Scientists at the University of Arizona and Pacific Northwest National Laboratories used a broadly tunable swept-wavelength external cavity quantum cascade laser (swept-ECQCL) operating in the midwave infrared (MWIR) spectral region to measure transmission through explosive fireballs generated from charges of four different explosive types detonated in an enclosed chamber. The swept-ECQCL’s broad wavelength tuning range allowed the rapid measurement of multiple chemical substances and large molecules in the fireballs. The high-energy explosives were placed in a specially designed chamber to contain the fireball. The researchers directed a laser beam from the swept-ECQCL through this chamber while rapidly varying the laser light’s wavelength. The laser light transmitted through the fireball was recorded throughout each explosion to measure changes in the way infrared (IR) light was absorbed by the molecules in the fireball. Using an instrument built in their lab, the researchers were able to measure explosive events at faster speeds, at higher resolutions, and for longer time periods than previously possible using IR laser light. This image shows how a swept-wavelength external cavity quantum cascade laser measures rapid changes in infrared light absorbed by molecules inside an explosive detonation. The ability to measure and monitor the changes during explosions could help scientists understand and even control them. Courtesy of Mark C. Phillips. “The swept-ECQCL approach enables new measurements by combining the best features of high-resolution tunable laser spectroscopy with broadband methods such as FTIR,” researcher Mark Phillips said. Carbon dioxide, carbon monoxide, nitrous oxide, and other substances were detected by the characteristic way in which they absorbed IR light. Detailed analysis of the results from the swept-ECQCL provided the researchers with information about temperature and concentrations of these substances throughout the explosive event, including initial detonation, fireball expansion and cooling, and diffusive mixing in the chamber. The scientists also used spectral analysis to measure absorption and emission of IR light from tiny solid particles (soot) created by the explosion. The swept-ECQCL measurements could provide a new way to study explosive detonations, and the swept-ECQCL approach could have other uses. In future studies, the researchers hope to extend the measurements to more wavelengths, faster scan rates, and higher resolutions. Scientists from the University of Illinois at Urbana-Champaign also participated in the research. The research was published in the Journal of Applied Physics (https://doi.org/10.1063/1.5107508).