New High Efficiency Deep UV Raman Spectrometer for Standoff Detection

Abstract

UV resonance Raman spectroscopy is uniquely suitable for standoff measurements due to its high sensitivity and selectivity. When excitation wavelength falls within an electronic transition of a molecule, Raman band intensities associated with the chromophore vibrations are significantly enhanced. This resonance Raman Effect, as well as negligible fluorescence interference in the deep UV, enable the detection and investigation of enhanced species at trace concentrations at a distance. We developed a state-of-the-art, high-efficiency standoff deep UV Raman spectrometer. This spectrometer is based on a custom deep UV F/8 Cassegrain telescope with a 200 mm primary mirror. This telescope is equipped with an electric secondary focus operating from infinity to 3 m distance. The UV Raman spectrograph utilizes high-efficiency deep UV transmission grating and custom Rayleigh rejection filter. As an excitation source for Raman measurements, we utilized a recently developed 228 nm compact solid state deep UV laser. The 228 nm resonance excitation enhances the Raman intensities of vibrations of NOx groups, peptide bonds, aromatic amino acid side chains, and DNA/RNA nucleotides. We used this novel spectrometer for detection of NOx-based explosive materials at trace concentrations at a stand-off distance.