Ultraviolet Resonant Raman Enhancements in the Detection of ...

measurements made at 532 nm. Their study also included reported strong fluorescence
of many background materials at 532 and 785 nm excitation with a significant reduction
in background fluorescence at 248 nm.
Comanescu et al. recently published a comprehensive study on the resonance
Raman spectra of TNT, RDX, HMX, and PETN using 40 UV wavelengths from 210 to
280 nm using a collection geometry [32]. This study includes tables that detail
excitation wavelengths that produced the strongest Raman bands for HE samples in
acetonitrile solutions. The strongest Raman bands tend to be the symmetric NO2 stretch
and the symmetric phenyl stretch, which is consistent with Raman selection rules.
B. STANDOFF RAMAN
In 1992, Angel et al. demonstrated standoff Raman detection of ferrocyanide at
distances of 20 m using 488 nm and 809 nm continuous wave (CW) lasers, a 40 mm
collection optic, spectrograph, and a liquid-nitrogen cooled CCD [33].
In 2000, Ray et al. at Brookhaven National Laboratory developed a UV Raman
lidar system for the standoff detection of contaminants at distances of 30 m or less using
a pulsed Nd:YAG laser operating at 266 nm with an average intensity of 200 mW/cm 3 ,
six inch Newtonian telescope, spectrograph, and CCD camera [34]. In the article, Ray
and coworkers used cyclohexane, acetonitrile, and Teflon ® for his proof of concept.
In 2005, Carter at Lawrence Livermore National Laboratory, in cooperation with
Angel and other researchers at the University of South Carolina, performed the first
Raman standoff detection measurements of high explosives in a silica matrix [19]. Using
a pulsed 10 Hz Nd:YAG laser at 532 nm, an 8 in Schmidt–Cassegrain telescope,
spectrograph and a gated ICCD camera, various explosive samples were detected at
standoff distances up to 50 meters. This work also included a power density study which
demonstrated linear Raman intensities up to ~3 x 10 6 W/cm 2 . Additionally, in some HE
samples photodecomposition or other signal degrading effects were observed.
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