Miniaturized Shock Waves Can Study Molecular Dynamics

"We have developed a versatile technique for generating and probing shock waves in virtually any material," said Dana Dlott, a U. of I. professor of chemistry. "Our technique opens the door for any research group equipped with a moderate-power, ultrafast laser system to study the effects of shock waves on complex systems of interest in chemistry, biology and medicine."

To generate their miniature shock waves at high repetition rates, Dlott, postdoctoral research associate Selezion Hambir and graduate student Jens Franken use a tabletop picosecond laser system and a special, multilayered shock array target.

First, the laser pulse ­ focused to an intense spot about 100 microns in diameter ­ is aimed at the thin outer layer of the target. This "shock generation" layer consists of an absorbing dye and an energetic binder. The molecules absorb the laser energy and explode, sending a shock wave through the sample below. The target is then moved slightly by a motorized platform and the process is repeated ­ at up to 100 times per second. The resulting shock waves, which have a duration of a few nanoseconds, are called "nanoshocks."

"With nanoshocks, a great deal of energy can be pumped into a sample in a very short time," Dlott said. "Nanoshock pulses can suddenly drive the material to extreme conditions of high pressure, high temperature or large mechanical deformation. These phenomena can then be probed by optical or vibrational spectroscopy, which allows molecular-level behavior to be investigated."

The ultrafast nature of the nanoshock technique makes it a powerful tool for studying the molecular dynamics of complex systems. For example, nanoshocks produce a very fast temperature jump that can initiate a thermochemical reaction. The shock wave rapidly compresses the sample, causing intense heating. Then, as the material springs back, it cools quickly on a subnanosecond time scale.

"It thus becomes possible to obtain spectra of materials that react or decompose too quickly to study by conventional means," Dlott said. "Large organic and biomolecular systems, which have never been studied at high temperatures and pressures, are now within reach."

Shock-wave experiments have typically been carried out at large facilities such as government or military laboratories, Dlott said. "Shocks are generated using explosive charges, gas-driven projectiles or high-energy lasers, but the repetition rate is low and the cost per shock is high. Our nanoshocks are relatively inexpensive and very easy to reproduce."

The researchers described the new nanoshock technique at the American Chemical Society national meeting in Boston, Aug. 23-28.