energy relaxation cascade in x ray-pumped targets typically seek to maximize the intensity and homogeneity of target heating. These two goals are necessarily compromised in studies where they conflict with other experimental constraints, such as in x-ray diffraction studies that require high-energy incident photons to reach large momentum transfers or XANES for hard x-ray binding energies. Here we explore structured target design as a new technique for increasing the volumetric density of energy deposition in XFEL-heated targets across a wide variety of experimental contexts. Our design consists of nanostructured low- and mid-Z target materials clad with dense, high-Z elements. Using simulations with the Monte Carlo code PENELOPE we show that energy deposition in the target is enhanced via nonlocal heating by multiple-keV Auger and photoelectrons produced in the high-Z cladding. This enhancement is particularly strong for low-Z targets and high incident photon energies; for instance, it reaches a factor of nearly 100 in a 150 nm-thick layered gold-diamond-gold target stimulated by 7 keV electrons. We predict that this target design opens a wider thermodynamic parameter space using pure x-ray heating and also gives new ways to interrogate finite-temperature electronic structure and, more broadly, the relaxation cascade following XFEL heating of a solid state system. This effort forms an important part of the scientific basis for beamrum LK20 at MEC in January 2016.