Decomposition of nitramine energetic materials in excited electronic states: RDX and HMX

Ultraviolet excitation (8-ns duration) is employed to study the decomposition of RDX (1,3,5-trinitro-1,3,5-triazacyclohexane) and HMX (1,3,5,7-tetranitro-1,3,5,7-tetrazacyclooctane) from their first excited electronic states. Isolated RDX and HMX are generated in the gas phase utilizing a combination of matrix-assisted laser desorption and supersonic jet expansion techniques. The NO molecule is observed as one of the initial dissociation products by both time-of-flight mass spectroscopy and laser-induced fluorescence spectroscopy. Four different vibronic transitions of NO are observed: A (2)Sigma(v(') = 0)<--X (2)Pi(v(") = 0,1,2,3). Simulations of the NO rovibronic intensities for the A<--X transitions show that dissociated NO from RDX and HMX is rotationally cold (approximately 20 K) and vibrationally hot (approximately 1800 K). Another potential initial product of RDX and HMX excited state dissociation could be OH, generated along with NO, perhaps from a HONO intermediate species. The OH radical is not observed in fluorescence even though its transition intensity is calculated to be 1.5 times that found for NO per radical generated. The HONO intermediate is thereby found not to be an important pathway for the excited electronic state decomposition of these cyclic nitramines.