Femtosecond and nanosecond LIBS studies of nitroimidazoles: correlation between molecular structure and LIBS data

Abstract

In the present study, seven novel explosive molecules (nitroimidazoles) have been investigated for laser induced breakdown (LIB) spectral signatures of molecular and atomic species in air and argon atmospheres utilizing both femtosecond (fs) and nanosecond (ns) laser pulse excitation. The molecular emissions were observed to be stronger in the fs spectra whereas atomic emissions were prominent in the ns spectra recorded in both air and argon atmospheres. The C₂ Swan band was strongest in an argon atmosphere while the CN violet band was strongest in ambient environment. The LIB spectra were analysed for understanding (a) the influence of molecular structure i.e. type of bonds (C–C, CC, C–N and CN) on atomic (C, H, N and O) and molecular (CN, C₂ and NH) emissions, (b) effect of surrounding atmosphere on the fs and ns LIB spectra, (c) correlation between stoichiometric and intensity ratios of molecular as well as atomic species, (d) effect of the number of substituents and their position in the ring on the fragmentation pathways and (e) correlation between oxygen balance and LIB spectra. Furthermore, time resolved spectroscopic studies of the plasma induced by fs and ns laser pulses have been carried out to understand the temporal evolution and possible reaction mechanisms of various molecular species. An attempt has been made to correlate the spectral emission with the chemical structure for this series of energetic materials. Our detailed studies and analyses clearly suggest that the atomization/fragmentation ratio could serve as a performance metric for high energy materials.