Time- and space-resolved studies of shock compression molecular dynamics

Abstract

Two kinds of experiments are described, that measure the dynamical effects of shock compression on molecules with high time and space resolution. Time res- olution is obtained using ultrafast nonlinear coherent vi- brational spectroscopy. Space resolution greater than the diffraction limit is obtained by building in nanostructures of known dimensions. In the first experiments, 100 ps laser flash heating is used to suddenly vaporize Al nanoparti- cles embedded in reactive oxidizing polymers nitrocellu- lose (NC) and Teflon. The hot nanoparticles react with a surrounding shell of oxidizer and generate spherical shock waves >10 GPa. The propagation of shock-induced chem- istry in time and over distances ranging from 50 to 1,000 nm is measured and discussed. In the second experiment, fem- tosecond laser-driven planar shock waves run through a molecular monolayer of linear hydrocarbon chains. The methyl -CH3 groups that terminate the chains form a plane ∼1.5 u A thick. The C-H stretching vibrations of these groups are monitored as the shock front passes over. A combination of experiment and molecular simulations shows that chains with odd (15) numbers of carbon atoms become shorter by bending behind the shock front, whereas chains with even numbers (18) of carbon atoms undergo mechanical failure and shorten by forming gauche defects.