Deep-Ultraviolet Resonance Raman Excitation Profiles of NH₄NO₃, PETN, TNT, HMX, and RDX

Abstract

We measured the dispersion of the absolute-differential Raman crosssections 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.

Recent Developments in Spectroscopic Techniques for the Detection of Explosives

“…It was found that the spectral signal increases significantly with the decrease of excitation wavelength [114]. …”

Advances in explosives analysis—part II: photon and neutron methods

“…Different Nd:YAG harmonics (266, 355, and 532 nm) have been compared for optimizing Raman versus fluorescence signals for explosives detection [104]. Deep-ultraviolet (DUV) resonant excitation increases the Raman cross section significantly, and can be performed at such short wavelengths that the interfering fluorescence occurs at wavelengths outside the range of the Raman spectrum [105][106][107][108][109]. DUV excitation is strongly absorbed and thus only probes the surface of materials, so there is a tradeoff between the higher cross section of the DUV excitation and the larger number of molecules excited with visible excitation [101].…”

Recent Advances and Remaining Challenges for the Spectroscopic Detection of Explosive Threats

“…His work is very extensive in the area of UV Raman for explosives detection, and over the past three years, his work has covered signatures, spectral profiles, cross sections, and solid solution analysis. [22][23][24][25] While his work has not been in collaboration with ECBC, we have shared data and openly discussed the challenges of acquiring Raman cross sections of solid materials. These combined measurements have been used to assess the optimal excitation wavelength(s) for the detection of explosives from the deep UV to the near infrared (NIR) and determine differences in cross sections and signatures in the solid state (Table I).…”

“…A summary table of the Raman cross sections of explosives is given in Table I, including both solution and solid-phase cross sections (in parentheses) as a function of wavelength. The values listed in this table were taken from Ghosh et al 25 and Emmons et al 26,27 It is expected that the Raman spectra and cross sections of the solid phase explosives will differ from those obtained in solution due to differences in molecular conformation. For example, the most stable conformation for gas-phase RDX molecules is believed to be the chair conformation with all three nitro groups pointing parallel to the axis of the ring, the so-called AAA conformation (the conformation adopted in crystalline b-RDX).…”