Broadband Mid-Infrared Stand-Off Reflection–Absorption Spectroscopy Using a Pulsed External Cavity Quantum Cascade Laser

Broadband mid-infrared molecular spectroscopy is essential for detection and identification of many chemicals and materials. In this report, we present stand-off mid-infrared spectra of 1,3,5-trinitro-1,3,5-triazine or cyclotrimethylene trinitramine (RDX) residues on a stainless-steel surface measured by a broadband external cavity quantum cascade laser (QCL) system. The pulsed QCL is continuously scanned over 800 cm⁻¹ in the molecular fingerprint region and the amplitude of the reflection signal is measured by either a boxcar-averager-based scheme or a lock-in-amplifier-based scheme with 1 MHz and 100 kHz quartz crystal oscillators. The main background noise is due to the laser source instability and is around 0.1% of normalized intensity. The direct absorption spectra have linewidth resolution around 0.1 cm⁻¹ and peak height sensitivity around 10⁻² due to baseline interference fringes. Stand-off detection of 5–50 µg/cm² of RDX trace adsorbed on a stainless steel surface at the distance of 5 m is presented.