Substrate-related effects on molecular and atomic emission in LIBS of explosives

Abstract

Femtosecond laser induced breakdown spectroscopy (LIBS) has been shown to be sensitive to a variety of ERC's (explosive-related compounds) deposited on substrates. In LIBS, surface material is excited by a high-powered laser pulse forming a plasma. The optical emission from this plasma is collected and spectrally analyzed to determine the surface species entrained in the excitation event. The detection of explosive related compounds in the field presents many challenges, one of these being the wide variety of materials surfaces that might be covered with ERC's. Results from femtosecond and nanosecond LIBS of ERC's of metal, glass, and polymer substrates show that the optical properties of the substrate play a large role in the observed emission. Results indicate that nanosecond LIBS of ERC's on metal surfaces yield strong atomic emission while nanosecond LIBS of ERC's on glass results in some molecular emission. Molecular emission is also present in femtosecond LIBS spectra of ERC's on all surfaces but is particularly strong for metal substrates. In particular emission from the CN molecular fragment could provide a means to understand the effect of the substrate on the excitation event in nitroaromatic compounds since it is present in both nanosecond LIBS spectra of the TNT/glass system and femtosecond LIBS spectra of the TNT/Al system. The origins of this CN molecular fragment are currently being studied since fragmentation and reaction processes in LIBS events are not fully understood at this time.