Standoff reflection–absorption spectra of surface adsorbed explosives measured with pulsed quantum cascade lasers

We demonstrate a standoff detection scheme of surface adsorbed explosives based on the broadband quantum cascade laser (QCL) infrared reflection–absorption spectroscopy and ultraviolet laser photodecomposition. We utilized a sub-second rapid-scan data acquisition scheme to record continuously scanned spectra which revealed the broad-band absorption features of surface adsorbed explosive molecules such as cyclotrimethylene trinitramine (RDX) and trinitrotoluene with a surface concentration of 1 μg/cm2. The standoff QCL spectra perfectly reproduce the reflectance spectra obtained with a conventional Fourier transform infrared spectroscopy instrument. The conformation conversion of RDX molecules is also observed and studied.