Project of the month, April 2009

Resonant Raman Software for Standoff Detection

Resonant Raman spectroscopy is a powerful technique used worldwide to probe structure and function of molecular materials and condensed matter. In the biomedical area Raman detection makes it is possible to extend imaging to cells and molecules where most of the understanding about diseases, and their early-stage development, is to be found. Other important applications include point detection of foreign substances in gaseous form or in interaction with substrates. Operatation at ultra-low intensities is most often required in order to detect single molecules or microscopic objects for which the basic cross section generally is very small. The low Raman scattering cross sections can be compensated for (by several orders of magnitude) by applying conditions of electronic resonance or plasmon field enhancements. However, these conditions aggravate the analysis for the Raman signal in terms of molecular signatures and struture-to-property relations as the underlying theoretical descriptions then become more complex. In the present project we have presented a generalization of the microscopic expressions connecting molecular properties and Raman scattering intensities to include conditions of electronic resonances. An expression for resonant molecular Raman scattering in terms of the complex electric-dipole polarizability tensor is presented and implemented with first principles complex polarization propagator theory. The theory reveals the strong role of phase delays of the detected Raman radiation in the resonant case.

Adopting the resonant-convergent complex polarization propagator approach we are able to predict resonant Raman intensities entirely from first-principles. The software can favorably be used in conjunction with hardare for stand-off detection of foreign substances. These substances can be of gaseous form in ultra-low concentrations as due to emissions from sealed objects, like parcels, bags, bottles, any concealing package or open source, like human breath. Substances of interest for detection can be drugs, explosives, poison, microbes, and viruses. With the present proposal we lay the basis for a firm identification of low-concentration substances by establishing fingerprints in the form of a resonance Raman spectrum. We also provide the absolute cross sections for the resonant Raman signal, which is a crucial piece of information in order to derive the technical specification of the detection instruments and the geometrical set up for stand-off detection.

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