We measured the dispersion of the absolute-differential Raman cross-sections of ammonium nitrate (NH4NO3), pentaerythritol tetranitrate (PETN), trinitrotoluene (TNT), nitroamine (HMX), and cyclotrimethylene-trinitramine (RDX) in acetonitrile and water solutions between 204 and 257 nm. The ultraviolet (UV) resonance Raman/differential Raman cross-sections of NH4NO3, PETN, TNT, HMX, and RDX dramatically increase as the excitation wavelength decreases deep into the UV to 204 nm. NH4NO3, PETN, and RDX are best resonance-enhanced by the 204 nm excitation used here, while the optimum excitation wavelength for TNT and HMX is ∼230 nm. The excitation profile of TNT roughly follows its absorption band shape. The excitation profiles for the different Raman bands of each explosive molecule differ, indicating that multiple-excitation wavelength spectra are not redundant and can offer additional information on the species present. We see no evidence of any nonlinear spectral response or sample degradation at the fluences and spectral accumulation times used here. However, we previously observed such phenomena at longer spectral accumulation times and higher fluences. These results are promising for the development of standoff deep-UV Raman methods for explosive molecule determinations.
Manash Ghosh, Luling Wang, and Sanford A. Asher, "Deep-Ultraviolet Resonance Raman Excitation Profiles of NH4NO3, PETN, TNT, HMX, and RDX," Appl. Spectrosc. 66, 1013-1021 (2012)