Ultrafast Molecular Dynamics at a Shock -Compressed Metal -Liquid Interface

169 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2004.A novel experimental technique has been developed for investigating molecular dynamics at a mechanically perturbed metal-liquid interface with picosecond time resolution and Angstrom spatial resolution. Self-assembled monolayers of thiols chemisorbed to gold serve as a model metal-liquid interface. This interface is compressed by a ∼5 GPa laser-driven shock wave. Time resolved vibrational sum-frequency (SF) spectroscopy is employed to probe the detailed molecular dynamics of this shock compression and release cycle. SF spectroscopy is only sensitive to the terminal methyl group of an alkanethiol, providing depth resolution of ∼2 A. Passage of the shock front reorients the plane of methyl groups within ∼4 ps through molecular tilts and generation of gauche conformational defects. Relaxation occurs during the subsequent ∼100 ps. The recovery dynamics change considerably when subtle changes are made to the interfacial structure. Recovery is more reversible for an odd-chain (15 carbon) thiol than an even-chain (18 carbon) thiol. Molecular modeling was used to simulate the compression process. These simulations suggest the even chains respond to compression by adopting gauche conformations. The odd-length chains remain all-trans throughout the cycle. Because the ends of the thiol molecules move laterally in response to compression, this experimental technique is relevant to tribology. This approach could be used to investigate additional systems, including shock induced chemical reaction dynamics.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD