First-principles study of laser absorption characteristics of five typical explosives

The ultrafast high peak powers of femtosecond laser ablation can be used to safely process explosives by precisely controlling the laser energy and focus. The laser absorption mechanisms and capacities are wavelength dependent due to the different compositions and structures of explosives. The absorption characteristics directly affect the efficiency and safety of femtosecond laser processing. Therefore, the optical absorption mechanisms and characteristics of five typical explosives excited by femtosecond lasers at different wavelengths have been studied. Absorption spectra of β-phase octahydro-1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane (β-HMX), 1,3,5-triamino-2,4,6-trinitrobenzene (TATB), cyclotrimethylenetrinitramine (RDX), pentaerythritol tetranitrate (PETN) and 2,4,6-trinitrotoluene (TNT) were obtained by quantum chemistry calculation. With infrared (IR) lasers, explosives absorb energy mainly via vibrational excitation in functional groups, which subsequently leads to bond breaking. Calculations indicated that the absorption maxima attributed to vibrations in these explosives were all concentrated between 3125 and 3571 nm. Ultraviolet–visible (UV–vis) lasers could be used to excite the explosives via valence electron transitions, which could trigger chemical reactions. β-HMX, RDX and PETN had narrow absorption spectra with strong peaks concentrated around 170–225 nm, while TATB and TNT had broad absorption ranges with strong peaks around 230–300 nm. These bands were far from the widely used 810 nm near-infrared femtosecond laser.