I-Yin Sandy Lee

Molecular dynamics observed 60 ps behind a solid‐state shock front

Microfabricated monolithic shock target arrays with embedded thin layers of dye‐doped polymer films, termed optical nanogauges, are used to measure the velocity and pressure (Us=3.5 km/s; P=2.1 GPa) of picosecond‐laser‐driven shock waves in polymers. The 60 (±20) ps rise time of absorbance changes of the dye in the nanogauge appears to be limited by the transit time of the shock across the 300 nm thick gauge. The intrinsic rise time of the 2 GPa shock front in poly‐methyl methacrylate must therefore be ≤60 ps. These measurements are the first to obtain picosecond resolution of molecular dynamics induced by the passage of a shock front through a solid. Good agreement was obtained between the nanosecond time scale shock‐induced adsorption redshift of the dye behind the P=2 GPa shock front, and the redshift of a nanogauge, under conditions of static high pressure loading in a diamond anvil cell at P=2 GPa. Transient effects on the ≊100 ps time scale are observed in the dye spectrum, primarily on the red abso...

Preexponential-limited solid state chemistry: Ultrafast rebinding of a heme-ligand complex in a glass or protein matrix

Ultrafast spectroscopy is used to investigate the temperature dependence of a bimolecular chemical reaction occurring at reaction centers embedded in a glycerol:water glass. The reaction centers consist of carbon monoxide bound to protoheme (PH–CO), or to myoglobin at pH=3 (Mb3–CO), a protein containing PH–CO with a broken proximal histidine–Fe bond. These systems have in common a small energetic barrier for rebinding of the photodissociated ligand. In the glass, the ligand is caged, so that only geminate rebinding is possible. Rebinding is not exponential in time. For t≳20 ps, the survival fraction of deligated heme N(t)∝t−n(n≥0). Below 100 K, rebinding is dominated by an inhomogeneous distribution of activation enthalpy P(ΔH‡) and n is temperature dependent. Inhomogeneous means that every site has a unique barrier. Above 150 K, n becomes independent of temperature. In this high temperature limit, the distribution of preexponential factors, attributed to a distribution of activation entropy P(ΔS‡), domin...

Localization of laser-induced breakdown in aggregates of silver nanoshells

Abstract Nanoshells are useful in near-infrared (NIR) applications because of the red shift of the Mie resonance into NIR. Silica/gold/silver nanoshells were prepared via gold anchoring and characterized by Mie-theoretical calculations and transmission electron microscopy, which indicated a broad and enhanced NIR absorption and a partially coated, mottled Ag/Au structure on the silica core. The mottled nanoshells were found to spontaneously aggregate in solution to form irregular islands. Their surface roughness was examined by the fractal dimension analysis. When the aggregates were irradiated with 1064-nm laser pulses, highly localized emission due to laser-induced breakdown was observed, which is attributed to the non-uniform metal distribution over the aggregate surface and the localization of the optical field induced by the disordered arrangement of the nanoshells in the aggregate.

Laser-induced breakdown spectroscopy at metal–water interfaces

Visible light emission from metal–water interfaces at laser-induced breakdown (LIB) has been observed for aluminium, titanium and platinum. The spectra are found to consist of broadband continuum without any discrete atomic lines, which may be useful as a pulsed light source for spectroscopy. A simple thermal diffusion model combined with blackbody radiation is described, the numerical result of which agrees fairly well with the observed spectra.

Ultrafast spectroscopy of temperature and pressure jump and shock waves in molecular materials

This paper discusses some dynamical aspects of shock wave excitation and shock wave induced chemical reactions in molecular materials. Just behind the shock front lies a zone of nonequilibrium material which is not in mechanical, thermal, or chemical equilbrium. The thickness of this zone depends on the nature of the material. In order to obtain picosecond time resolution of the nonequilibrium dynamics, it is necessary to probe the behavior of the material behind the shock front with nanometer spatial resolution. Progress in understanding these issues and in fabricating and characterizing optical shock nanogauges is described here.