Nanoshocks Reveal Molecular Behavior

Robert C. Pini

What is the first step in an explosion? In energetic materials, the question focuses on the dynamics of molecular behavior. Empirically, we know that we can break bonds and make energy, but how does this violent process start? At the University of Illinois, chemistry professor Dana Dlott has been asking such fundamental questions. And he has developed a technique to help look for answers.
The molecular dynamics involved in breaking and forming bonds are examples of what chemists call large amplitude motions, and to study them, Dlott uses ultrafast laser pulses to produce shock waves that heat and cool materials on a subnanosecond timescale. Delivering the pulsed shock waves, or nanoshocks, heats samples to temperatures of 400 °C and pressures of up to 40,000 atmospheres, and cools them to ambient pressure and temperature in a few nanoseconds.
In fact, such bursts of energy produce thermochemical and thermomechanical processes involving large amplitude motions. Dlott produces the nanoshocks up to 100 times per second using a mode-locked, Q-switched Nd:YLF laser operating at 1053 nm. According to Dlott, the "homemade" laser is based on a Quantronix 117 Nd:YLF model. It pumps two dye lasers and is also cavity dumped to make a 70-ps shock pulse with 1 mJ of energy. The shock pulse is followed by two probe pulses of 30 ps total.

On/off switch
The technique is a bit like using an on/off switch to start and stop thermochemical reactions. Typically, a sample is coated with light-absorbing dye mixed with an energetic binder to generate the shocks. To observe the effects of the shock front, Dlott varies the delay of the probe pulse to match the time the shock takes to move through the sample.
Dlott and his colleagues use coherent anti-Stokes Raman to monitor the results. "We use Raman because it gives vibrational spectra that provide a fingerprint of the molecular behavior," he explained. "Ordinary Raman is weak and incoherent. Using two lasers, we can do coherent Raman." They observe the shock pulse with a Spex 1702 monochromator and a Princeton Instruments charge-coupled device detector.
"Dlott's work has the potential to answer fundamental questions behind processes," said professor Yogendra M. Gupta, director of the Institute for Shock Physics at Washington State University in Pullman.