Hightlight Paper

Ab Initio Study of Coherent Anti-Stokes Raman Scattering (CARS) of the 1,3,5-Trinitro-1,3,5-Triazacyclohexane (RDX) Explosion Molecule.

Advances in coherent anti-Stokes Raman scattering (CARS) over the recent years have turned this technique into a unique and powerful tool used worldwide to  probe structure and function of molecular or composite  materials in biology. In the biomedical area, CARS detection makes it is possible to extend imaging of plant cells and tissue and provides exciting possibilities to gain information on processes at the microscopic scale. CARS microscopy is also expected to have potential applications in the field of stand off-detection of foreign substances in gaseous form or in interaction with substrates. Operation at ultra-low intensities is most often required in order to detect single molecules or microscopic objects for which the basic Raman cross section generally is very small. 

Despite that CARS nowadays is a widely used technique with a well established theoretical background ab initio studies of CARS spectra of molecular systems have been lacking. Very recently this state of affairs was radically changed by the presented formalism that allows for fully analytical calculations of CARS spectra. The formalism  put forward  refrers to a matrix based open-ended theory for the calculation of frequency-dependent molecular response properties of arbitrary order, including also contributions from perturbation-dependent basis sets. This work opens new possibilities for practical applications of CARS, as it lends itself for design of CARS labels and also a priori identification of CARS fingerprints in label free probes. Furthermore, the ability to make precise determination of absolute CARS cross sections makes it possible to predetermine the requirements for technical parameters in measurments, for instance for the lasers and detectors at any a given geometrical set up in standoff detection. In order to illustrate and confirm these new possibilities we have applied the method for a common explosive molecule of interest in stand-off detection technology, namely 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) C₄H₆N₆O₆. The results indicate a very favourable agreement with features that are measured.

References

A. Mohammed, H. Ågren, A. J. Thorvaldsen and K. Ruud, manuscript in preparation

A. J. Thorvaldsen, L. Ferrighi, K. Ruud, and H. Ågren, Phys. Chem. Chem. Phys. 11, 2293  (2009)

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