Temporal Evolution of the LIBS Spectra of a Representative Nitro Compound

LIBS has been investigated extensively for the detection of explosives, normally by the measurement of N, O and C lines and their ratios. There have been fewer studies on the potential of molecular fragments (e.g. CN, C2, NO) for explosive identification. Even less information is available regarding the temporal evolution of these molecules’ emission within the plasma. We have investigated the early (first two microseconds) evolution of the CN and C2 spectra from LIBS plasmas formed on 3-nitrobenzoic acid (NBA), a simulant for energetic nitro compounds. This is available in pure, solid form and contains a nitro-substituted benzene ring. It should be noted that molecular LIBS is interesting for other applications, for example, detection of biological samples or isotopic

analysis. We have measured the integrated CN violet band areas as a function of time during the first two microseconds of plasma evolution. This behavior is compared to that of graphite/NH4NO3 admixture plasmas. In addition to the spectral evolution, time resolved atomic excitation and vibrational temperatures are compared. The CN violet spectra are generally well-fit by a single vibrational temperature at times >1µs. Early spectra, however, are not well fit by varying the equilibrium temperature. These results may indicate non-equilibrium conditions or differences between rotational and vibrational temperatures. Alternatively, an optically thick plasma may cause the discrepancies between computed and measured spectra. We will describe the results of experiments currently underway to distinguish between these possible scenarios.