This technology measures directly the characteristic vibrational spectrum of the sample under investigation. This molecular specific fingerprint allows direct identification of the substance under investigation. Standoff measurement of solids has been shown on a distance of up to 50m. Difficulties arise from the weakness of the Raman signal as well as from competing processes such as elastic scattering (Rayleigh) and fluorescence.

An area of Raman that is seeing renewed interest is standoff Raman spectroscopy, driven largely by NASA’s planetary exploration program . In standoff Raman, a pulsed laser is used to interrogate a sample from a distance from which the inelastically Raman scattered photons are collected with a large optic and detected with a dispersive spectrograph system.

Raman Lidars based on large mounted telescopes and high-powered lasers have been used for two decades for atmospheric measurements at kilometer distances.  Raman measurements of solids, liquids, and gases have been demonstrated at distances up to several hundred meters using a medium-powered UV laser and a large collection optic.  

Recent Raman spectrometers for detection of explosives have been of the dispersive type with charged coupled device (CCD) detection and of the Fourier-transform type with excitation at 1064 nm and a near infrared detector . The dispersive/CCD type Raman instruments have been made portable (<10 kg total weight) facilitating field use. Fluorescence interferences and the extremely weak nature of the Raman effect make the use of Raman for trace detection of explosives problematic unless resonance or surface enhancement methods are used. 

Despite this Raman has been already used successfully to detect residual explosives in fingerprint samples, among other small sample detection schemes. While Raman has the capability to detect very small solid samples of pure material, detection limits are too large for Raman to be used for vapour phase detection at very low levels.

However, Raman signals can be enhanced using UV wavelength excitation via both the 1/(lambda4) wavelength dependence and resonance enhancement as electronic transitions in the target molecules are approached. Wu et al. have utilized UV resonance enhancement in a mini-Raman LIDAR (light detection and ranging) apparatus for stand off detection of explosives. Their device has been applied to solid and liquid targets, but not vapours, to date (RIS). In OPTIX, the research performed in RAMAN together with the enabling technologies (Laser and Spectrometry) will lead to an improved detection system that will increase the ability for detection and identification of the explosives:  IN OPTIX, for RAMAN detection, 3 excitation wavelengths will be generated by one Nd:YAG laser to be developed in the project.

The potential of the Nd:YAG laser developed in this project will be fully exploited. Harmonic generation modules for 532, 355 and 266 nm will be available and it is planned to make use of this unique instrumental capability: Using multiple excitation wavelengths an increased probability to excite in resonance Raman will be achieved.

Furthermore, by sequential use of these excitation lines adaption to different scenarios will be possible.  Beyond the overall increase in UV Raman cross sections over those of infrared or visible, the UV cross sections can also increase due to resonance or near resonance at an allowed electronic transition. When the laser excitation frequency approaches an electronic transition of a molecule, an enormous enhancement of the Raman scattering cross section can occur for those vibrational modes that are coupled to the transition. However, when the resonance or near-resonance condition is met, the enhancement provides not only increased detection sensitivity but also simplification of the Raman spectrum, since only those vibrational modes coupled to the excitation transition undergo an enhancement.  Thus, as the Nd:YAG laser enables excitation with 532, 355 and 266 nm, particular Raman modes of different peroxide and nitroxide explosives residues will be enhanced, due to pre- or resonance Raman effects. The 266 nm laser line offers the advantage of a mainly fluorescence-free window. However, by using laser lines to record resonance Raman spectra fluorescence emission inconveniences, either from the analyte itself or from the matrix, occur. The issue of biofluorescence in particular is relevant to ground contamination assessment.  Fluorescence rejection will be addressed by ultrafast intensified CCD.

By making use of short laser pulses and time-gated detection, will be possible to record the Raman signals during the pulse while blocking most of the fluorescence. Of course both the laser pulse and the detector response need to be short relative to the fluorescence lifetime (typically in the low ns range). Thus, OPTIX approach for fluorescence rejection, will be the use of an ultrafast intensified CCD camera. The fluorescence rejection efficiency depends mainly on the closing slope of the gate, which is about 80-100 ps. This allows to improve the Raman/fluorescence ratio with several orders of magnitude.