Long range trace detection by Radar REMPI

We demonstrate the feasibility of using microwave scattering from free electrons generated by resonantly enhanced multi-photon ionization (REMPI) for trace species detection. The laser is tuned to ionize only the selected molecular trace species in ambient air. We achieve detection of parts-per-billion of NO in atmospheric pressure nitrogen, in dry air and in laboratory air with 50% humidity. In addition, we performed at-range measurements in order to prove the feasibility of using the Radar REMPI detection technique in a remote configuration. We obtained reliable backscattered microwave signal 1m away while focusing a laser from 10m distance from the target. We have extended the use of the Radar REMPI detection scheme to more complicated molecular systems by pre-dissociating the molecule into smaller fragments which can be detected with high specificity. We demonstrate the detection of SF₆ by laser dissociation of the SF₆ molecule, and measuring the Radar REMPI signals obtained from the SF₂ product. The SF₂ is produced by the UV REMPI laser pulse itself, and a scattered microwave signal is detected from the SF₂ molecule. Significant enhancement is achieved using a pre-ionizing pulse from a Nd YAG laser shortly before the measurement. The short time between fragmentation and detection allows transient fragments and fragments in vibrational nonequilibrium to be detected. This approach may allow for the identification of complex molecules by remotely detecting even short lived molecular constituents or fragments which are produced either during or just shortly before the Radar REMPI measurement